Effects of beta adrenoceptor down-regulation on the cardiovascular responses to the stereoisomers of dobutamine

Effects of prolonged in vivo infusion of either saline (control) or isoproterenol (beta adrenoceptor desensitization) on acute cardiovascular responses to (+) (beta agonist)-, (-) (alpha agonist)- and (+/-)-dobutamine were studied in pithed rats. Each form of dobutamine resulted in comparable dose-dependent increases in maximum left ventricular dP/dt (LVdP/dtmax) in control animals. Effects of (+)-dobutamine were blocked by propranolol whereas those of l-dobutamine were sensitive to prazosin; both alpha and beta antagonists were required to block the inotropic effects of the racemic mixture. Contractile responses to (+)- and (+/-)-dobutamine were accompanied by tachycardia (characteristic of beta adrenoceptor stimulation) whereas (-)-dobutamine enhanced LVdP/dtmax without altering heart rate (characteristic of alpha adrenoceptor stimulation). Isoproterenol infusion resulted in a pronounced desensitization to the inotropic effects (LVdP/dtmax) of (+/-)- and (+)-dobutamine. Ed30 values for (+/-)- and (+)-dobutamine were increased by approximately 15- and 50-fold, respectively, and maximal responses to both drugs were severely attenuated. Prazosin further blunted remaining inotropic responses to (+/-)-dobutamine and propranolol resulted in a complete block. Responses to (+)-dobutamine were only sensitive to propranolol. Attenuation of heart rate responses paralleled those observed for LVdP/dtmax. By contrast, the inotropic effects of (-)-dobutamine in either control or desensitized rats were both qualitatively and quantitatively comparable; responses were blocked by the alpha-1 antagonist, prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of prolonged isoproterenol infusion on cardiac and vascular responses to adrenoceptor agonists

In vitro responses of cardiac and vascular smooth muscle to both adrenoceptor agonists and phosphodiesterase inhibitors were studied in tissues from either saline- or isoproterenol-infused rats. After chronic isoproterenol infusion the sigmoidal relationship between concentration of acutely administered isoproterenol and inotropic response of cardiac muscle was shifted to the right; the maximum response was decreased by approximately 40%. Inotropic responses were attenuated further by the beta adrenoceptor antagonist, propranolol. By contrast, quantitatively comparable inotropic responses to phenylephrine were not altered after isoproterenol infusion. However, they were blocked by the selective alpha adrenoceptor antagonist, prazosin, but were not affected by propranolol. Inotropic effects of the phosphodiesterase inhibitor, isobutylmethylxanthine, were comparable in tissues from either saline- or isoproterenol-infused rats. Similar results were obtained in vascular tissues. Portal veins and aortas from isoproterenol-infused rats were less responsive to the acute relaxant properties of the beta adrenoceptor agonists, isoproterenol and salbutamol. However, as in cardiac muscle, relaxant effects to phosphodiesterase inhibitors (isobutylmethylxanthine and papaverine) were not attenuated. In addition, contraction to norepinephrine was comparable in tissues from either saline- or isoproterenol-infused rats. These data indicate that isoproterenol infusion attenuates beta adrenoceptor-mediated responses of vascular and cardiac muscle to similar degrees but does not alter responses to either alpha adrenoceptor agonists or phosphodiesterase inhibitors.     

Effects of dopamine, (+/-)-dobutamine and the (+)- and (-)-enantiomers of dobutamine on cardiac function in pithed rats

The effects of dopamine, (+/-)-dobutamine (racemic mixture) and the (+)- and (-)-enantiomers of dobutamine on myocardial function were evaluated in pithed rats. Dopamine and (+/-)-dobutamine produced effects on cardiac function in pithed rats that were qualitatively similar to those reported for these compounds in humans. The increase in cardiac output produced by dopamine and (+/-)-dobutamine was due mainly to an increase in stroke volume, with increases in heart rate contributing to a significant but lesser degree. For both dopamine and (+/-)-dobutamine, the increase in stroke volume appears to result from an increase in myocardial contractility as assessed by increases in left ventricular (LV) dp/dt. Dopamine produced a marked increase in mean arterial blood pressure, whereas (+/-)-dobutamine only modestly increased blood pressure. The (-)-enantiomer of dobutamine, which possesses mainly alpha-1 adrenoceptor agonist activity, produced dose-dependent increases in cardiac output, stroke volume, LVdp/dt and mean arterial blood pressure, but did not significantly increase heart rate except at high doses. Thus, the increase in cardiac output produced by (-)-dobutamine was derived almost exclusively from an augmentation in stroke volume resulting from an increase in myocardial contractility. In contrast, (+)-dobutamine, which possesses predominantly beta-1 and beta-2 adrenoceptor agonist activity, elicited only a modest increase in cardiac output which was due both to an increase in heart rate and stroke volume. Mean arterial blood pressure was not significantly affected by (+)-dobutamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enantiomers of dobutamine increase the force of contraction via beta adrenoceptors, but antagonize competitively the positive inotropic effect mediated by alpha-1 adrenoceptors in the rabbit ventricular myocardium

Experiments were carried out to characterize the pharmacological properties of enantiomers and racemic mixture of dobutamine to modulate the myocardial contractility through alpha and beta adrenoceptors in the rabbit papillary muscle. Dobutamine caused the concentration-dependent positive inotropic effect: the rank order of potency was R-(+)- greater than (+/-) - greater than S-(-)-dobutamine. The positive inotropic effect of (+)-, (-)- and (+/-)-dobutamine was antagonized by a beta adrenoceptor antagonist, (+/-)-bupranolol in a competitive manner, but was not affected by an alpha-1 adrenoceptor antagonist, prazosin. The concentration-response curve for (-)-phenylephrine mediated by alpha adrenoceptors in the presence of 10(-6) M (+/-)-bupranolol was shifted by enantiomers of dobutamine to the right in a concentration-dependent manner. Thus, enantiomers of dobutamine antagonized the positive inotropic effect of (-)-phenylephrine in a competitive manner, and pA2 values [negative logarithm of the dissociation constant (KB)] for (+)- and (-)-dobutamine were 6.67 and 5.99, respectively. The specific binding of [3H]prazosin to membrane fractions of rabbit ventricular myocardium was displaced by dobutamine with a high potency: the -log Ki values for (+)- and (-)-dobutamine were 6.43 and 5.97, respectively, which correspond well with pA2 values of these compounds for functional modification. These findings indicate that enantiomers of dobutamine elicit the positive inotropic effect through activation of beta adrenoceptors, whereas both enantiomers behave as the competitive antagonist of myocardial alpha adrenoceptors mediating the positive inotropic effect in the isolated rabbit papillary muscle.     

Inotropic selectivity of dobutamine enantiomers in the pithed rat

The inotropic selectivities of the (-)- and (+)-enantiomers of dobutamine were assessed in pithed rat by comparing the relative ability of each enantiomer to increase left ventricular contractility (left ventricular dp/dt) and heart rate. The (-)-enantiomer of dobutamine, which is predominantly an alpha-1 adrenoceptor agonist, displayed greater inotropic selectivity than the (+)-enantiomer, which is predominantly a beta-1 and beta-2 adrenoceptor agonist. Pretreatment with the alpha-1 adrenoceptor antagonist prazosin significantly inhibited the effect of (-)-dobutamine on left ventricular dp/dt, but did not affect the chronotropic activity of this enantiomer. As such, pretreatment with prazosin decreased the inotropic selectivity of (-)-dobutamine. In contrast, the inotropic activity and selectivity of (+)-dobutamine were not affected by prazosin pretreatment. These results indicate that the inotropic effects of (-)-dobutamine are mediated, at least in part, by alpha-1 adrenoceptors. We conclude, based on the marked inotropic activity of (-)-dobutamine and the greater inotropic selectivity of (-)-dobutamine over (+)-dobutamine, that alpha-1 adrenoceptors may play a role in the inotropic activity and selectivity of racemic dobutamine used clinically. The possible involvement of both myocardial alpha-1 and beta-1 adrenoceptors in the inotropic activity of dobutamine must be considered.     

In vivo cardiovascular responses to isoproterenol, dopamine and tyramine after prolonged infusion of isoproterenol

Effects of prolonged in vivo infusion of isoproterenol on acute cardiovascular responses to isoproterenol, dopamine and tyramine were studied in pithed rats. Isoproterenol infusion resulted in a significant decrease in control values for maximum left ventricular dP/dt; heart rate and left ventricular systolic blood pressure were not altered. This treatment also depleted both atrial and ventricular stores of norepinephrine and caused cardiac hypertrophy. Isoproterenol infusion resulted in a desensitization of drug-induced cardiovascular responses. The acute in vivo effects of isoproterenol on maximum left ventricular dP/dt, heart rate and left ventricular systolic blood pressure responses to isoproterenol were severely attenuated. The ED50 for maximum left ventricular dP/dt was increased 36-fold and maximal responses were reduced by half; changes in heart rate occurred in a parallel fashion. By contrast, ED50 values for inotropic responses to tyramine and dopamine were increased 14- and 4-fold, respectively, whereas increases in heart rate were blunted. Tyramine and dopamine-mediated increases in heart rate were completely attenuated by desensitization; chronotropic effects were again evident after pretreatment with the selective alpha-1 blocker prazosin. In addition, prazosin blocked the inotropic responses to tyramine and dopamine after desensitization and this antagonism was only slightly enhanced by addition of propranolol (prazosin + propranolol); propranolol alone was ineffective. These results are consistent with the down-regulation of beta adrenoceptors after prolonged exposure to catecholamines and indicate that under such conditions the alpha-mediated cardiovascular responses may be unmasked. Compared to pure beta agonists, agents with a degree of alpha-1 activity might be superior inotropes in heart failure patients who characteristically present with depleted stores of myocardial norepinephrine and minimal beta adrenoceptor reserve.     

Selective desensitization of cardiac beta adrenoceptors by prolonged in vivo infusion of catecholamines in rats

The effects of prolonged in vivo infusion of isoproterenol (400 micrograms/kg/hr) or norepinephrine (200 micrograms/kg/hr) from a minipump on the physiological reactivity and binding properties of cardiac beta and alpha-1 adrenoceptors were tested in rats. Infusion of either catecholamine significantly reduced the in vitro inotropic and chronotropic potency of isoproterenol in isolated left and right atria, respectively; desensitization was near maximal as early as after 2 hr of infusion. No significant change in the density of [3H]dihydroalprenolol-labeled beta receptors was evident at this time point in either atrial or ventricular tissue, although isoproterenol did decrease binding site density after 7 days of infusion. There was no change in the binding affinity or physiological blocking potency of dihydroalprenolol after isoproterenol infusion. The inotropic potency of phenylephrine in the presence of dihydroalprenolol was unaffected by infusion of either isoproterenol or norepinephrine and methoxamine failed to increase right atrial rate either in control or in isoproterenol-infused rats. There was also no change in the density and affinity of [3H]prazosin binding sites after isoproterenol infusion. These results indicate selective desensitization of cardiac beta receptors without changes in alpha-1 receptors by prolonged in vivo stimulation with catecholamines. This reaction pattern is different from the well documented effects of hypothyroidism, which include decreased sensitivity of cardiac beta and increased sensitivity of cardiac alpha-1 receptor-mediated responses in rats. Thus, the mechanisms responsible for altered receptor function in the two conditions appear to be different.     

Effect of aromatic fluorine substitution on the alpha and beta adrenoceptor-mediated effects of 3,4-dihydroxytolazoline in the pithed rat

The alpha and beta adrenoceptor-mediated effects of the catecholimidazoline, 3,4-dihydroxytolazoline, and its 2-, 5- and 6-aromatic fluorine-substituted derivatives have been studied in the cardiovascular system of the pithed rat. All four compounds produced vasopressor responses in beta adrenoceptor blocked (propranolol, 3 mg/kg i.v.) animals. The pressor responses produced by all four compounds were antagonized by the selective alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg i.v.), and were relatively unaffected by the selective alpha-2 adrenoceptor antagonist, rauwolscine (0.5 mg/kg i.v.), indicating that vasoconstriction produced by the fluorinated imidazolines was mediated exclusively by postjunctional vascular alpha-1 adrenoceptors. The rank order of potency at the alpha-1 adrenoceptor was: 5-fluoro greater than 2-fluoro greater than desfluoro greater than 6-fluoro. At higher doses, 3,4-dihydroxytolazoline and its fluorinated derivatives produced an alpha-2 adrenoceptor-mediated inhibition of neurogenic tachycardia in animals pretreated with prazosin, with all four compounds being equipotent. In rats with complete alpha adrenoceptor blockade [phenoxybenzamine (3 mg/kg i.v.), prazosin (0.1 mg/kg i.v.) and rauwolscine (1 mg/kg i.v.)] whose blood pressure was elevated by constant infusion of angiotensin II (150 ng/kg/min i.v.), high doses of the 2-fluoro-, but not the 5-, 6- or desfluoro catecholimidazoline derivatives, produced a beta-2 adrenoceptor-mediated vasodepressor response. All four compounds produced a beta-1 adrenoceptor-mediated positive chronotropic response in pithed rats with the rank order of potency being: 2-fluoro = 5-fluoro greater than desfluoro greater than 6-fluoro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effect of alpha-1 adrenoceptor stimulation on cardiac sympathetic neurotransmission in pithed normotensive rats

In the present study we investigated the inhibitory effect of the selective alpha-1 adrenoceptor agonists cirazoline, amidephrine and St 587 on the cardiac sympathetic neurotransmission in pithed normotensive rats. Increases in heart rate were elicited by electrical stimulation of the cardiac sympathetic nerves or by i.v. administration of norepinephrine, isoproterenol or tyramine. Intravenous pretreatment of the animals with cirazoline, amidephrine or St 587 diminished the heart rate response to sympathetic stimulation significantly. However, the tachycardia produced by norepinephrine, isoproterenol or tyramine was also inhibited significantly by the selective alpha-1 adrenoceptor agonists. The selective alpha-1 antagonist prazosin blocked the sympathoinhibitory effect to alpha-1 adrenoceptor stimulation significantly. However, the inhibitory effect of cirazoline and St 587 was not suppressed completely by a maximally effective dose of prazosin. In contrast, the sympathoinhibitory action of amidephrine was antagonized completely by prazosin. However, the selective alpha-2 antagonist rauwolscine also produced a significant, albeit modest, attenuation of the sympathoinhibitory effect to amidephrine. The results of the present study indicate that alpha-1 adrenoceptor agonists, at relatively high doses, inhibit the sympathetic neurotransmission in rat heart. This sympathoinhibitory effect is mediated largely by alpha-1 adrenoceptors which are localized postjunctionally rather than prejunctionally.     

Alpha adrenoceptor-mediated vasoconstriction in rat hindlimb: innervated alpha-2 adrenoceptors in the saphenous arterial bed

The alpha adrenoceptor subtypes mediating vasoconstriction to exogenous agonists and to spinal sympathetic nerve stimulation have been characterized in the autoperfused (constant flow) femoral (predominantly skeletal musculature) and saphenous (predominantly cutaneous) vascular beds of the pithed rat. Intra-arterial infusion of the alpha-1 adrenoceptor agonist, methoxamine, increased perfusion pressure in both vascular beds over the same range of infusion rates, and the maximum responses were similar. The selective alpha-2 adrenoceptor agonist, B-HT 933, also increased perfusion pressure in both beds, although the maximum response to B-HT 933 in the saphenous bed was approximately twice that observed in the femoral bed. Responses to methoxamine were blocked by the alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg), but not the alpha-2 adrenoceptor antagonist, rauwolscine (1 mg/kg), or by the selective postjunctional alpha-2 adrenoceptor antagonist, SK&F 104078 (1 mg/kg). Conversely, responses to B-HT 933 were blocked by rauwolscine and by SK&F 104078, but not by prazosin. Vasopressor responses to B-HT 933 in both vascular beds of the rat hindlimb also were reduced markedly by the calcium channel blocker, nifedipine (1 mg/kg), whereas responses to methoxamine were relatively resistant to inhibition by nifedipine. In the femoral bed, as in the systemic arterial circulation, responses to sympathetic nerve stimulation were strongly inhibited by prazosin, were potentiated by rauwolscine and were unaffected by SK&F 104078. In contrast, in the saphenous arterial bed, the responses to sympathetic nerve stimulation were inhibited by all three antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sympathoadrenal modulation of stress-activated signaling in burn trauma

Burn injury stimulates stress-responsive components, p38 mitogen-activated protein kinase (MAPK)/c-Jun N-terminal kinases (JNK)/nuclear factor (NF)-kappaB. p38 MAPK plays a role in postburn cardiomyocyte tumor necrosis factor-alpha secretion and cardiac dysfunction. Since burn trauma increases circulating catecholamine levels, which in turn modulate inflammatory cytokine production, we hypothesized that increased sympathetic activity after major burn trauma may trigger postburn cardiac p38 MAPK activation via an adrenergic receptor-mediated phenomenon. We examined adrenergic receptor populations involved in burn-activated cardiac stress signaling. Sprague Dawley rats were divided into six groups: 1) control, 2) control plus alpha1-adrenergic agonist phenylephrine (2 microg/kg, intravenous), 3) control plus beta-adrenergic agonist isoproterenol (1 microg/kg, intravenous), 4) burn (fluid resuscitation with lactated Ringer's 4 ml/kg/% burn), 5) burn plus alpha1-adrenergic antagonist prazosin (1 mg/kg, by mouth), and 6) burn plus beta-adrenergic antagonist propranolol (3.3 mg/kg, by mouth). Phenylephrine, but not isoproterenol, increased cardiac p38 MAPK/JNK/NF-kappaB activation. Burn trauma activated p38 MAPK, JNK, and NF-kappaB, and this stress response was blocked by either prazosin or propranolol. Thus, stimulation of the adrenergic pathway may constitute one upstream activator of stress response in burn.     

Characterization of adrenoceptor subtypes in cat cutaneous vasculature

Experiments were undertaken to characterize the relative contribution of adrenoceptor subtypes in mediating vasoconstriction to exogenous agonists in the digital cutaneous vascular bed of the anesthetized cat using laser-Doppler flowmetry. Intra-arterial administration of (-)-epinephrine and (-)-norepinephrine into the brachial artery caused a dose-related vasoconstriction (decreased flow) with ED50 values of 7 and 21 ng, respectively. Blockade of beta adrenoceptors with propranolol did not alter the response to (-)-epinephrine nor did i.a. isoproterenol produce a significant vasodilation. Vasoconstrictor responses elicited by (-)-epinephrine and (-)-norepinephrine were antagonized by treatment with phentolamine (2.5 mg/kg i.v.) and by yohimbine (0.5 mg/kg i.v.) but were only marginally blocked by prazosin (0.1 mg/kg i.v.). A dose-related depression of cutaneous blood flow was also caused by clonidine at doses virtually identical to those of (-)-norepinephrine. Clonidine-induced vasoconstriction was antagonized by rauwolscine (0.5 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Dose-response curves to a variety of additional adrenoceptor stimulants were constructed with the potency rank order for all agonists being: (-)-epinephrine greater than B-HT 920 = (-)-norepinephrine = clonidine much greater than (-)-phenylephrine much greater than B-HT 933 greater than methoxamine. Treatment with prazosin (0.1 mg/kg i.v.) antagonized methoxamine induced cutaneous vasoconstriction but not the decreased blood flow caused by B-HT 933. In contrast, rauwolscine (0.5 mg/kg i.v.) blocked the responses to B-HT 933 but not methoxamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduction of cardiac outward currents by alpha-1 adrenoceptor stimulation: a subtype-specific effect?

In rat ventricular myocytes, the effects of alpha adrenoceptor stimulation on outward currents were studied by means of the whole cell voltage-clamp technique. Phenylephrine (30 microM) in the presence of propranolol (1 microM) to block beta adrenoceptors reduced voltage-activated transient outward current. Both components of transient outward current were affected, i.e., peak current (Ipeak) was reduced by 25.3 +/- 1.8%, the outward current at the end of a clamp step (Ilate) was reduced by 39.1 +/- 3.5% (n = 5; holding potential -40 mV, clamp step to +20 mV). In order to describe the alpha-1 adrenoceptor subtypes involved in this action, the effect of phenylephrine was also investigated after pretreatment of the cells with various antagonists. Pretreatment with prazosin (0.3 microM) abolished completely the phenylephrine effect. The alpha-1A adrenoceptor subtype-selective antagonists 5-methylurapidil and (+)-niguldipine (0.1 microM each) and the irreversible alpha-1B adrenoceptor subtype antagonist chloroethyl-clonidine (100 microM) blocked the phenylephrine effect on Ipeak, but merely attenuated the effect on Ilate, whereas pretreatment with a combination of chloroethylclonidine and (+)-niguldipine suppressed the phenylephrine-induced effect on both outward current components just like prazosin did. In conclusion, stimulation of both adrenoceptor subtypes is required for reduction of Ipeak, but stimulation of either alpha-1A or alpha-1B subtype is sufficient for reduction of Ilate. Therefore, stimulation of both alpha-1 adrenoceptor subtypes contributes to the phenylephrine-induced reduction in transient outward currents of isolated rat myocytes.     

Alpha adrenoceptor regulation of coronary artery blood flow in normal and stenotic canine coronary arteries

The response of systemic blood pressure, heart rate, lead II ECG and left circumflex (LCX) coronary artery blood flow to left cardiac sympathetic nerve stimulation was measured in pentobarbital-anesthetized, open chest, spinal transected and vagotomized dogs. After beta adrenoceptor blockade, left cardiac sympathetic nerve stimulation produced frequency dependent decreases in LCX blood flow. Selective alpha-2 adrenoceptor blockade with idazoxan produced a greater inhibition of this decrease in LCX blood flow than did selective alpha-1 adrenoceptor blockade with prazosin. In an additional population of dogs which were similarly prepared but were not spinally transectioned or pretreated with a beta adrenoceptor antagonist, left cardiac sympathetic nerve stimulation produced an increase in LCX blood flow in all animals which reached a maximum within 40 sec, and then began to decline slowly. However, after beta adrenoceptor blockade, identical stimulation parameters produced only a decline in LCX blood flow which returned to the level of control resting blood flow by the end of the stimulation period. Both selective alpha-2 adrenoceptor blockade with idazoxan and selective alpha-1 adrenoceptor blockade with prazosin produced an inhibition of the LCX blood flow decrease provoked by left cardiac sympathetic nerve stimulation in dogs pretreated with beta adrenoceptor antagonists. Idazoxan produced a slightly greater inhibition of the LCX blood flow decrease than did prazosin, suggesting a greater role for postjunctional vascular alpha-2 adrenoceptors in LCX blood flow regulation during cardiac sympathetic nerve stimulation. The presence of a severe coronary artery stenosis reduced, but did not inhibit, the increase in LCX blood flow in response to cardiac sympathetic nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on the mechanism of arterial vasodilation produced by the novel antihypertensive agent, carvedilol

The mechanism(s) responsible for arterial vasodilation observed following acute administration of racemic carvedilol, a novel vasodilator/beta adrenoceptor antagonist, has been investigated in rats. In conscious spontaneously hypertensive rats, carvedilol (0.03-3.0 mg/kg, iv) produced a dose-dependent reduction in blood pressure with no significant effect on heart rate. Because cardiac output was relatively unaffected, the antihypertensive response of carvedilol was associated with a dose-dependent reduction in total peripheral vascular resistance. Submaximal antihypertensive doses of carvedilol were chosen for mechanism of action studies in pithed rats. Carvedilol (0.3 mg/kg, iv) produced a significant inhibition of the beta 1 adrenoceptor mediated positive chronotropic response to isoproterenol. This same dose of carvedilol also inhibited, but to a lesser degree, the beta 2 adrenoceptor mediated vasodepressor response to salbutamol in pithed rats whose blood pressure was elevated by a constant intravenous infusion of angiotensin II. Thus, carvedilol blocks both beta 1 and beta 2 adrenoceptors at antihypertensive doses, with modest selectivity being observed for the beta 1 adrenoceptor subtype. Carvedilol produced significant inhibition of the alpha 1 adrenoceptor mediated pressor response to cirazoline in the pithed rat, but had no effect on the alpha 2 adrenoceptor mediated pressor response to B-HT 933, suggesting that carvedilol is also an alpha 1 adrenoceptor antagonist at antihypertensive doses. Carvedilol had no effect on the pressor response elicited by angiotensin II, indicating a lack of nonspecific vasodilator activity. The vasopressor response to the calcium channel activator, BAY-K-8644, which is mediated through the opening of voltage dependent calcium channels and the subsequent translocation of extracellular calcium, was significantly inhibited by carvedilol (1 mg/kg, iv), suggesting that carvedilol is also a calcium channel antagonist, consistent with our previous in vitro studies. In anesthetized spontaneously hypertensive rats, the antihypertensive activity of carvedilol was nearly abolished by combined pretreatment of the rats with high doses of the alpha 1 adrenoceptor antagonist, prazosin (1 mg/kg, iv), and the nonselective beta adrenoceptor antagonist, propranolol (3 mg/kg, iv), suggesting that the majority of the antihypertensive response produced by carvedilol may be accounted for by blockade of beta and alpha 1 adrenoceptors. We therefore conclude that carvedilol, at antihypertensive doses, is an antagonist of beta 1, beta 2, and alpha 1 adrenoceptors, and also of calcium channels in vascular smooth muscle.(ABSTRACT TRUNCATED AT 400 WORDS)     

Alpha-2 adrenoceptors on nerves and muscles of rat uterus

To test the hypothesis that functional postsynaptic alpha-2 adrenoceptors exist in rat myometrium, we examined whether specific binding sites for [3H]rauwolscine were present on microsomal membranes from myometrium of nonpregnant, day 16 pregnant and delivering rats and whether an alpha-2 adrenoceptor agonist has functional effects. The myometrium of rats at term undergoes a physiological denervation, confirmed in this study by ultrastructural examination of uterine samples and by [3H]saxitoxin binding studies. Binding sites for rauwolscine of similar Kd (11-15 nM) were present in all groups of myometrium and were localized on plasma membranes. There was no significant change in the density of rauwolscine binding sites in membranes from day 16 animals compared to nonpregnant ones, but a significant fall (38%) from these values at term. Strips of uterine circular or longitudinal muscle from nonpregnant, day-16 or day-22 pregnant rats failed to respond to the selective alpha-2 agonist, BHT-920, in the presence of propranolol; i.e., BHT-920 neither caused contraction nor inhibited contractions induced by oxytocin. BHT-920 did not affect the inhibitory effects of isoproterenol which were antagonized by propranolol. However, it antagonized contractions to norepinephrine in the presence of propranolol with a pKB value of 5.6 to 5.7. These contractions were phentolamine-sensitive. BHT-920 displaced rauwolscine from binding to all groups of myometria (IC50 = 2 to 3 x 10(-6) M) and displaced prazosin (Kd = 0.65 nM) from binding to myometria of nonpregnant rats (IC50 value congruent to 2 x 10(-4) M). Phentolamine also displaced rauwolscine from binding (IC50 = 2 x 10(-8) M). 5-Hydroxytryptamine displaced rauwolscine from binding only at higher concentrations (IC50 greater than 10(-5) M). We conclude that binding sites for alpha-2 adrenoceptor antagonists in myometrial microsomes were located primarily on smooth muscle plasma membrane. A smooth muscle alpha-2 adrenoceptor agonist appeared to occupy a site on muscle with the same affinity as it displayed toward rauwolscine binding site and competitively inhibited effects of an alpha-1 agonist. Our data suggest that alpha-2 adrenoceptor binding sites may exist on smooth muscle without coupling to contractile function, but their occupancy competitively prevents occupancy of alpha-1 agonist receptor activation sites.     

Modulation of pyloric motor activity via adrenergic receptors

The influences of adrenergic nerves on pyloric motor function and the locus and types of receptors involved were examined. Using glyoxylic acid fluorescence a dense adrenergic innervation of the inner pyloric muscle ring was demonstrated. Pyloric motor activity was monitored while close i.a. injections of a selective alpha-1 adrenoceptor agonist, phenylephrine, or a selective alpha-2 adrenoceptor agonist, B-HT-920, were given. Neither agonist affected basal pyloric motor activity, but both inhibited pyloric activity when it was stimulated by duodenal field stimulation or by intraduodenal acid infusion. The actions of each of the inhibitory agonists, phenylephrine or B-HT-920, were blocked selectively by prazosin or rauwolscine, respectively. Injection of isoproterenol usually had no effect or excited basal pyloric activity. This excitation could be antagonized selectively by propranolol or by atropine. Injection of isoproterenol after neural blockade by tetrodotoxin inhibited pyloric motor activity. Receptor binding studies carried out with subcellular nerve or muscle enriched membrane fractions of canine pyloric muscle with [3H]prazosin, [3H]rauwolscine and [125I]cyanopindolol revealed a dual location of alpha-1, alpha-2 and beta receptors on both nerve membranes and smooth muscle membranes. These results suggest that adrenergic effects on the pyloric muscle can be exerted by pre- and postsynaptic beta receptors which, respectively, excite by releasing acetylcholine and inhibit by acting on receptors on the pyloric muscle. Also inhibitory alpha-1 and alpha-2 receptors are present on cholinergic nerves. The functions of postsynaptic alpha-1 and alpha-2 adrenoceptor binding sites on smooth muscle are so far unknown.     

Adrenergic mechanisms underlying cardiac and vascular responses to cocaine in conscious rats.

The contribution of adrenergic receptors to the cardiovascular responses to cocaine (5 mg/kg i.v.) were examined in conscious, free-moving rats instrumented for continuous measurement of arterial pressure, heart rate and blood flows in the mesentery and hindquarters or ascending aorta. Cocaine elicits an immediate (peak) and sustained pressor response with a concomitant reduction in heart rate. Prazosin (0.1 mg/kg i.v.) pretreatment significantly reduced both the peak and sustained pressor responses by attenuating the increases in systemic, mesenteric and hindquarters vascular resistances. Idazoxan pretreatment (1 mg/kg i.v.) attenuated the peak increase in hindquarters vascular resistance. Whereas propranolol pretreatment (1 mg/kg i.v.) attenuated the peak pressor response, the sustained pressor response was enhanced due to increased hindquarters and systemic vascular resistances. Metoprolol pretreatment (1 mg/kg i.v.) enhanced the sustained pressor response to cocaine, in part due to increased heart rate and mesenteric vascular resistances. Upon examination of the cardiac effects of cocaine, a sustained bradycardic response was observed, whereas stroke volume and cardiac output were relatively unaffected. The bradycardic response to cocaine was attenuated by yohimbine (0.1 mg/kg i.v.), prevented by prazosin and converted to a tachycardia after idazoxan (1 mg/kg) pretreatment. After propranolol pretreatment, cocaine substantially decreased cardiac output and stroke volume. Our results demonstrate that cocaine produces a biphasic pressor response in conscious rats and that the mechanisms underlying the dual responses vary in intensity and mode of action in different vascular beds, but are primarily dependent upon alpha-1 adrenergic receptor-mediated vasoconstriction.     

Muscarinic cholinergic and beta-adrenergic contribution to hindquarters vasodilation and cardiac responses to cocaine

Cocaine produces a pressor response associated with an initial hindquarters vasoconstriction followed by a prolonged vasodilation in conscious rats. Propranolol pretreatment prevented the vasodilation and enhanced the pressor response, whereas atropine methylbromide pretreatment reduced the increase in systemic vascular resistance. We studied the role of selective muscarinic and beta-adrenoceptor antagonists on responses to cocaine in rats with an increase in systemic vascular resistance to cocaine (vascular responders). Arterial blood pressure and ascending aortic and distal descending aortic blood flow using pulsed Doppler flowmetry were measured. In conscious rats, cocaine (5 mg/kg i.v.) elicited consistent pressor responses but variable systemic and hindquarters vascular resistance responses that were directly correlated, suggesting that skeletal muscle resistance responses comprise an important component of systemic vascular resistance. ICI 118551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)-amino]-2-butanol] (0.5 mg/kg i.v.) pretreatment prevented the hindquarters vasodilation, enhancing the increase in systemic vascular resistance and the pressor response while further depressing the cardiac output response, similar to the effects of propranolol. Atenolol (1 mg/kg) pretreatment attenuated the stroke volume and cardiac output responses while enhancing the increase in systemic vascular resistance without affecting the hindquarters responses. In contrast, M2 antagonist methoctramine (0.3 mg/kg) pretreatment had similar effects as atropine in reducing the decrease in cardiac output by reducing the increase in systemic vascular resistance, whereas the M1 antagonist pirenzipine (0.02 mg/kg) did not alter responses. Therefore, the cocaine-induced pressor response is ameliorated by beta2-adrenoceptor mediated skeletal muscle vasodilation, whereas the decrease in cardiac output and the increase in systemic vascular resistance are dependent on M2-cholinoceptor activation.     

Pharmacodynamic Studies of Beta Adrenergic Antagonism Induced in Man by Propranolol and Practolol

The pharmacodynamic activities of two beta adrenergic antagonists, propranolol and practolol, were compared in eight hypertensive patients. The activity of each antagonist was established in relation to its blood concentration at maximal and submaximal adrenergic blockade defined by inhibition of exercise tachycardia. Maximal inhibition of exercise tachycardia was comparable with both drugs and averaged 74+/-7% of the control value during drug treatment. This inhibition was achieved with a blood concentration of 2.5+/-0.4 mug/ml practolol and 0.10+/-0.08 mug/ml propranolol. The antagonist activities of these drugs against adrenergic stimulation with isoproterenol infusion indicated a much greater relative potency of propranolol against this stimulus, and in vivo estimates of PA(2) values differed by more than 600-fold. Relative antagonist activity of practolol during isoproterenol stimulation was equivalent both at cardiac (inotropic and chronotropic) and at vascular adrenergic receptors, whereas greater antagonist activity of propranolol was observed at vascular receptors than at cardiac receptors. Thus, the activity of practolol was not limited to cardiac receptors as previously suggested. Practolol did not reduce cardiac output at any dose level and the effect on resting blood pressure was small. Both practolol and propranolol had much greater hypotensive activity during exercise. These studies have defined the differing pharmacodynamic activities on the cardiovascular system of two effective beta adrenergic receptor antagonists and have established the blood levels of these antagonists necessary to achieve effective adrenergic blockade.