Α-adrenergic Antagonist Phentolamine Research Articles

Skeletal Muscle Oxygen Delivery and Utilization during Exercise in Heart Failure with Preserved Ejection Fraction: Role of Sympathetic (α‐adrenergic) Vasoconstriction

BACKGROUNDHeart Failure with preserved ejection fraction (HFpEF) is characterized by severe exercise intolerance underpinned by low aerobic capacity (VO2peak). It is likely low VO2peak in HFpEF is not only related to deficits in cardiac function, but also peripheral vascular and skeletal muscle dysfunction that may disrupt the balance between O2 delivery and utilization. There is emerging evidence for sympathetic nervous system overactivity in patients with HFpEF that results in excessive vasoconstriction in the peripheral circulation, and may therefore contribute to dysregulation of exercising muscle blood flow. We tested the hypothesis that pharmacological blockade of sympathetic (α‐adrenergic) vasoconstriction would increase blood flow, O2 delivery and utilization of exercising skeletal muscle to a greater extent in HFpEF compared to healthy control subjects.METHODSFour healthy control (CON) (2 M, 69±3 yrs) and three HFpEF (1 M, 71±3) volunteers participated in this study. Femoral blood flow (FBF) was assessed by ultrasound Doppler. Catheters were placed in the femoral artery and vein for drug infusions and determination of arterial and venous O2 content (CaO2 and CvO2, respectively). O2 utilization of the exercising leg was calculated via the Fick equation: VO2 = FBF∙(CaO2‐CvO2), and O2 delivery was calculated as FBF∙CaO2. Measurements were made at rest and during dynamic single‐leg knee‐extension (KE) exercise (5 W) before and after intra‐arterial infusion of the α‐adrenergic antagonist phentolamine (PHEN).RESULTSBaseline FBF, O2 delivery, and VO2 were similar between groups. At rest, PHEN increased FBF and O2 delivery similarly in both groups, while the increase in VO2 tended to be smaller in patients with HFpEF compared to CON (+3±2 vs. +14±5 ml/min; respectively, P=0.09). During KE exercise, a ≍20‐30% decrement in FBF (2052±261 vs. 2529±230 ml/min; HFpEF vs. CON, P=0.23), O2 delivery (282±27 vs. 392±35 ml/min, HFpEF vs. CON, P=0.07), and VO2 (193±25 vs. 284±31 ml/min; HFpEF vs. CON, P=0.09) were observed in patients with HFpEF. PHEN increased exercising muscle FBF (+243±45 vs. +247±58 ml/min; HFpEF vs. CON) and O2 delivery (+51±9 vs. +34±19 ml/min; HFpEF vs. CON) similarly between groups, while the PHEN‐induced increase in VO2 tended to be greater in patients with HFpEF (+20±12 vs. +6±7 ml/min; HFpEF vs. CON, P=0.32).CONCLUSIONTaken together, these preliminary data suggest a reduction in FBF, O2 delivery and utilization during small muscle mass exercise in patients with HFpEF that may be, in part but not fully, explained by excessive sympathetic (α‐adrenergic) vasoconstriction in this patient group.

α1-adrenergic stimulation increases ventricular action potential duration in the intact mouse heart

The role of α1-adrenergic receptors (α-ARs) in the regulation of myocardial function is less well-understood than that of β-ARs. Previous reports in the mouse heart have described that α1-adrenergic stimulation shortens action potential duration in isolated cells or tissues, in contrast to prolongation of the action potential reported in most other mammalian hearts. It has since become appreciated, however, that the mouse heart exhibits marked variation in inotropic response to α1-adrenergic stimulation between ventricles and even individual cardiomyocytes. We investigated the effects of α1-adrenergic stimulation on action potential duration at 80% of repolarization in the right and left ventricles of Langendorff-perfused mouse hearts using optical mapping. In hearts under β-adrenergic blockade (propranolol), phenylephrine or noradrenaline perfusion both increased action potential duration in both ventricles. The increased action potential duration was partially reversed by subsequent perfusion with the α-adrenergic antagonist phentolamine (1 μmol L−1). These data show that α1-receptor stimulation may lead to a prolonging of action potential in the mouse heart and thereby refine our understanding of how action potential duration adjusts during sympathetic stimulation.

The vasopressor action of angiotensin II (ANG II) in ball pythons (Python regius)

Angiotensin II (ANG II) is part of the renin-angiotensin system (RAS) in vertebrates and exert vasoconstriction in all species studied. The present study examines the vasopressor effect of ANG II in the ball python (Python regius), and examines whether ANG II exert its effect through direct angiotensin receptors or through an activation of α-adrenergic receptors. The studies were conducted in snakes with chronic arterial catheters that had recovered from anesthesia. In addition to demonstrating a clear and pronounced dose-dependent rise in arterial blood pressure upon repeated injections of boluses with ANG II (0.001–1 μg/kg), we demonstrate that the pressor response persisted following α-adrenergic blockade using the α-adrenergic antagonist phentolamine (2.5 mg/kg). Unfortunately, it proved impossible to block the ANG receptors using losartan (1, 3 or even 10 mg/kg). The pressor response to ANG II was associated with a significant rise in heart rate at the higher dosages, pointing to a resetting of the barostatic mechanism for heart rate regulation. The responses were similar in fasting and digesting pythons despite the expected rise in baseline values for blood pressure and heart rate of the digesting snakes.

A Role for Adrenergic Receptors in the Uterotonic Effects of Ergometrine in Isolated Human Term Nonlaboring Myometrium

Ergometrine is a uterotonic agent that is recommended in the prevention and management of postpartum hemorrhage. Despite its long-standing use, the mechanism by which it acts in humans has never been elucidated fully. The objective of this study was to investigate the role of adrenoreceptors in ergometrine's mechanism of action in human myometrium. The study examined the hypothesis that α-adrenoreceptor antagonism would result in the reversal of the uterotonic effects of ergometrine. Myometrial samples were obtained from women undergoing elective cesarean delivery. The samples were then dissected into strips and mounted in organ bath chambers. After the generation of an ergometrine concentration-response curve (10 to 10 M), strips were treated with increasing concentrations of ergometrine (10 to 10 M) alone and ergometrine (10 to 10 M) in the presence of phentolamine (10 M), prazosin (10 M), propranolol (10 M), or yohimbine (10 M). The effects of adding ergometrine and the effect of drug combinations were analyzed using linear mixed effects models with measures of amplitude (g), frequency (contractions/10 min), and motility index (g×contractions/10 min). A total of 157 experiments were completed on samples obtained from 33 women. There was a significant increase in the motility index (adding 0.342 g × counts/10 min/μM; 95% confidence interval [CI], 0.253-0.431, P < .001), amplitude (0.078 g/μM; 95% CI, 0.0344-0.121, P = 5e-04), and frequency (0.051 counts/10 min/μM; 95% CI, 0.038-0.063, P < .001) in the presence of ergometrine. The α-adrenergic antagonist phentolamine and the more selective α1-adrenergic antagonist prazosin inhibited the ergometrine mediated increase in motility index, amplitude, and frequency (-1.63 g × counts/10 min/μM and -16.70 g × counts/10 min/μM for motility index, respectively). These results provide novel evidence for a role for α-adrenergic signaling mechanisms in the action of ergometrine on human myometrial smooth muscle in the in vitro setting. Information that sheds light on the mechanism of action of ergometrine may have implications for the development of further uterotonic agents.

OP.8C.08] INTERACTION BETWEEN PERIPHERAL ALPHA-ADRENERGIC RECEPTORS AND NEURONAL NITRIC OXIDE SYNTHASE

Objective: Vascular tone is influenced by release of norepinephrine (NE) from sympathetic nerves and by nitric oxide (NO) released from perivascular neuronal NO synthase (nNOS). nNOS appears a more important physiological determinant of vascular tone than endothelial NO synthase (eNOS), particularly when stimulated by stress. Thus vascular tone is determined mainly by the balance of the two neurally released mediators, NE tending to constrict and NO to dilate vascular smooth muscle. Indirect observations suggest there may be cross-talk between peripheral α-adrenergic receptors and nNOS signalling. Design and method: We therefore examined local forearm vasodilator responses to intra-brachial artery infusion of the non-selective α-adrenergic antagonist phentolamine in the presence and absence of S-methyl-L-thiocitrulline (SMTC), a specific inhibitor of nNOS. Normotensive participants (n = 12, 7 female, mean ± SD age 33 ± 13 years) were studied on two separate occasions in a two-visit single-blind, cross-over study, with each visit separated by at least 7 days. SMTC (0.2 μmol/min, a dose that inhibits vasodilation to mental stress but not to other NO donors or to calcium channel antagonists) on one visit and saline vehicle on the other were co-infused with a rising dose infusion of phentolamine (10, 30 and 100 μg/ml/min). Results: Phentolamine increased forearm blood flow less (from 3.39 ± 0.58 to 6.01 ± 1.10 ml/min/100 ml) when co-infused with SMTC compared to saline vehicle (increase from 3.37 ± 0.45 to 8.74 ± 1.16 ml/min/100 ml, P < 0.05 for difference in blood flow response). Conclusions: These data suggest that peripheral α-adrenergic receptors inhibit nNOS activation, an action likely to potentiate vasoconstrictor responses and to oppose antithrombotic and antiplatelet effects of nNOS-derived NO and which may contribute to nNOS dysfunction in hypertension and in other conditions associated with increased sympathetic activation.

Impaired neuronal nitric oxide synthase-mediated vasodilator responses to mental stress in essential hypertension.

Neuronal NO synthase (nNOS) regulates blood flow in resistance vasculature at rest and during mental stress. To investigate whether nNOS signaling is dysfunctional in essential hypertension, forearm blood flow responses to mental stress were examined in 88 subjects: 48 with essential hypertension (42±14 years; blood pressure, 141±17/85±15 mm Hg; mean±SD) and 40 normotensive controls (38±14 years; 117±13/74±9 mm Hg). A subsample of 34 subjects (17 hypertensive) participated in a single blind 2-phase crossover study, in which placebo or sildenafil 50 mg PO was administered before an intrabrachial artery infusion of the selective nNOS inhibitor S-methyl-l-thiocitrulline (SMTC, 0.05, 0.1, and 0.2 μmol/min) at rest and during mental stress. In a further subsample (n=21) with an impaired blood flow response to mental stress, responses were measured in the presence and absence of the α-adrenergic antagonist phentolamine. The blood flow response to mental stress was impaired in hypertensive compared with normotensive subjects (37±7% versus 70±8% increase over baseline; P<0.001). SMTC blunted responses to mental stress in normotensive but not in hypertensive subjects (reduction of 40±11% versus 3.0±14%, respectively, P=0.01, between groups). Sildenafil reduced the blood flow response to stress in normotensive subjects from 89±14% to 43±14% (P<0.03) but had no significant effect in hypertensive subjects. Phentolamine augmented impaired blood flow responses to mental stress from 39±8% to 67±13% (P<0.02). Essential hypertension is associated with impaired mental stress-induced nNOS-mediated vasodilator responses; this may relate to increased sympathetic outflow in hypertension. nNOS dysfunction may impair vascular homeostasis in essential hypertension and contribute to stress-induced cardiovascular events.

Regulation of Vascular Tone and Pulse Wave Velocity in Human Muscular Conduit Arteries

Arterial tone in muscular conduit arteries may influence pressure wave reflection through changes in diameter and pulse wave velocity. We examined the relative specificity of vasodilator drugs for radial artery and forearm resistance vessels during intrabrachial arterial infusion. The nitric oxide (NO) donors, nitroglycerine and nitroprusside, and brain natriuretic peptide were compared with the α-adrenergic antagonist phentolamine, calcium-channel antagonist verapamil, and hydralazine. Radial artery diameter was measured by high resolution ultrasound, forearm blood flow by strain gauge plethysmography, and pulse wave velocity by pressure recording cuffs placed over the distal brachial and radial arteries. Norepinephrine was used to constrict the radial artery to generate a greater range of vasodilator tone when examining pulse wave velocity. Despite dilating resistance vasculature, phentolamine and verapamil had little effect on radial artery diameter (mean dilation &lt;9%). By contrast, for comparable actions on resistance vessels, nitroglycerine and nitroprusside but not brain natriuretic peptide had powerful actions to dilate the radial artery (dilations of 31.3±3.6%, 23.6±3.1%, and 9.8±2.0% for nitroglycerine, nitroprusside, and brain natriuretic peptide, respectively). Changes in pulse wave velocity followed those in arterial diameter irrespective of the signaling pathway used to modulate arterial tone ( R =−0.89, P &lt;0.05). Basal tone in human muscular arteries is relatively unaffected by α-adrenergic or calcium-channel blockade, but is functionally or directly antagonized by NO donors. The differential response to NO donors suggests that there is potential to manipulate the downstream pathway to confer greater specificity for large arteries with a resultant decrease in pressure wave reflection and systolic blood pressure.

Spontaneous oscillations in intracellular Ca2+ concentration via purinergic receptors elicit transient cell swelling in rat parotid ducts

Using multiphoton microscopy, we established that rat parotid ductal cells exhibit spontaneous oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). These oscillatory Ca(2+) responses were observed during continuous perfusion with a physiological salt solution at 37 degrees C in the absence of calcium mobilizing agonist stimulation. The timing and patterns of these spontaneous Ca(2+) oscillations varied among individual ductal cells, and the average number of Ca(2+) responses in a single responding ductal cell was 2.1 in a 10-min recording period. High-speed scanning (0.6 s/image) revealed that most spontaneous elevations in [Ca(2+)](i) were initiated at the luminal side of ductal cells and spread toward the basal side within 2 s. Electron microscopic analysis after Ca(2+) imaging indicated that spontaneously oscillating ducts contained numerous granules at the luminal side, which is characteristic of granular ducts. These Ca(2+) oscillations were completely blocked by the purinergic receptor inhibitors 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS) and suramin but were not blocked by the muscarinic antagonist atropine or the alpha-adrenergic antagonist phentolamine. Simultaneous observation of fura-2 fluorescence and differential interference contrast (DIC) images showed that spontaneous elevations of [Ca(2+)](i) were well correlated with changes in shape of ductal cells. Using a plasma membrane fluorescence probe, SynaptoGreen C4, we found that the changes in DIC images reflected spontaneous cell swelling of ductal cells. Our findings present the possibility that purinergic receptors mediate spontaneous Ca(2+) oscillations in parotid ductal cells and regulate electrolyte reabsorption from the primary saliva in the resting state.

Adenine nucleotide changes in the remnant liver: An early signal for regeneration after partial hepatectomy

Liver regeneration after partial hepatectomy (PHx) is orchestrated by multiple signals from cytokines and growth factors. We investigated whether increased energy demand on the remnant liver after PHx contributes to regenerative signals. Changes in the tissue's energy state were determined from adenine nucleotide levels. Adenosine triphosphate (ATP) levels in remnant livers decreased markedly and rapidly (to 48% of control by 30 seconds post-PHx) and remained significantly lower than those in sham-operated controls for 24 to 48 hours. The ATP decrease was not reflected in corresponding increases in adenosine diphosphate (ADP) and adenosine monophosphate (AMP), resulting in a marked decline in total adenine nucleotides (TAN). We found no evidence of mitochondrial damage or uncoupling of oxidative phosphorylation. Multiple lines of evidence indicated that the decline in TAN was not caused by increased energy demand, but by ATP release from the liver. The extent of ATP loss was identical after 30% or 70% PHx, whereas fasting or hyperglycemia, conditions that greatly alter energy demand for gluconeogenesis, affected the ATP/ADP decline but not the loss of TAN. Presurgical treatment with the alpha-adrenergic antagonist phentolamine completely prevented loss of TAN, although changes in ATP/ADP were still apparent. Importantly, phentolamine treatment inhibited early signaling events associated with the priming stages of liver regeneration and suppressed the expression of c-fos. Pretreatment with the purinergic receptor antagonist suramin also partly suppressed early regenerative signals and c-fos expression, but without preventing TAN loss. The rapid loss of adenine nucleotides after PHx generates early stress signals that contribute to the onset of liver regeneration.

Adrenergic Regulation of Bacterial Virulence

Adrenergic Regulation of Bacterial Virulence

Autocrine actions of macrophage-derived catecholamines on interleukin-1 β

Previous studies indicate that norepinephrine and epinephrine modulate production of interleukin-1 β (IL-1 β) by activated macrophages, but it is not known if macrophage-derived catecholamines affect IL-1 β. In this study, recruited peritoneal macrophages from CBA/J female mice were activated with lipopolysaccharide (LPS) and treated with vehicle or adrenergic receptor antagonists for 24 h. Extracellular and intracellular levels of IL-1 β were measured with ELISA. Treatment with the β-adrenergic receptor antagonists propranolol or ICI 118,551 increased LPS-induced production of IL-1 β, whereas treatment with the α-adrenergic antagonists phentolamine or yohimbine decreased IL-1 β. These findings demonstrate that adrenergic receptor antagonists unmask autocrine actions of macrophage-derived catecholamines on IL-1 β that may influence the inflammatory response.

Effects of social stress on blood leukocyte distribution: the role of α- and β-adrenergic mechanisms

Social stress in mammals has repeatedly been shown to cause substantial alterations in the distribution pattern of immune cells in the peripheral blood. The studies described here investigated the role of adrenergic mechanisms in mediating stressor-induced changes in blood leukocyte numbers using a social confrontation procedure in the rat. Experimental manipulations were carried out to eliminate the stress-associated release of adrenal hormones or to block the binding of endogenous catecholamines to α- and β-adrenergic receptors. Adrenalectomy completely abolished the stressor-induced decreases in circulating numbers of T helper cells (CD4 +), cytotoxic T cells (CD8 +) and B cells but was ineffective in preventing neutrophil granulocytosis, monocytosis and an increase in natural killer (NK) cells. Treatment with the α-adrenergic antagonist phentolamine (PHE) was highly effective in preventing granulocytosis and partially blocked lymphopenia, but failed to abolish monocytosis and an increase in NK cells. Treatment with the β-adrenergic antagonists propranolol (PROP) or nadolol (NAD) entirely blocked the increases in monocytes and NK cells. In addition, β-adrenergic blockade also significantly reduced neutrophilia, with PROP being more effective than NAD. The results presented here provide evidence that catecholamines play an important role in the redistribution of blood leukocytes during social stress. In particular, the mobilization of cells of the innate immune response seems to be regulated by adrenergic mechanisms.

Central neurokinin 3 receptors increase systemic oxytocin release: interaction with norepinephrine

Stimulation of central tachykinin receptors contributes to neuroendocrine functions of the hypothalamo-neurohypophyseal system. However, the specific role of each tachykinin receptor subtype has not been completely characterized. Specifically, while neurokinin 3 (NK3) receptor stimulation increases systemic vasopressin, the effects on oxytocin (OT) are not known. Therefore, the present studies investigated the effect of central NK3 receptor stimulation with senktide on release of systemic and central OT. Furthermore, since central NK3 receptors activate noradrenergic systems, which contribute to OT release, the effects of α-adrenergic receptor blockade on senktide-induced changes in OT release were evaluated. Female rats were implanted with a cannula in the third cerebral ventricle, and changes in plasma OT concentration determined before and following central administration of senktide in vehicle-treated rats, and animals following central administration of the α-adrenergic antagonist phentolamine. Other rats were implanted with microdialysis probes adjacent to the paraventricular nucleus (PVN), and dialysate and plasma OT concentrations were determined before and during administration of senktide through the dialysis probe. Central senktide increased systemic OT release, which was prevented by pretreatment with phentolamine. Furthermore, there was no detectable change in extracellular OT concentration in the PVN during dialysis administration of senktide. These data demonstrate that activation of central NK3 receptors stimulates systemic release of OT by activation of central noradrenergic systems, apparently without increasing intranuclear OT release in the PVN.

Neuromodulation of enteropathogen internalization in Peyer's patches from porcine jejunum

Jejunal Peyer's patches (JPP) are innervated sites of immune induction and enteropathogen infection. We investigated the role of enteric nerves in modulating pathogen entry into porcine JPP. Presumptive norepinephrine (NE)-containing nerve fibers were localized in JPP domes and follicle-associated villi by secondary immunofluorescence histochemistry. NE or the neuronal conduction blocker saxitoxin increased intracellular internalization of pathogenic Salmonella choleraesuis and Escherichia coli O157:H7, but not nonpathogenic E. coli, into isolated JPP mucosa. NE action was prevented by the α-adrenergic antagonist phentolamine. Withdrawal of enteric neural activity or NE administration appears to modulate JPP interactions with pathogenic bacteria.

Catecholamines modulate Escherichia coli O157:H7 adherence to murine cecal mucosa.

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is an important food-borne pathogen. While the molecular mechanisms governing E. coli O157:H7 pathogenesis have been intensively investigated, the role of host factors has received less attention. In this study, we tested the hypothesis that the enteric catecholamines norepinephrine (NE) and dopamine (DA) modulate interactions of the cecal mucosa with E. coli O157:H7. Full-thickness sheets of murine cecum were mounted in Ussing chambers and short circuit current and tissue electrical conductance were periodically determined to assess active transepithelial ion transport and ionic permeability, respectively. Neurochemicals and stationary-phase E. coli O157:H7 were exposed respectively to the contraluminal and luminal aspects of the mucosa. Epithelial adherence of E. coli O157:H7 was quantified by a bacterial adhesion assay after 90 min of luminal E. coli O157:H7 exposure. DA and NE increased E. coli O157:H7 adherence relative to untreated control tissues at 50% effective concentrations of 3.8 microM and 4.2 microM respectively. Pretreatment of tissues with either the alpha-adrenergic antagonist phentolamine or the beta-adrenergic antagonist propranolol prevented the action of NE. The effect of DA was prevented by the dopamine antagonist haloperidol. The drugs did not impair tissue viability or transepithelial conductance. The present findings suggest that enteric catecholamines modulate E. coli O157:H7 adherence to the cecal epithelium. Conditions associated with elevated catecholamine release, such as stress exposure, may influence host susceptibility to E. coli O157:H7 infection.

Isoproterenol inhibits resistin gene expression through a G S-protein-coupled pathway in 3T3-L1 adipocytes

Resistin was recently identified as a hormone secreted by adipocytes which leads to insulin resistance in vivo and in vitro and might therefore be an important link between obesity and diabetes. To clarify the regulation of resistin gene expression, 3T3-L1 adipocytes were treated with various agents known to modulate insulin sensitivity, and resistin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, isoproterenol treatment reduced the level of resistin mRNA to 20% of non-treated control cells. This effect was dose-dependent with significant inhibition occurring at concentrations as low as 10 nM isoproterenol. Moreover, pretreatment of adipocytes with the β-adrenergic antagonist propranolol almost completely reversed the inhibitory effect of isoproterenol, whereas addition of the α-adrenergic antagonist phentolamine did not have any effect. Furthermore, the effect of isoproterenol could be mimicked by activation of G S-proteins and adenylyl cyclase. Thus, both cholera toxin and forskolin decreased resistin mRNA expression in a dose-dependent fashion by up to 90% of control levels. Taken together, these results suggest that resistin gene expression is regulated by a protein kinase A-dependent pathway in 3T3-L1 adipocytes.

Metabolic Responses to Epinephrine Stimulation in Goldfish Hepatocytes: Evidence for the Presence of α-Adrenoceptors

The effect of epinephrine on various aspects of cellular metabolism was studied in hepatocytes from the goldfish Carassius auratus. Epinephrine increased cytosolic free calcium ([Ca2+]i) from a baseline value of 108 ± 22 nM to a peak value of 577 ± 127 nM in suspensions of hepatocytes. Responses of single cells ranged from a single spike (66% of hepatocytes) to variable oscillatory patterns (34%). The increase in [Ca2+]i was independent of the presence of extracellular Ca2+ and was prevented by the α-adrenergic antagonist phentolamine. Cellular glucose release induced by epinephrine (1.7- to 3.2-fold) was significantly reduced in Ca2+-depleted cells and in the presence of phentolamine, providing evidence for the co-occurrence of α-adrenoceptors and a Ca2+-independent, presumably β-adrenergic, system in these cells. Furthermore, epinephrine stimulated oxygen consumption in a Ca2+-dependent manner, which was not due to stimulated Na+ pump activity. An increased rate of acid secretion of 50%, evoked by epinephrine, appears to be mediated by enhanced Na+/H+ exchange but did not result in intracellular alkalization.

The Effects of Atropine and Adrenergic Antagonists on Behavioral Arousal in Rats

Two experiments investigated the mechanism for changes in measures of behavioral arousal inhibition in rats following administration of atropine. In Experiment 1, 40-day-old rats were given administrations of atropine sulfate, the α-, β-adrenergic blocker labetalol, or both. The drugs, either alone or in combination, increased transport response intensity, whereas both together increased dorsal immobility durations. In Experiment 2, rats were given atropine, the β-adrenergic antagonist propranolol, the α-adrenergic antagonist phentolamine, or a combination of two of the drugs. Propranolol blocked atropine-induced increases in transport response, and phentolamine was without effect. Phentolamine, when combined with atropine, increased dorsal immobility durations. Results are discussed with respect to aspects common to both transport response and dorsal immobility.

Supplemental oxygen does not modulate responses to acetylcholine or ascorbic acid in the forearm of patients with congestive heart failure

Despite providing symptomatic relief in patients with congestive heart failure (CHF), supplemental oxygen (O2) has been demonstrated to increase total peripheral resistance. The present study investigated the possibility that O2 inhalation reduces nitric oxide (NO) bioavailability, using endothelium-dependent (acetylcholine) and -independent (phentolamine) vasodilators, and the antioxidant ascorbic acid. Ten patients (nine male and one female) with primary left ventricular failure participated in the study. Forearm venous occlusion plethysmography was used to study blood flow responses to acetylcholine and the α-adrenergic antagonist phentolamine during inhalation of either room air or 100% O2, with and without the simultaneous infusion of ascorbic acid. Neither O2 inhalation (3.9±0.4 compared with 3.8±0.3 ml⋅min-1⋅100 ml-1) nor ascorbic acid infusion (5.2±0.4 compared with 5.5±0.4 ml⋅min-1⋅100 ml-1) affected resting forearm blood flow. The percentage increase from basal blood flow after acetylcholine infusion was not altered by either O2 inhalation or ascorbic acid infusion (room air, 140±55%; O2, 118±46%; ascorbic acid, 147±39%; ascorbic acid+O2, 109±31%). O2 inhalation did, however, reduce the dilation induced by phentolamine (room air, 131±24%; O2, 80±14%; P &lt; 0.05). These data indicate that oxygen inhalation does not increase forearm vascular resistance. Secondly, preservation of reactivity to acetylcholine during O2 inhalation suggests that degradation of NO by O2-derived free radicals is not enhanced. Attenuation of phentolamine-induced vasodilation during O2 inhalation, however, implies increased adrenergic activity, which may possibly exacerbate the detrimental effects of elevated sympathetic activity in CHF.

Adrenergic inhibition of endogenous acetylcholine release on postganglionic cardiac vagal nerve terminals

The aim was to examine the adrenergic modulation of endogenous acetylcholine (ACh) release from vagal nerve terminals in the in vivo heart. Using dialysis technique in anesthetized cats, we investigated the influence of exogenous noradrenaline on dialysate ACh response. Dialysis probes were implanted in the left ventricular myocardium and perfused with Krebs-Henseleit buffer containing eserine (10(-4) M) at 3 microl/min. Dialysate ACh concentration was measured as an index of ACh release from cardiac vagal nerve terminals. The dialysate ACh response to vagal nerve stimulation was examined before and after local administration of noradrenaline (10(-5) M) through dialysis probes. Noradrenaline significantly attenuated the dialysate ACh response to vagal nerve stimulation (10 Hz) from 9.5+/-1.8 to 5.4+/-1.2 nM (n=7). In the presence of the alpha-adrenergic antagonist phentolamine (10(-4) M), noradrenaline did not attenuate the dialysate ACh response (from 9.8+/-2.7 to 9.4+/-2.8 nM, n=6). The N-type Ca2+ channel blocker omega-conotoxin GVIA (10(-5) M) significantly attenuated the dialysate ACh response from 9.6+/-1.2 to 4.5+/-0.7 nM (n=8). In the presence of omega-conotoxin GVIA, noradrenaline did not attenuate the dialysate ACh response (from 3.8+/-1.4 to 3.5+/-1.3 nM, n=7). Our results suggest the presynaptic adrenergic inhibition of ACh release on postganglionic cardiac vagal nerve terminals. Adrenergic inhibition of Ca2+ influx through the N-type Ca2+ channels could play a predominant role in the decrease in ACh release.