Effects Of Alpha-adrenergic Stimulation Research Articles

P.3.e.009 Cognitive enhancement in schizophrenia: the role of dopamine and noradrenaline in the prefrontal cortex

The present experiments were designed to examine the effects of alpha-1 adrenergic stimulation and inhibition on memory encoding and to investigate whether the alpha-1 adrenergic and muscarinic cholinergic systems interact in the regulation of spatial navigation behavior in the Morris water maze test and we also studied the effects of d-cycloserine, a partial agonist at the glycine binding site on the N-methyl-d-aspartate (NMDA) receptor complex, on the performance of scopolamine-treated rats in this task. Pre-training subcutaneous administration of St-587 (a putative alpha-1 agonist) at 1000 μg kg−1 or 1500 μg kg−1 improved water maze navigation to a hidden platform. Prazosin (an alpha-1 antagonist), 300–2000 μg kg−1, did not significantly impair the spatial navigation performance. Pre-training administration of prazosin 1000 μg kg−1, but not 300 μg kg−1, slightly potentiated the deficit in water maze navigation seen after scopolamine (200 μg kg−1, pre-training intraperitoneal injection). Pre-training administration of St-587 at a dose 1500 μg kg−1, but not 500 μg kg−1, slightly ameliorated the scopolamine-induced (200 μg kg−1) impairment in performance of rats. Pre-training administration of prazosin at doses 300 or 1000 μg kg−1 or St-587 at doses 500 μg kg−1 or 1500 μg kg−1 did not have any significant influence on the scopolamine-induced (200 μg kg−1) increase of swimming speed. Furthermore, d-cycloserine at the dose of 300 μg kg−1 but not 1000 or 3000 μg kg−1 reversed the scopolamine (200 μg kg−1)-induced deficit in acquisition of the water maze task but not the increase in motor output (increased swimming speed). These results indicate that the stimulation of alpha-1 adrenoceptors may facilitate the encoding of new information. These findings suggest that alpha-1 adrenergic mechanisms do not participate or at least are not the most critical part of the noradrenergic system in the interaction between noradrenaline and muscarinic receptors in the modulation of learning and memory. In addition, these results suggest that d-cycloserine may be effective in alleviating states of central cholinergic hypofunction.

HERG K(+) channel activity is regulated by changes in phosphatidyl inositol 4,5-bisphosphate.

Autonomic stimulation controls heart rate and myocardial excitability and may underlie the precipitation of both acquired and hereditary arrhythmias. Changes in phosphatidyl inositol bisphosphate (PIP2) concentration results from activation of several muscarinic and adrenergic receptors. We sought to investigate whether PIP2 changes could alter HERG K(+) channel activity in a manner similar to that seen with inward rectifier channels. PIP2 (10 micromol/L) internally dialyzed increased the K(+) current amplitude and shifted the voltage-dependence of activation in a hyperpolarizing direction. Elevated PIP2 accelerated activation and slowed inactivation kinetics. When 10 micromol/L PIP2 was applied to excised patches, no significant change in single channel conductance occurred, indicating that PIP2-dependent effects were primarily due to altered channel gating. PIP2 significantly attenuated the run-down of HERG channel activity that we normally observe after patch excision, suggesting that channel run-down is due, in part, to membrane depletion of PIP2. Inclusion of a neutralizing anti-PIP2 monoclonal antibody in whole cell pipette solution produced the opposite effects of PIP2. The physiological relevance of PIP2-HERG interactions is supported by our finding that phenylephrine reduced the K(+) current density in cells coexpressing alpha1A-receptor and HERG. The effects of alpha-adrenergic stimulation, however, were prevented by excess PIP2 in internal solutions but not by internal Ca(2+) buffering nor PKC inhibition, suggesting that the mechanism is due to G-protein-coupled receptor stimulation of PLC resulting in the consumption of endogenous PIP2. Thus, dynamic regulation of HERG K(+) channels may be achieved via receptor-mediated changes in PIP2 concentrations.

Alpha2-adrenoceptors inhibit antidiuretic hormone-stimulated Na+ absorption across tight epithelia (Rana esculenta).

In the present study we examined the effect of alpha-adrenergic regulation of active transepithelial Na+ absorption across the isolated frog skin epithelium. alpha-Adrenergic stimulation was achieved by addition of the adrenergic agonist noradrenaline in the presence of the beta-adrenergic blocker propranolol. alpha-Adrenergic stimulation inhibited basal as well as antidiuretic hormone (ADH)-stimulated Na+ transport. The ADH-induced increase in Na+ transport was accompanied by a membrane depolarisation due to an increase in the apical Na+ permeability. The subsequent application of noradrenaline inhibited the Na+ transport and repolarised the membrane potential, suggesting that alpha-adrenergic stimulation had reduced the apical Na+ permeability. The inhibition was abolished by the alpha2-adrenergic antagonist yohimbine whereas it was insensitive to the alpha1-adrenergic antagonist prazosin. alpha-Adrenergic stimulation had no effect on the cytosolic free [Ca2+] ([Ca2+]i). Incubation of the epithelium in the presence of ADH increased the cellular adenosine 3',5'-cyclic monophosphate (cAMP) content, an increase which was abolished by alpha-adrenergic activation. The effect of alpha-adrenergic stimulation on cAMP production was abolished by the alpha2-adrenergic antagonist yohimbine. We conclude that the noradrenaline-induced inhibition of the ADH-stimulated Na+ absorption and cAMP content is mediated by activation of alpha2-adrenoceptors. The data further indicate that the principal cells of the epithelium do not express alpha1-adrenoceptors. The noradrenaline-induced inhibition of the ADH-stimulated Na+ transport was concentration dependent, with 0.24+/-0.03 microM eliciting a half-maximal response. This alpha2-adrenergic-mediated down-regulation of Na+ absorption is achieved at a concentration of noradrenaline which begins to activate the NaCl secretion via the skin glands. The alpha2-adrenoceptors therefore appear to have considerable physiological importance.

Effects Of Alpha-Adrenergic Stimulation On Grafted Internal Mammary Artery Flow In Patients Receiving Milrinone Versus Nitroglycerin

Introduction. The internal mammary artery (IMA) is a living conduit subjected to the influence of vasoactive agents. Both nitroglycerin (TNG) and milrinone (M) are known to dilate the IMA. [1,2] Alpha agonists are frequently used to treat excessive vasodilatation after CPB and may decrease IMA flow despite TNG or M, thus risking myocardial ischemia. [3] The purpose of this study was to measure changes in grafted IMA flow after the administration of phenylephrine in patients receiving either TNG, milrinone, or both drugs concomitantly. [3] Methods. After approval from the institutional review board and informed consent, 30 adult patients undergoing coronary artery bypass grafting (CABG) with IMA placement were studied. IMA flow was measured with a calibrated laser Doppler probe (Transonic Inc, Ithaca NY). The IMA was anastomosed to the left anterior descending artery with a continuous 8-0 prolene suture in all cases. After separation from CPB, patients were then randomized to receive either a loading dose of milrinone (M, 50 [micro sign]g/kg), an infusion of nitroglycerin (TNG, 2 [micro sign]g/kg/min), or a loading dose of milrinone followed by TNG (M+TNG). Phenylephrine was then administered by infusion and titrated to raise the mean arterial pressure (MAP) by 20% or to 60 mm Hg. IMA flow was measured prior to the administration of the drugs, 10 minutes following either the beginning of the TNG infusion or the conclusion of the M bolus, and after the target MAP was reached. Statistical analysis was performed with ANOVA of repeated measures. A p < 0.05 was considered significant. Results. The administration of phenylephrine in the presence of TNG increased mean IMA flow in 6/10 patients (25 +/- 10 mL/min to 52 +/- 12 mL/min; p < 0.001); however, IMA flow decreased in 4/10 patients (33 +/- 16 mL/min to 24 +/- 17 mL/min; p < 0.04). Phenylephrine increased mean IMA flow from 55 +/- 15 mL/min to 87 +/- 13 mL/min in the M group (p < 0.01) and from 48 +/- 10 mL/min to 67 +/- 12 mL/min in the TNG+M group (p<0.02). No patients receiving M or TNG+M exhibited a reduction in IMA flow. Discussion. In patients receiving milrinone, alone or in combination with TNG, the use of alpha adrenergic agonists after CPB appears safe since IMA flow is preserved or enhanced. In the presence of TNG, stimulation of alpha receptors may actually decrease IMA flow in some patients and may be a risk factor for post-CPB myocardial ischemia. (Figure 1)Figure 1

Effects of alpha1-adrenergic stimulation on L-type Ca2+ current in rat ventricular myocytes.

The effect of alpha1-adrenergic stimulation on L-type Ca2+ current (ICa,L) in adult rat ventricular myocytes was investigated using three different methods of current recording. During conventional whole-cell recordings with 5 mm-BAPTA included in the pipette solution, phenylephrine (20 microM) did not increase ICa,L after 10 min of application. With nystatin perforated-patch whole-cell recordings, phenylephrine potentiated ICa,L, although there were variations among myocytes. The most frequent response was a transient suppression of peak ICa,L at approximately 2 min of exposure followed by a sustained increase of current amplitude evident after 5-10 min exposure. The relative current amplitude 10 min after phenylephrine application was 1.08+/-0.05 compared to control (n=14 cells,P<0.05). During cell-attached single channel recordings, phenylephrine (1 microM) increased the L-type Ca2+ channel open probability (NPo) by 2.25+/-0.31-fold (n=21,P<0.01). It potentiated NPo by increasing the number of openings per sweep and also by promoting longer openings. These effects developed slowly in approximately 10 min. Phenylephrine had no on unitary current amplitude. The potentiation was also elicited by methoxamine (5 microM) and was blocked by prazosin (1 microM), indicating that it was mediated by alpha1-adrenergic receptor stimulation. The increase in NP(o) was suppressed by chelerythrine, a protein kinase C inhibitor. Our results demonstrate that ICa,L can be enhanced by alpha1-adrenergic stimulation, and stress the importance of not disturbing the intracellular environment during studies of the modulation of cardiac ICa,L by alpha1-adrenergic stimulation.

Alpha1-Adrenergic activation of L-type Ca current in rat ventricular myocytes: perforated patch-clamp recordings.

alpha1-Adrenergic stimulation has little effect on L-type Ca2+ channel current (ICa,L) in adult cardiac myocytes measured using conventional whole cell voltage-clamp techniques. In this study using perforated-patch techniques, we reevaluated the effect of alpha1-adrenergic stimulation on ICa,L in adult rat ventricular myocytes. Action potentials and ICa,L were examined in the presence of 1 microM nadolol, a beta-adrenergic antagonist, in myocytes internally dialyzed with Na+- and K+-free solutions (Cs+ and tetraethylammonium as substitutes). Phenylephrine (PE; 30 microM) increased the action potential duration measured at 25 and 70% of repolarization by 104 and 86%, respectively. In the perforated-patch configuration, PE elicited a transient decrease followed by a approximately 60% increase in ICa,L, whereas only the transient decrease in ICa,L was observed in myocytes when the conventional whole cell configuration was used. The PE-induced increase in ICa,L was reversibly blocked by 1 microM prazosin, an alpha1-adrenergic antagonist. These results suggest that alpha1-adrenergic stimulation enhances cardiac ICa,L and that obligatory intracellular mediators for this action are lost during whole cell recordings.

Stimulation of alpha-1 adrenergic receptors facilitates spatial learning in rats

The present experiments were designed to examine the effects of alpha-1 adrenergic stimulation and inhibition on memory encoding and to investigate whether the alpha-1 adrenergic and muscarinic cholinergic systems interact in the regulation of spatial navigation behavior in the Morris water maze test and we also studied the effects of d-cycloserine, a partial agonist at the glycine binding site on the N-methyl- d-aspartate (NMDA) receptor complex, on the performance of scopolamine-treated rats in this task. Pre-training subcutaneous administration of St-587 (a putative alpha-1 agonist) at 1000 μg kg −1 or 1500 μg kg −1 improved water maze navigation to a hidden platform. Prazosin (an alpha-1 antagonist), 300–2000 μg kg −1, did not significantly impair the spatial navigation performance. Pre-training administration of prazosin 1000 μg kg −1, but not 300 μg kg −1, slightly potentiated the deficit in water maze navigation seen after scopolamine (200 μg kg −1, pre-training intraperitoneal injection). Pre-training administration of St-587 at a dose 1500 μg kg −1, but not 500 μg kg −1, slightly ameliorated the scopolamine-induced (200 μg kg −1) impairment in performance of rats. Pre-training administration of prazosin at doses 300 or 1000 μg kg −1 or St-587 at doses 500 μg kg −1 or 1500 μg kg −1 did not have any significant influence on the scopolamine-induced (200 μg kg −1) increase of swimming speed. Furthermore, d-cycloserine at the dose of 300 μg kg −1 but not 1000 or 3000 μg kg −1 reversed the scopolamine (200 μg kg −1)-induced deficit in acquisition of the water maze task but not the increase in motor output (increased swimming speed). These results indicate that the stimulation of alpha-1 adrenoceptors may facilitate the encoding of new information. These findings suggest that alpha-1 adrenergic mechanisms do not participate or at least are not the most critical part of the noradrenergic system in the interaction between noradrenaline and muscarinic receptors in the modulation of learning and memory. In addition, these results suggest that d-cycloserine may be effective in alleviating states of central cholinergic hypofunction.

Alpha 1-adrenergic stimulation inhibits 3,5,3'-triiodothyronine-induced expression of the rat heart sarcoplasmic reticulum Ca2+ adenosine triphosphatase gene.

The interactions between the beta-adrenergic system and thyroid hormone (T3) on cardiac function have been investigated in detail. In addition to beta-adrenoceptors, alpha 1-adrenergic receptors are present in the mammalian heart. The interactions between T3 and the alpha 1-adrenergic system remain, however, poorly understood. T3 stimulates the expression and transcription of the sarcoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA2) gene, a protein vital in the control of cardiac calcium transients and contractility. We show that in rat cardiac myocytes, the stimulatory effect of T3 on SERCA2 messenger RNA expression and gene transcription is inhibited by an alpha 1-adrenergic agonist. We demonstrate that direct activation of the alpha 1-adrenergic signaling pathway, using a mutant constitutively active G protein (Gq) similarly down-regulated the T3 effect on SERCA2 transcription. The combined effect of thyroid hormone receptor and retinoid X receptors on T3-stimulated SERCA2 gene transcription was also markedly attenuated by alpha 1-adrenergic stimulation. These results suggested that activation of the alpha 1-adrenergic signaling pathway has an inhibitor effect on T3-dependent SERCA2 gene transcription. As this inhibitory effect of alpha 1-adrenergic stimulation occurs when only one thyroid hormone response element (TRE) drives reporter expression, it is most likely mediated by an alteration of the nuclear factors binding to the TRE or by influencing the interaction of the TRE complex with the basal transcriptional machinery.

Alpha 1-adrenergic stimulation of isolated rat atria results in discoordinate increases in natriuretic peptide secretion and gene expression and enhances Egr-1 and c-Myc expression.

We studied the effects of alpha1-adrenergic stimulation on atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) secretion and gene expression in isolated right atria. The early-response genes Egr-1 and c-myc were also studied as potential markers of transcriptional activation after alpha1-adrenergic stimulation. Isolated right atria from rats were stimulated for up to 8 h by the alpha1-adrenergic agonist phenylephrine (PE). PE at 10, 50, or 100 microM stimulated the secretion of immunoreactive (ir) ANF, beginning at 0.5 h and peaking after 1.5 h, IrANF secretion remained significantly elevated for 8 h with 100 microM PE, reached control levels after 5 h with 10 microM PE, and after 6 h microM PE with 50 microM PE, PE at 50 or 100 microM stimulated irBNP secretion after 15 min, which peaked at 1 h, and thereafter remained above control levels. Calculation of irANF/irBNP ratios revealed that their stimulated secretion was not coregulated. PE caused significant changes in steady state transcript levels for the genes studied. After 6 h, 50 microM PE caused a 49% increase in ANF messenger RNA (mRNA) levels. BNP mRNA levels were increased by 135% after 6 h and by 77% after 8 h. Egr-1 mRNA levels were increased by 81% after 4 h, 167 after 6 h, and 40% after 8 h of treatment, mRNA levels of c-myc were increased by 49% after 4 h and 53% after 6 h. PE-induced increases in secretion and gene expression were inhibited by the alpha1-adrenergic receptor antagonist prozosin (10 microM). We conclude that both ANF and BNP secretion from atria can be stimulated by PE, and that their secretion is not coregulated. The kinetics of enhanced natriuretic peptide gene expression and secretion did not change in parallel, suggesting that these processes are not acutely coordinated. The enhanced expression of Egr-1 and c-myc suggests that they may be involved in the modulation of atrial gene expression in response to alpha1-adrenergic stimulation. The results presented suggest that compensatory adrenergic activation such as those seen in several clinical entities may be one of the factors that provide long-term enhanced natriuretic peptide production, thus contributing to the maintenance of cardiovascular homeostasis.

Inotropic effects of alpha 1-adrenergic agonists in myocardium from rats with postinfarction heart failure.

In clinical and experimental heart failure, the inotropic response to beta-adrenergic receptor stimulation is depressed. Therefore, non-beta-adrenergic mechanisms may assume increasing importance for summoning inotropic reserve in the failing heart. To test the integrity of the inotropic pathway mediated by alpha 1-adrenergic receptor stimulation in a model of chronic ischemic heart failure, we administered phenylephrine to noninfarcted left ventricular papillary muscles isolated from sham-operated rats (n = 10) and rats with large (> 40% left ventricular circumference) anterior myocardial infarctions (n = 9). Isometric force was monitored, and intracellular Ca2+ (Cai2+) transients were recorded with the bioluminescent protein aequorin. Compared with muscles from sham-operated rats, contractility of muscles from rats with postinfarction heart failure was depressed at extracellular Ca2+ concentrations between 0.5 and 3.0 mM. Phenylephrine produced comparable dose-dependent increases in developed tension (126 +/- 4 vs. 125 +/- 7% of baseline) and peak rate of tension rise (125 +/- 4 vs. 140 +/- 9% of baseline) in muscles from sham and infarcted rats, respectively. There was no significant change in the time course of the isometric twitch or in the time course or amplitude of the Cai2+ transient after phenylephrine administration in muscles from either group. No evidence of Cai2+ overload, as defined by spontaneous Ca2+ release, was observed during phenylephrine administration in muscles from normal or failing hearts. The density of alpha 1-adrenoceptors measured with [3H]prazosin binding in crude membranes isolated from noninfarcted left ventricular tissue was not different in control and infarcted hearts (48 +/- 5 vs. 53 +/- 4 fmol/mg protein). These data indicate that the positive inotropic effect of alpha-agonists may be preserved in chronic ischemic heart failure. In both normal and failing myocardium, the inotropic effects of alpha 1-adrenergic stimulation occurred with little or no increase in Cai2+ availability and no apparent adverse effects on myocardial relaxation. Therefore, agents that act by similar mechanisms may have certain therapeutic advantages over traditional inotropic agents in patients with heart failure.

Effects of alpha adrenergic stimulation on time independent potassium current of isolated ventricular myocytes

The role of alpha adrenergic receptor stimulation on ventricular electrical activity is controversial. The aim of the present paper was to study a wide range of concentrations of alpha adrenergic agonists on the electrical properties of guinea pig's heart isolated ventricular myocytes. The experiments were performed according to the single electrode voltage clamp technique. Phenylephrine and epinephrine (in the presence of propranolol) were used at concentrations from 1 × 10 −9 to 10 −5M. It was observed that both agonists induce an increase in the time independent inward rectifying potassium current (i k1), that could explain the shortening of the action potential. All the observed effects were dose-dependent and disappeared during washout. These results could explain, at least partially, some of the electrical changes observed during ischemia.

Effect of alpha adrenergic stimulation and carnitine palmitoyl transferase I inhibition on hypertrophying adult rat cardiomyocytes in culture.

Long-term, serum supplemented cultures of rat adult ventriculocytes were utilized to study the tropic effects of the alpha-agonist phenylephrine and of the carnitine palmitoyltransferase I inhibitor etomoxir. Cell protein and the rate of incorporation of phenylalanine were measured, corrected for cellular DNA content and utilized as an index for hypertrophy and of anabolic activity of the cells, respectively. The mRNA level of ANF was utilized as an index for the pathological phenotypic change (i.e., switch to fetal gene program), and that of the Na-channel--a constantly expressed gene in normal and hypertrophic cardiomyocytes--served as an internal control. Both mRNAs were quantified at various stages in culture by competitive reverse transcriptase PCR. The size of control myocytes steadily increased for over 3 weeks. The cells were completely redifferentiated and reached a maximum of anabolic activity 2 weeks after plating. Secretion and mRNA levels of ANF were increased severalfold after 7-8 days. Addition of 10 microM phenylephrine considerably speeded up cell growth. Maximum anabolic activity and complete redifferentiation were reached already after 1 week. Levels of mRNA and of ANF release increased 30-40 fold. Interestingly, induction of ANF gene transcription lagged behind the redifferentiation of the cells. Ten microM etomoxir inhibited the oxidation of palmitic acid and stimulated that of exogenous glucose by adult cardiomyocytes. In spite of its clear effect on fuel utilization, etomoxir had no direct hypertrophic effect on the myocytes in culture and did not inhibit the stimulatory action of alpha-agonists. Reactivation of the fetal gene program, as visualized by ANF production, was not reversed by etomoxir.

Release of atrial natriuretic factor.

The cardiac atria synthesize and store a hormone termed atrial natriuretic factor (ANF). ANF is released into the systemic circulation, and the circulating 28 amino acid peptide can be measured by radioimmunoassay. The hormone participates in body fluid homeostasis through its effect on renal sodium excretion and by inducing a shift of circulating fluid to the interstitial space. Release of ANF is mainly regulated by mechanical changes in the left and right atrial wall. It has been demonstrated that ANF release is related to changes in atrial wall tension occurring during each atrial cycle, and therefore, release of ANF will increase with increasing heart rate. Not only the increase in wall tension during passive atrial distension (v wave), but also the increase in tension during atrial systole (a wave) are determinants of ANF release. The mechanochemical transducer is most likely located in the atrial myocytes, but its nature is unknown. There is no evidence to suggest that efferent cardiac nerves are essential in the regulation of ANF release. Humoral factors have been suggested as regulators of ANF release, particularly catecholamines and angiotensin II. A receptor-mediated direct stimulatory effect of alpha-adrenergic stimulation and an inhibitory effect of beta-adrenergic stimulation have been demonstrated, but these direct effects are small compared to the effect of changes in atrial wall tension. Circulating catecholamines and angiotensin II stimulate ANF release mainly through their haemodynamic effects.

Alpha 1- and beta-adrenergic interactions on L-type calcium current in cardiac myocytes.

We investigated the mechanism by which alpha 1-adrenergic activation regulates basal and stimulated whole cell L-type Ca current (ICa) in rat ventricular myocytes using the physiological neurotransmitter, norepinephrine (NE, 10 microM). Stimulation of alpha 1-adrenoceptors, achieved by NE + 10 microM esmolol (a beta-receptor antagonist), had no significant effect on basal ICa. alpha 1-adrenergic activation had a marked inhibitory effect on ICa elevated by beta activation (NE + 1 microM) prazosin, an alpha 1-receptor antagonist) or activation of adenylyl cyclase by forskolin (25 microM); the inhibitory effect was reversible upon washout. However, alpha 1-adrenergic stimulation had no significant effect on ICa previously increased by intracellular application of cAMP (25 microM). The inhibitory effect seen on ICa elevated by NE showed no significant shift of either I-V or inactivation curves. It is unlikely that the inhibitory effect of alpha 1-adrenergic stimulation on NE or forskolin-elevated ICa is mediated through activation of Ca-dependent protein kinase C or changes in intracellular free Ca (pCa = 8.5, EGTA 5 mM) or cAMP-dependent phosphodiesterase. We conclude that alpha 1-adrenergic inhibition of beta-adrenergic stimulated-ICa is probably mediated through an as yet unknown G-protein. This inhibitory effect could serve as a regulatory feedback mechanism in physiological and pathophysiological settings.

Effect of alpha-adrenergic stimulation on regional contractile function and myocardial blood flow with and without ischemia.

The effect of alpha-adrenergic receptor activation on regional contractile function and transmural myocardial blood flow is controversial. Accordingly, the effects of selective alpha 1-(methoxamine) and alpha 2-(BHT 933) receptor stimulation on regional contractile function and transmural myocardial blood flow distribution were studied in 15 anesthetized open-chest dogs. The alpha-adrenergic agonists were separately infused into the cannulated left circumflex coronary artery during control and ischemic conditions in the same animal. Mean coronary perfusion pressure was held constant by a servocontrolled pump in an extracorporeal circuit. Ischemia was created by reducing coronary perfusion pressure to the level at which percent systolic wall thickening (%WT) decreased by 54%. Contractile function during control conditions was unchanged, whereas under ischemic conditions a further significant decrease in %WT of 27% occurred with either alpha 1- or alpha 2-receptor stimulation without any change in the anterior (control) wall function. Both alpha 1- and alpha 2-receptor stimulations during control conditions resulted in a relatively uniform transmural decrease in blood flow with no change in the subendocardial-to-subepicardial blood flow ratio. With alpha 1-stimulation during ischemia (n = 13), there was a tendency toward decreased subepicardial blood flow with no change in subendocardial flow, resulting in an increased subendocardial-to-subepicardial blood flow ratio (0.61 +/- 0.23 to 0.82 +/- 0.40, p less than 0.05). alpha 2-Receptor stimulation during ischemia (n = 12) produced a significant decrease in subepicardial blood flow (0.45 +/- 0.20 to 0.35 +/- 0.12 ml/min/g, p less than 0.01) with no change in subendocardial blood flow, also resulting in an increased subendocardial-to-subepicardial blood flow ratio. These results indicate the selective vasoconstriction in outer wall layers during ischemia mediated by either alpha 1- or alpha 2-receptors can cause a decrease in regional contractile function despite unchanged subendocardial blood flow and improved subendocardial-to-subepicardial flow ratio. This suggests an adverse effect of alpha-adrenergic vasoconstriction during ischemia in this coronary perfusion pressure-controlled canine model.

Effect of alpha-1 adrenergic stimulation on action potential duration are time-dependent: E. Drouin, F. Charpentier, H. Le Marec. Laboratory of cardiology, UA CNRS 1340, Nantes, France

Effect of alpha-1 adrenergic stimulation on action potential duration are time-dependent: E. Drouin, F. Charpentier, H. Le Marec. Laboratory of cardiology, UA CNRS 1340, Nantes, France

Abnormal automatic rhythms in ischemic Purkinje fibers are modulated by a specific alpha 1-adrenergic receptor subtype.

Recent advances in adrenergic pharmacology have made possible the identification of alpha 1-adrenergic receptor subtypes using the specific blockers chloroethylclonidine and WB 4101. In the present study, we used these two blockers to determine the mechanisms responsible for automatic rhythms occurring during simulated ischemia and reperfusion of isolated canine Purkinje fibers. Experiments were done in the presence of propranolol to minimize beta-adrenergic contributions to the rhythms studied. In the control situation, all fibers showed membrane potentials greater than -90 mV and normal automatic rhythms. During simulated ischemia, membrane potential depolarized to the -60 mV range. Abnormal automaticity was seen in 20% of fibers not treated with phenylephrine and in 50% of those superfused with 1 x 10(-7) M phenylephrine. The incidence of abnormal automaticity was reduced to 0% by WB 4101 (which blocks phosphoinositide metabolic effects of alpha 1-adrenergic stimulation in the heart) and was increased to 90% by chloroethylclonidine (which blocks Na-K pump-stimulating effects of alpha-agonists). Moreover, the ischemic fibers were significantly more hyperpolarized during superfusion with WB 4101 than with chloroethylclonidine. Triggered activity induced by delayed or early after depolarizations was not seen in any experiment. Automatic arrhythmias induced by alpha 1-adrenergic stimulation during simulated ischemia may be attributed to a specific alpha 1-adrenergic receptor subtype that is blocked by WB 4101. These results have important implications with respect to the induction of arrhythmias in the setting of ischemia and the means for their prevention or treatment.

Effects of alpha-adrenergic stimulation on intracellular sodium activity and automaticity in canine Purkinje fibers.

Approximately 60% of adult canine Purkinje fibers respond to alpha 1-adrenergic stimulation with a decrease in automaticity. Recent studies of disaggregated Purkinje myocytes have suggested that this negative chronotropic effect results from alpha 1-adrenergic activation of the Na-K pump. In this study we evaluated 1) whether Na-K pump activation is associated with the negative chronotropic effect of alpha 1-adrenergic stimulation in adult canine Purkinje fibers and 2) if the effect of alpha-agonists on the pump is direct or mediated by an increase in intracellular sodium activity (aNai). We used sodium selective microelectrodes to determine the effects of 5 x 10(-9) and 5 x 10(-8) M phenylephrine on aNai. Phenylephrine decreased automaticity in five of eight Purkinje fibers while an increase occurred in the other three. The rate decrease was always accompanied by a decrease in aNai (-3.9 mM; p less than 0.05), whereas in fibers showing an increase in rate, aNai was unchanged. To evaluate the effect of phenylephrine in the absence of changes in automaticity, 10 Purkinje fibers were studied during pacing. A clear-cut reduction in aNai (-2.8 mM) was present in six fibers; no change was seen in the other four. The effect of phenylephrine was blocked by prazosin but not by propranolol. We conclude that the effect of alpha 1-adrenergic stimulation to reduce aNai is consistent with activation of the Na-K pump. Moreover, this action of alpha 1-adrenergic stimulation is closely linked to its negative chronotropic effect.

Effects of Alpha Adrenergic Stimulation and Blockade on Early Afterdepolarizations Induced by Cesium in Canine Cardiac Purkinje Fibers

Alpha Adrenergic Stimulation and EADs. The effects of alpha adrenergic stimulation and three alpha adrenoceptor blockers on early afterdepolarizations (EADs) were examined in canine card diac Purkinje fibers. In the first group of 18 preparations, EADs were induced by superfusion with 7.5 mM cesium (Cs) dissolved in low potassium (2.7 mM KCl) Tyrode's solution. During alpha adrenoceptor stimulation (norepinephrine 1 μM and propranolol 1 μM) to enhance EADs, the effects of phentotamine and two new alpha 1 adrenoceptor antagonists, benoxathian and WB 4101 (1, 3, and 10 μM), were examined. WB 4101 (1 μM) suppressed EADs in all six preparations. Benoxathian required 3 μM in five preparations and 10 μM in one to suppress EADs. Phentolamine (10 μM) suppressed EADs in two of six preparations. In the second group of 21 preparations, the effects on cesium‐induced EADs of the three alpha adrenoceptor hlockers (10 μM) were examined without alpha adrenoceptor stimulation. WB 4101 (10 μM) suppressed EADs in seven of seven preparations, while benoxathian (10 μM) suppressed EADs in five of seven. Phentolamine (10 μM) enhanced EADs in six of seven preparations. In the third group of 24 Purkinje fibers, the direct effects (without alpha adrenoceptor stimulation) of these three antagonists on the characteristics of normal Purkinje fiber action potentials superfused with normal Tyrode's solution were examined. Phentolamine (1, 3, and 10 μM, n = 8) prolonged action potential duration at 90% repolarization (APD90) by 5.3%± 1.3%, 10.0%± 1.8%, and 14.3%± 2.7%, respectively. Benoxathian (n = 8) and WB 4101 (n = 8) shortened APD90 equally: 6.8%± 1.1% vs 7.7%± 1.3% at 1 μM, 13.6%± 1.0% vs 14.5%± 1.6% at 3 μM, and 21.1%± 1.2% vs 20.8%± 2.1% at 10 μM, respectively. We conclude that in cesium treated Purkinje fibers: (1) Alpha adrenoceptor stimulation enhanced EADs; (2) Phentolamine was least effective in suppressing EADs, probably because of a direct effect that prolonged MM)90,: and (3) Although benoxathian and WB 4101 had similar etTects on APD90, WB 4101 was more effective in suppressing EADs at lower concentrations than henoxathian.

Developmental changes in guanine nucleotide regulatory proteins in the rat myocardial alpha 1-adrenergic receptor complex.

During development, the cardiac alpha 1-adrenergic chronotropic response changes from positive in the neonate to negative in the adult. The negative chronotropic effect of alpha 1-adrenergic stimulation in the adult depends on maturation of sympathetic innervation and the presence of a pertussis toxin (PT)-sensitive guanine nucleotide-binding (G) protein. To examine the possibility of a developmental change in coupling of a PT-sensitive G protein to the alpha 1-adrenergic receptor, radioligand binding experiments with the iodinated alpha 1-selective radioligand [125I]-I-2-[beta-(4-hydroxyphenyl)ethylaminomethyl]tetralone ([ 125I]-IBE 2254) were performed on membranes prepared from control and PT-treated neonatal and adult rat hearts. Scatchard analysis showed fewer alpha 1-adrenergic receptors in the adult than in the neonate (168 +/- 10 fmol/mg protein in the neonate vs. 124 +/- 13 fmol/mg protein in the adult), but similar affinities (equilibrium dissociation constant 124 +/- 29 pM in the neonate vs. 140 +/- 34 pM in the adult). PT treatment did not alter the results. In both the neonate and adult, 5'-guanylylimidodiphosphate [Gpp(NH)p, 500 microM] shifted the l-epinephrine competition curve to the right and increased the slope factor toward unity. PT had no effect on the l-epinephrine competition curve in the neonate. However, in the adult PT itself caused a partial shift in the agonist competition curve, reducing but not eliminating the effect of Gpp(NH)p. Consistent with the results from the binding experiments, PT did not have any effect on the alpha 1-adrenergic-mediated positive chronotropic response in the neonate, whereas in the adult the alpha 1-adrenergic-mediated negative chronotropic response was completely converted to a positive one after PT treatment. These results indicate the presence of a PT-insensitive G protein in the neonatal and adult rat heart and the acquisition of a PT-sensitive G protein linked to the negative chronotropic response during development.