Isoproterenol Dose-response Curve Research Articles

Beta-Adrenergic and ATP Sensitive Potassium Channel Effects on Ventricular Action Potential Duration: Alterations with Exercise Training

Regular exercise-training is known to reduce the incidences and severity of ischemic heart disease, but the mechanistic nature is unknown. Recently, we showed that regular exercise prolongs the left ventricular action potential duration (APD) at resting heart rate (HR) while shortening it at high HR. These adaptations enhance cardiac contractility at rest while preventing Ca2+ overload and/or inadequate filling during exercise. Here we test the hypothesis that exercise-training induced changes in the APD are due to a reduced responsiveness to β-adrenergic regulation, and increased ATP sensitive potassium channel (KATP) function. Female and male Sprague-Dawley rats were randomly assigned to voluntary wheel running or control groups. After 6-8 weeks training, cardiac myocytes were isolated from the apex and base regions of the left ventricle with collagenase–protease dispersion technique. APD were measured with glass micro-electrode at 1 or 10 Hz stimulation. At 1 Hz, the non-selective β agonist Isoproterenol shortened the APD an effect largely reversed by selective β1-AR blocker Atenolol (0.1 μM), but not selective β2-AR antagonist ICI 118,551 (0.1 μM). Wheel running shifted the Isoproterenol dose-response curve (0.01-100 nM) rightward compared to controls, with LogIC50 decreased by exercise-training in both male and female rats by ∼ one log unit. The addition of KATP inhibitor glibenclamide (2 µM) prolonged APD at 10 Hz, with a greater prolongation observed in myocytes from exercise-trained rats. In conclusion, the exercise program down regulated the β1-AR to isoproterenol an adaptation that would contribute to prolonged action potential in non-stressed hearts where the activation of the sympathetic nervous system is low, while the exercise-training induced decline in the APD under energy demanding conditions (e.g. high HR) seems caused at least in part by increased KATP repolarizing current.

Blood Pressure and Mesenteric Vascular Reactivity in Spontaneously Hypertensive Rats 7 Months After Gonadectomy

Sexual dimorphism in the degree of high blood pressure (BP) has been observed in both animal and human hypertension. However, the mechanisms are still poorly understood. We tested the hypothesis that long-term loss of sex steroids promotes changes in mesenteric vascular reactivity that impact the maintenance of hypertension in the spontaneously hypertensive rats (SHR). Male SHR were sham operated (M-SHAM) or castrated (M-CX), and female SHR were sham-operated (F-SHAM) or ovariectomized (F-OVX) at 3 weeks of age. Seven months later, BP was measured in anesthetized rats, and vascular responsiveness was evaluated in the isolated perfused mesentery. Mean arterial BP (mm Hg) was significantly greater in M-SHAM (186 ± 6) compared with F-SHAM (159 ± 5). Gonadectomy reduced BP in male SHR (M-CX: 160 ± 4) but had no significant effect in female SHR (F-OVX: 153 ± 7). Norepinephrine-induced constriction was similar in all groups. Gonadectomy attenuated serotonin-induced vasoconstriction in the mesentery. Acetylcholine (ACh)- and isoproterenol (ISO)-induced vasodilation was greater in female than male SHR. Ovariectomy of female SHR blunted ACh and ISO dilatory responses. ISO dose-response curves were shifted to the left in castrated male SHR. Gonadectomy exerts long-term effects on mesenteric vascular reactivity and hypertension in the SHR.

Parasympathetic but not sympathetic cardiac dysfunction at early stages of Parkinson’s disease

Autonomic cardiovascular dysfunction is common in Parkinson's disease (PD). Imaging studies suggest loss of cardiac sympathetic nerves even in the absence of clinical signs of autonomic dysfunction. Aim of the study was to investigate the functional significance of autonomic cardiovascular denervation at early stages of PD. Seven PD patients (Hoehn and Yahr class 1 or 1.5) without clinical signs of autonomic dysfunction and seven age-matched healthy control subjects were studied. To evaluate the pre- and post-synaptic components of sympathetic innervation, dose-response curves of isoproterenol (no neuronal uptake) and epinephrine (neuronal uptake) on heart rate, contractility, cardiac output and systemic vascular resistance were determined echocardiographically. Additionally, measurements of baroreflex sensitivity and 24-h heart rate variability were done. The chronotropic and inotropic responses during stimulation with isoproterenol and epinephrine were similar in PD patients and control subjects. Assessment of baroreflex sensitivity yielded no difference. Of the parameters of 24-h heart rate variability, only measures of high-frequency heart rate variation that more purely reflect parasympathetic activity were significantly depressed in PD patients as compared with control subjects. The results of our study using direct determination of catecholamine-mediated chronotropic and contractile responses provide evidence against a functionally relevant sympathetic dysfunction. Possibly, sympathetic denervation is incomplete and the remaining fibers are sufficient for the maintenance of autonomic control. In contrast, the depression of several parameters of heart rate variability supports a significant change of parasympathetic activity at an early stage of PD with subclinical autonomic failure.

P38 MAP Kinase Inhibitor Prevents Diastolic Dysfunction in Rats Following HIV gp120 Injection In vivo

HIV infection in patients is associated with a surprisingly high frequency of diastolic dysfunction followed by the development of a dilated cardiomyopathy. Potential mechanisms include direct effects of HIV proteins, including gp120. We have previously reported direct inotropic and p38 MAP kinase signaling effects of HIV gp120 on isolated cardiac myocytes in vitro. We now report effects of a single injection of HIV gp120 on cardiac hemodynamics in vivo. HIV gp120 (50 microg/kg) was injected intravenously and hemodynamics assessed at 1, 24, 48 and 72 h in freely ambulatory, awake rats. Rats injected with gp120 demonstrated a blunted diastolic response to increasing intravenous (IV) injections of the beta-adrenergic agonist, isoproterenol (ISO), compared with similarly instrumented controls at 48 h (gp120 vs. control, P < 0.01; n = 6, for each). Pre-treatment with the p38 MAP kinase inhibitor, SB 203580, prevented the diastolic dysfunction (gp120 + SB vs. control, P = NS; n = 6, for each). Hemodynamic changes correlated with inhibition of both p38 MAP kinase and troponin I phosphorylation by SB in vivo. There were no differences in ISO dose-response curves between gp120-treated and control rats at 1, 24 or 72 h (P = NS; n = 6, for each). Thus, evidence of diastolic dysfunction is apparent in vivo in rats following a single dose of HIV gp120. To our knowledge, this is the first report of a hemodynamic effect of an HIV protein in vivo. These findings lend further support to the hypothesis that the HIV virus, itself, may contribute to myocardial dysfunction in patients infected with HIV via a p38 MAP kinase mechanism.

Diastolic Dysfunction in Rats Following Intravenous Injection of HIV gp120

HIV infection in patients is associated with a surprisingly high frequency of diastolic dysfunction followed by the development of a dilated cardiomyopathy. Potential mechanisms include direct effects of HIV proteins, including gp120. We have previously reported direct inotropic and signaling effects of HIV gp120 on isolated cardiac myocytes in vitro. We now report effects of a single injection of HIV gp120 on cardiac hemodynamics in vivo. HIV gp120 (50ug/kg) was injected intravenously and hemodynamic measurements were made at 1 hr, 24 hrs and 48 hrs in freely ambulatory, awake rats. Rats injected with gp120 demonstrated a blunted diastolic response to increasing doses of intravenous infusion of the beta‐adrenergic agonist, isoproterenol(ISO), compared with similarly instrumented controls at 48 hrs (gp120 vs control, p&lt;0.01; n=6, for each). Pre‐treatment with the p38 MAP kinase inhibitor, SB 203580, prevented the diastolic dysfunction (gp120+ SB vs control, P=NS; n=6, for each). Western analyses confirmed the inhibition of p38 MAP kinase by SB in vivo. There were no differences in ISO dose response curves or p38 MAP kinase phosphorylation between gp 120‐treated and control rats at 1 or 24 hrs. (P=NS; n=6, for each). Thus, evidence of diastolic dysfunction is apparent in rats following a single dose of HIV gp120. These findings lend further support to the hypothesis that the HIV virus, itself, may contribute to myocardial dysfunction in HIV infected patients.

α2B-adrenoceptor agonist ST-91 antagonizes β2-adrenoceptor-mediated relaxation in rat mesenteric artery rings

We sought an isolated vascular preparation and experimental setting where the function of α2B-adrenoceptors could be demonstrated by non-recombinant technique. ST-91 (2-[2,6-diethylphenylamino]-2-imidazoline), an α2B-adrenoceptor agonist with a mixed α adrenergic receptor type/subtype selection profile antagonized the relaxant effect of isoproterenol in endothelium-denuded rat mesenteric artery rings precontracted with phenylephrine. At 10−7 M of ST-91, the antagonism was characterized by a rightward shift of isoproterenol dose–response curve (A50=6.81±1.40 e−7 (n=4) vs the control 1.29±0.25 e−7 M (n=4)) with no Emax depression. At 10−6 M the Emax depression was prevalent (36.1±7.0% (n=4) vs the control 79.9±5.1% (n=4)); both actions could be antagonized by the α2-adrenoceptor antagonist yohimbine. The not subtype-selective α2-adrenoceptor agonist xylazine (10−7 M) did not affect the relaxant action of isoproterenol. Present findings are discussed in the light of previously reported hemodynamic effects attributed to α2B-adrenoceptors in receptor subtype-knockout animals.

Targeted Overexpression of Sarcolipin in the Mouse Heart Decreases Sarcoplasmic Reticulum Calcium Transport and Cardiac Contractility

The role of sarcolipin (SLN) in cardiac physiology was critically evaluated by generating a transgenic (TG) mouse model in which the SLN to sarco(endoplasmic)reticulum (SR) Ca(2+) ATPase (SERCA) ratio was increased in the ventricle. Overexpression of SLN decreases SR calcium transport function and results in decreased calcium transient amplitude and rate of relaxation. SLN TG hearts exhibit a significant decrease in rates of contraction and relaxation when assessed by ex vivo work-performing heart preparations. Similar results were also observed with muscle preparations and myocytes from SLN TG ventricles. Interestingly, the inhibitory effect of SLN was partially relieved upon high dose of isoproterenol treatment and stimulation at high frequency. Biochemical analyses show that an increase in SLN level does not affect PLB levels, monomer to pentamer ratio, or its phosphorylation status. No compensatory changes were seen in the expression of other calcium-handling proteins. These studies suggest that the SLN effect on SERCA pump is direct and is not mediated through increased monomerization of PLB or by a change in PLB phosphorylation status. We conclude that SLN is a novel regulator of SERCA pump activity, and its inhibitory effect can be reversed by beta-adrenergic agonists.

Insulin-induced changes in β-adrenergic response: An experimental study in the isolated rat papillary muscle

The aim was to investigate the ability of insulin to modulate the response to beta-adrenergic action on myocardial contractility, assessed as percentage changes of developed tension, in isolated rat papillary muscle. Dose-response curves for isoproterenol, calcium, and forskolin were constructed in an incremental fashion with the presence or absence of insulin at the dose of 50 muU/mL. Dose-response curves for isoproterenol on insulin background were also assessed in the presence and absence of a selective antagonist for beta(2)-adrenoceptor, ICI, at the dose of 5 x 10(-8) mol/L. Insulin did not modify the dose-response curve to calcium (EC(50): 1.4 +/- 0.4 mmol/Lfor insulin, n = 8 v 1.5 +/- 0.3 mmol/L for control, n = 8; P = not significant), whereas it was able to shift to the left the dose-response curve and reduce significantly the EC(50) of isoproterenol (EC(50): 0.2 +/- 0.2 nmol/L for insulin, n = 13 v 1.1 +/- 0.4 nmol/L for control, n = 12; P < .01). ICI shifted to the right dose-response curve of isoproterenol and increased about 10-fold the EC(50) value of isoproterenol, but insulin was still able to shift to the left dose-response curve of isoproterenol and to reduce significantly the EC(50) of isoproterenol also in the presence of ICI (EC(50): 11.0 +/- 1.5 nmol/L for ICI, n = 7 v 1.9 +/- 0.8 nmol/L for ICI + insulin, n = 7; P < .01). Insulin did not modify the dose-response curve to forskolin. Our results suggest that the insulin-induced modulation of contractility is calcium independent and insulin leads to a supersensitization on the beta(1)-adrenoceptors without effects on beta-adrenoceptor independent adenylate cyclase-related pathway.

Biphasic inotropic effects of methamphetamine and methylphenidate on ferret papillary muscles.

Methamphetamine and methylphenidate are structurally related agents that have direct negative inotropic (NIEs) and indirect positive inotropic effects (PIEs) in cardiac tissues. This study was designed to determine and compare cellular mechanisms of the NIEs of methamphetamine and methylphenidate by using a mammalian animal model. Isometric tension was measured in ferret papillary muscles in physiologic salt solution. A subset of muscles was loaded with the bioluminescent calcium indicator, aequorin. Further, to investigate the NIE of methamphetamine, the dose-response relations to isoproterenol, histamine, and calcium were measured with or without methamphetamine (2 x 10(-4) M), after adrenergic neuronal blockade with reserpine. Both methamphetamine and methylphenidate had direct NIEs at higher doses (>10(-4) M). A shift in the slope of the isoproterenol dose-response curve suggested involvement of the beta-adrenoceptor pathways. The direct NIE of methylphenidate was more prominent. Our results suggest that the negative inotropic effects of methamphetamine and methylphenidate may be important with clinically used and abused concentrations of these drugs and may be difficult to reverse with beta-adrenergic inotropic agents in cases of toxicity.

Effects of triiodothyronine administration on the adenylyl cyclase system in brown adipose tissue of rat.

This study was undertaken to investigate the effect of triiodothyronine (T3) administration to euthyroid rats on beta 3-adrenoceptor (beta 3-AR) expression and on the different components of the adenylyl cyclase (AC) system in brown adipose tissue (BAT). In rats treated with T3, the beta 3-AR density (assessed by the binding of [3H]CGP-12177) showed a decrease of 50%, as did their mRNA, as analyzed by reverse transcriptase-polymerase chain reaction. In hyperthyroid rats, compared with control rats, there was a 40% increase in G alpha s activity (stimulated by NaF or GTP gamma S) and a fourfold increase in the protein concentration (Western blotting). In contrast, the level of the pertussis toxin substrate Gi declined by 35% in response to T3. Analysis of dose-response curves for isoproterenol and CGP-12177 revealed that neither basal nor stimulated AC activities nor 50% stimulatory concentration for these agonists was changed by T3 administration. In conclusion, these results suggest that downregulation of the beta 3-AR by T3 was counter-balanced by changes in other components of the AC cascade (i.e., Gs and Gi), so no change occurred in the capacity of BAT to generate adenosine 3',5'-cyclic monophosphate.

Coronary artery stenosis in rats affects beta-adrenergic receptor signaling in myocytes.

The purpose of this study was to determine whether the early chronic ischemic cardiomyopathy produced by non-occlusive coronary artery constriction was characterized by alterations in the regulation of beta-adrenoreceptor (beta-AR) signaling. Coronary artery narrowing was surgically induced in rats and the animals sacrificed at 7 and 14 days. The changes in the biochemical properties of the multiple components of the beta-AR pathway were examined in enzymatically dissociated myocytes. Coronary stenosis, involving an average 55% reduction in luminal diameter, was associated with left ventricular failure and right ventricular dysfunction at both time intervals. A decrease in the quantity of beta-AR was detected at 7 days and preceded the loss of high-affinity binding sites. This regulatory modification was characterized by a reduction in beta 1 and beta 2 receptors and a shift in the isoproterenol dose response curve indicating a functional correlation between the decrease in beta-AR and attenuated inotropic support of the myocardium. The percentage of beta-AR binding agonist with high affinity decreased significantly at 14 days along with a further reduction in the density of beta 1 and beta 2 receptors. Reconstitution studies with cyc S49 lymphoma cells did not detect an impairment of Gs alpha functional activity, but the quantity of Gi alpha was increased at both intervals. Finally, activation of the catalytic unit of adenylyl cyclase by forskolin and GTP was not altered by coronary stenosis, however, basal cyclic AMP in myocytes was depressed at 14 days. Coronary stenosis induces distinct and progressive modifications in the beta-AR signaling cascade which may contribute to the impaired ventricular performance in this model of myocardial ischemia.

Clonidine for Major Vascular Surgery in Hypertensive Patients

The utility of clonidine for hypertensive patients presenting for major vascular procedures remains debatable.Twenty-one hypertensive patients presenting for aortic surgery were given clonidine (n = 11) or placebo (n = 10) in a double-blind, randomized manner. Clonidine was administered 6 micro gram/kg per os 120 min before induction of anesthesia and 3 micro gram/kg intravenously (IV) over 60 min from aortic declamping to skin closure. Anesthesia was induced with alfentanil 20 micro gram/kg, midazolam, and atracurium and maintained with nitrous oxide 70%, an alfentanil infusion (0.25 micro gram centered dot kg-1 centered dot min-1), and isoflurane. Anesthetic requirements, circulatory variables, interventions, and isoproterenol dose-response curves (pre- and postoperatively) were determined. Plasma concentrations of clonidine, alfentanil, and vasoactive hormones were measured. When the clonidine group was compared with the placebo group, (a) isoflurane, alfentanil, and midazolam requirements were reduced by 38%, 42%, and 41%, respectively (P = 0.04, 0.03, 0.0002, respectively); (b) supplemental circulatory and anesthetic adjustments were reduced by 51% (P = 0.0006); (c) interventions with vasopressors were not significantly increased (placebo: two; clonidine: five); (d) systolic and mean arterial pressures and heart rate were reduced; (e) increases in norepinephrine, epinephrine, and plasma renin activity were suppressed, whereas vasopressin surge was attenuated; and (f) chronotropic response to isoproterenol was unaffected. Clonidine was effective in reducing anesthetic requirements and in improving circulatory stability in hypertensive patients presenting for major vascular procedures. (Anesth Analg 1996;83:687-95)

No functional atypical β-adrenergic receptors in human omental adipocytes

Isoproterenol stimulates lipolysis in human omental adipocytes with an EC50 (concentration at which an agonist produces half-maximal stimulation) of 120 nM. CGP12177 (dl-4-3 [(1,1-dimethylethyl) amino]- 2-hydroxylpropoxy]1,3- dihydro-2H-benzimidazol-2-one hydrochloride), a potent β 1-β 2-adrenergic receptor (AR) antagonist but being an agonist for atypical β-AR, fails to stimulate lipolysis in these cells, even at a concentration as high as 0.1 mM. Since CGP12177 is a partial agonist, its failure to stimulate lipolysis may result from a poor stimulus-response coupling, so that it can not be excluded that atypical β-AR are actually present and even functional in these cells. To evaluate this hypothesis, we estimated the potency of CGP12177 to inhibit the isoproterenol-stimulated lipolysis. This inhibition curve reflects a single class of sites and the IC50- value (concentration at which an antagonist produces half-maximal inhibition) of CGP12177 (3.8 nM) is in good agreement with what should be expected for β1-AR/β2-AR. Moreover, metoprolol and atenolol, two β 1 - AR- selective antagonists, shift the isoproterenol dose-response curve to the right with high potency as well. These potencies are similar to the ones found for β1-AR in the human heart but appreciably higher than those which should be expected for atypical β-AR. The present study suggest that atypical β-AR are not functional in human omental adipocytes.

Burn-induced alterations in cardiac beta-adrenergic receptors.

Previous studies in our laboratory have demonstrated that burn injury (45% total body surface area, 3rd-degree scald burn) diminishes contractile and relaxation function in the isolated perfused guinea pig heart. The mechanisms responsible for the burn-mediated dysfunction are not well understood. Therefore the purpose of this study was to examine the inotropic response to isoproterenol, a beta-adrenergic agonist, and burn-induced alterations in beta-adrenergic receptors (beta-AR) in adult guinea pig hearts. Isoproterenol dose-response curves were generated in isolated perfused hearts from sham-burned and burned guinea pigs. In addition, binding studies were performed using [125I]iodocyanopindolol on hearts from sham-burned and burned guinea pigs. Both the functional response and sensitivity to isoproterenol were significantly diminished 24 h after burn injury. beta-AR density (binding capacity, Bmax) and affinity were determined by Scatchard analysis. Agonist competition curves were performed in the presence or absence of 0.1 mM 5'-guanylyl imidodiphosphate. There was no difference in Bmax in membranes from sham-burned and burned hearts; however, the affinity of beta-AR was significantly decreased after burn injury compared with sham burn [dissociation constant = 32.5 +/- 1.9 (mean +/- SE), n = 10, vs. 26.7 +/- 1.7 pM, n = 10, P = 0.039]. Furthermore, the fraction of receptors in a high-affinity state (those functionally coupled to Gs protein) was significantly decreased after burn injury compared with sham burn (41.2 +/- 4.7%, n = 9, vs. 54 +/- 2%, n = 9, P = 0.023).(ABSTRACT TRUNCATED AT 250 WORDS)

Compartmentation of cAMP in adult canine ventricular myocytes. Relation to single-cell free Ca2+ transients.

Isolated adult canine ventricular myocytes were used to study the role of compartmentation of cAMP in the diverse functional responses to various drugs that elevate cAMP. Myocytes presented with the beta-agonist isoproterenol accumulated cAMP with a half maximally effective concentration (EC50) of 3.55 x 10(-8) M. Approximately 45% of the total cAMP was recovered in the particulate fraction of digitonin-lysed myocytes under these conditions. With phosphodiesterase inhibition (10 microM isobutylmethylxanthine), isoproterenol-stimulated cAMP production was up to 3.4-fold greater, but the proportion of total cAMP residing in the particulate fraction declined to less than 20%. Similar results were obtained with forskolin, a direct activator of adenylate cyclase. Treatment with isoproterenol shortened the duration at 50% maximum peak height (T 1/2) and increased the peak fluorescence ratio of electrically triggered single-cell free Ca2+ transients in fura-2-loaded canine myocytes. Isoproterenol dose-response curves gave EC50 values of 1.7 x 10(-9) and 4.4 x 10(-9) M for effects on T 1/2 and peak height, respectively. Alterations in peak height and T 1/2 of Ca2+ transients also showed a dose dependency on isobutylmethylxanthine and forskolin. Comparison of myocyte cAMP content with the corresponding changes in free Ca2+ transients demonstrated a close correlation between particulate cAMP and the extent of shortening or increase in peak height of the fura-2 Ca2+ transients (r = 0.92 for each). However, when these two parameters were plotted as a function of total cAMP, the resulting curves were nonlinear and divergent for each agent tested. The results support the hypothesis that differences in responses to agents that augment cAMP can be explained in part by compartmentation of cAMP. Furthermore, Ca2+ mobilization seems to be most affected by cAMP located in the particulate compartment of canine cardiac myocytes.

Modulation of Adrenoceptor-Mediated Cardiovascular Effects by Short-Term In Vivo Infusion of Isoproterenol in Rats

After a 16-h in vivo infusion period of isoproterenol (400 micrograms/kg/h) from a minipump implanted subcutaneously (s.c.) in rats, we observed an increase in heart weight due to tissue edema. In isolated perfused heart preparations, the EC50s of isoproterenol to induce positive inotropy and increase in coronary flow were increased approximately 7 and 4 times, respectively. Isoproterenol dose-response curves performed in trachea preparations, were shifted to the right by about fivefold as compared with the control group. In the presence of phentolamine, the isoproterenol induced maximal relaxation in the isolated aorta from isoproterenol-pretreated animals was reduced from 46.5 to 9.2% as compared with saline-pretreated rats. In the absence of phentolamine, the epinephrine (EPI) and norepinephrine (NE) cumulative dose-response curves in this preparation were not affected by isoproterenol pretreatment with respect to EC50 and maximal effect. However, in the presence of phentolamine, the contractile response to a supramaximal dose of NE (10 microM) amounted to 30 and 70% in the saline- and isoproterenol-pretreated rats, respectively. In the presence of both phentolamine and propranolol, a similar response of 70% was observed by this supramaximal dose of agonist in the saline-pretreated rats as well. In anesthetized rats, 120 min after removal of the minipumps, sodium nitroprusside induced increase in heart rate (HR) was reduced after isoproterenol pretreatment, whereas for salbutamol the decrease in diastolic blood pressure (DBP) was also reduced. As compared with salbutamol, a marked increase was observed in the ratio of mean arterial blood pressure (MAP) to HR for SNP after isoproterenol pretreatment. The phenylephrine-induced increase in MAP was increased after isoproterenol pretreatment. We conclude that in the pathogenesis of heart failure the beta-adrenoceptor-mediated effects of catecholamines are attenuated and alpha-adrenoceptor-mediated effects become progressively important.

Anticholinergic Blockade of Beta-Blocker-Induced Bronchoconstriction

The mechanism of propranolol-induced bronchoconstriction (PIB) remains unclear. Previous uncontrolled studies have suggested that atropine antagonizes PIB in asthma. We conducted a randomized, double-blind, placebo-controlled study of the effect of a new anticholinergic agent, oxitropium bromide, on bronchoconstriction induced by inhaled propranolol in seven subjects with mild asthma 24 to 39 yr of age. Inhaled propranolol induced long-lasting reduction in specific airway conductance (SGaw) in all subjects. This was associated with airway beta-adrenoceptor blockade as demonstrated by a shift in the isoproterenol dose-response curve. Propranolol was less potent than methacholine, with a geometric mean dose causing a 35% fall in SGaw of 4.7 µmol for propranolol compared with 0.48 µmol for methacholine. Pretreatment with oxitropium 200 µg completely inhibited the fall in SGaw in response to inhaled propranolol in all subjects. Oxitropium also reversed PIB. Cholinergic receptor blockade by oxitropium inhibits...

Hyperthyroid adult rat cardiomyocytes. I. Nucleotide content, beta- and alpha-adrenoreceptors, and cAMP production

Ventricular myocytes isolated from the hypertrophied hearts of thyrotoxic adult rats have an increase in mean protein content per myocyte (6.3 +/- 0.2 vs. 4.4 +/- 0.2 ng) compared with euthyroid cells. Viability and adenine nucleotide profiles are similar in both populations, but NAD content of the hyperthyroid myocytes is depressed (4.9 +/- 0.2 vs. 5.5 +/- 0.2 nmol/mg for controls) and UTP is higher (1.2 +/- 0.09 vs. 0.9 +/- 0.04 nmol/mg). Binding of (-)-[125I]iodocyanopindolol to intact hyperthyroid myocytes is increased by 42% compared with controls, with no change in the dissociation constant (Kd). This elevation in beta-receptor number is correlated to enhanced beta-agonist-induced adenosine 3',5'-cyclic monophosphate (cAMP) production. The half-maximal effective concentration (EC50) for the euthyroid isoproterenol dose-response curve is 2.14 x 10(-7) M but is decreased to 2.51 x 10(-8) M in hyperthyroid cardiac cells. Basal adenylate cyclase activity is apparently not affected by thyroid hormones, since basal cAMP levels for both groups are identical (5 pmol/mg) and both rise roughly twofold in the presence of a phosphodiesterase inhibitor. Forskolin-induced cAMP production and cAMP-specific phosphodiesterase activity are similar as well. In contrast to beta-adrenergic response, there are no significant differences in alpha 1-antagonist [3H]prazosin binding parameters between hyperthyroid and euthyroid cardiomyocytes.

Beta-adrenergic supersensitivity of the transplanted human heart is presynaptic in origin.

An increase in cardiac beta-adrenergic sensitivity or beta-receptor density or both has been described in several animal species after denervating the heart. The transplanted human heart is also denervated and, therefore, may exhibit supersensitivity to beta-adrenergic agonists and an increase in beta-adrenergic receptor density. In 16 patients examined 1-3 months after orthotopic cardiac transplantation, beta-adrenergic receptor density measured by [125I]iodocyanopindolol binding in endomyocardial biopsy specimens was not significantly different in transplant recipients compared with normal controls (transplant = 1,429 +/- 199, control = 1,728 +/- 263 fmol/g wet wt; p = NS). However, when normalized to Lowry protein, the [125I]iodocyanopindolol in beta-adrenergic receptor density in biopsy tissue from transplant recipients was significantly lower than in tissue from controls (transplant = 58.1 +/- 6.2, control = 93.5 +/- 13.4 fmol/g Lowry protein; p = 0.011). Atrial sinus node activity of the denervated donor heart and the innervated atrial cuff of the native recipient heart could be detected on the surface electrocardiogram in six patients. In these six patients, the heart rate response to graded infusions of epinephrine (taken up by the adrenergic nerve terminals) and isoproterenol (not taken up by the adrenergic nerve terminals) was measured. The epinephrine dose-response curve in transplanted donor atria was significantly to the left of the native recipient atrial dose-response curve (p less than 0.0001). The isoproterenol dose-response curves for native and transplanted atria were not different. We conclude that myocardial beta-adrenergic receptors are not increased in human orthotopic cardiac allografts and that there is no evidence for beta-receptor-mediated supersensitivity of postsynaptic origin.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparaison chez le volontaire sain du blocage bêta-adrénergique systémique par les collyres bêta-bloquants

The isoproterenol dose-response curve was used to assess quantitatively the degree of systemic beta-adrenoceptor blockade induced by metipranolol (Bétanol ®) and betaxolol (Bétoptic ®) eye drops. The study was carried out in twelve healthy volunteers, aged 22 ± 1.4 yr. In a randomized doubleblind trial, each volunteer received, on separate occasions at least one week apart, one drop in each eye of either placebo (physiological saline) or either of the ophthalmic beta-blockers. The intraocular pressure (Pio), heart rate (f c), arterial systolic (Pa sys) and diastolic (Pa dia) pressures were measured before instillation of the eye drops after 15 to 30 min rest, and 3 h afterwards. The isoproterenol dose-response curve was studied 3 h after instillation of the drops. The CD 25 (the amount of isoproterenol needed to increase f c by 25 b · min −1) was obtained by extrapolation on the least square linear regression curve. Both beta-blockers gave a significant fall in Pio compared with placebo, metipranolol more than betaxolol (p < 0.02). There was also a significantly greater fall in f c with both metipranolol and betaxolol than with placebo. There were no changes in Pa sys and Pa dia. CD 25 was significantly increased with both beta-blocker eye drops as compared with placebo (p < 0.05 for betaxolol ; p < 0.01 for metipranolol), but there was no difference between the two. Systemic absorption after topical application of ocular beta-blockers was thus confirmed for both metipranolol and betaxolol. However, the degree of beta-adrenoceptor blockade was weaker than that observed with other older ocular beta-blockers (timolol and carteolol). Although these results suggest a greater safety margin with both metipranolol and betaxolol, anaesthesiologists should be aware and wary of this phenomenon.