Both alpha and beta adrenoceptor-mediated components contribute to final inotropic response to norepinephrine in rat heart

The classical approach of displacing the dose-response curve by an alpha adrenoceptor blocker has most often failed to demonstrate a contribution of an alpha adrenoceptor-mediated component in the final inotropic response to norepinephrine unless the beta adrenoceptors are extensively blocked. To unmask and quantify the inotropic components in the response to norepinephrine, we took a different approach by studying reversal responses to appropriate adrenoceptor blockers in isolated paced rat papillary muscles maximally stimulated by norepinephrine. The inotropic response to norepinephrine was rapidly reversed by simultaneous addition of the beta adrenoceptor blocker timolol and the alpha adrenoceptor blocker prazosin. When the adrenoceptor blockers were added sequentially, both alpha and beta adrenoceptor-mediated components in the inotropic response to norepinephrine could be demonstrated: one beta adrenoceptor-mediated component (timolol sensitive only) that represented about 75% of the inotropic response and one alpha adrenoceptor-mediated component (prazosin sensitive only) that represented about 25% of the inotropic response. When one adrenoceptor population was eliminated by giving one of the adrenoceptor blockers before norepinephrine, the inotropic response in this situation could be completely reversed by the other adrenoceptor blocker. The expression of both alpha and beta adrenoceptor-mediated components was significantly less during concomitant receptor stimulation than when the respective receptor populations were stimulated separately. Thus, there was apparently a mutual inhibition of one component upon the other. Although the beta adrenoceptor-mediated inotropic component clearly was the dominating one, the present observations will explain the difficulties in demonstrating an alpha adrenoceptor-mediated component during unopposed beta adrenoceptor stimulation in the inotropic response to norepinephrine in earlier studies.     

Alpha adrenoceptor antagonists selectively reduce thrombin-stimulated contraction in rabbit arteries

Thrombin-induced contractions and the influence on them of alpha adrenoceptor antagonists were studied in the rabbit femoral and central ear artery using in vitro methods. Maximum contraction with thrombin represented between 50 and 66% of the maximum arterial response to norepinephrine (NE). The thrombin dose-response relationship was complex and did not have a classic sigmoidal shape, whether the endothelium was functional or not. In the femoral artery, the nonselective alpha adrenoceptor antagonist phentolamine (10(-6) M) and the selective alpha-1 adrenoceptor antagonist prazosin (10(-8) and 10(-7) M) significantly reduced the contractions induced with concentrations of thrombin greater than 6 U X ml-1, and increased the concentration of thrombin required to produce maximal contraction. The selective alpha-2 adrenoceptor antagonist rauwolscine (10(-7) M) did not alter the contraction initiated by thrombin or by NE. In the rabbit ear artery, contraction to thrombin and NE could also be reduced with prazosin but not rauwolscine. Reserpine pretreatment did not alter the magnitude of the thrombin-induced contraction in the femoral and central ear artery, indicating that the response and its sensitivity to alpha adrenoceptor blockade was not related to the release of NE from nerve stores. Neither thrombin (0.1-16 U X ml-1) nor NE (10(-9)-10(-5) M) produced any significant relaxation in partially contracted rabbit femoral arteries, whether or not the endothelium was functional, i.e., exhibiting a 50 to 100% maximum relaxation with methacholine or when either alpha-1, alpha-2 or beta adrenoceptor antagonists were present.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition by 5-hydroxytryptamine of the beta adrenoceptor-mediated positive inotropic responses to catecholamines in rabbit papillary muscles: direct interaction with beta adrenoceptors.

The effects of 5-hydroxytryptamine (5-HT) on the positive inotropic responses to catecholamines were investigated in isolated rabbit papillary muscles. 5-HT produced a concentration-dependent positive inotropic effect, an effect which was antagonized by prazosin, but not by propranolol. The positive inotropic effect of 5-HT diminished greatly in muscles from rabbits pretreated with 6-hydroxydopamine. Thus, it is likely that 5-HT causes a release of norepinephrine and increases force of contraction indirectly through alpha-1 adrenoceptors. In the presence of prazosin, 5-HT exerted a concentration-dependent inhibition of the positive inotropic response to isoproterenol. The positive inotropic responses to tyramine and a beta-1 adrenoceptor agonist T-1583 were also inhibited by the addition of 5-HT. The inhibitory effect of 5-HT on the beta adrenoceptor-mediated responses was unaffected by methysergide, ketanserin, ICS 205-930 or atropine. Pretreatment with pertussis toxin did not block the inhibitory effect of 5-HT on the inotropic response to isoproterenol, while abolishing the cholinergic interaction against the isoproterenol response. In contrast to its antagonizing effect on the inotropic response to isoproterenol, 5-HT produced an additive effect on the positive inotropic response to norepinephrine. However, when neuronal amine uptake was blocked by cocaine, the positive intropic response to norepinephrine was suppressed by the addition of 5-HT. 5-HT inhibited (-)-[125I]iodocyanopindolol binding to the membranes from rabbit ventricles with a monophasic displacement curve.(ABSTRACT TRUNCATED AT 250 WORDS)     

Quantitative assessment of the pre- and postsynaptic alpha adrenoceptor antagonist potency of amitriptyline

The pre- and postsynaptic alpha adrenoceptor blocking affinity of amitriptyline was determined in isolated tissues by Schild regression analysis. In the absence of uptake-1 blockade with cocaine, amitriptyline treatment (3 X 10(-8)-3 X 10(-6) M) affected only marginally norepinephrine concentration-response curves in the rat anococcygeus muscle; a result suggesting opposing pharmacological effects (uptake-1 blockade and alpha blockade). After cocaine (3 X 10(-5) M) treatment, amitriptyline (3 X 10(-8)-3 X 10(-6) M) antagonized competitively concentration-response curves to norepinephrine, yielding a postsynaptic pKb of 7.51. A similar pKb was obtained when methoxamine was the agonist. Presynaptic alpha blocking affinity was determined by using the field-stimulated rat vas deferens. Stimulus conditions were chosen which minimized the inhibition of twitch height by high concentrations of cocaine. Using these conditions (10 Hz, 200 msec duration at 100-sec intervals), amitriptyline antagonized competitively clonidine inhibition of field-stimulated twitch contractions, yielding at pKb of 5.23. The presynaptic pKb was not changed in the presence of theophylline (10(-4) M). Amitriptyline was also observed to increase the release of [3H]norepinephrine from the field-stimulated rat anococcygeus muscle pretreated with cocaine. Although this effect primarily reflects alpha blockade, other biochemical and presynaptic mechanisms may be involved. Comparing the pre- and postsynaptic alpha blocking affinities indicates that amitriptyline has 191 X greater affinity for post- than presynaptic alpha adrenoceptors (i.e., alpha-1 much greater than alpha-2). The relevance of these observations to the mechanism of action and side effects of amitriptyline are discussed.     

Alkylation of alpha-1 receptors with a chemically reactive analog of prazosin reveals low affinity sites for norepinephrine in rabbit aorta

The effect of a newly synthesized irreversible blocker of the alpha-1 receptor [1-(4-amino-6,7-dimethoxy-2-quinazolnyl)-4-(2-bicyclo[2,2,2] octa-2,5-dienylcarbonyl)-piperazine; SZL-49] has been evaluated in contractile studies in rabbit aorta and binding studies in aorta and brain. SZL-49 produced long lasting inhibition of norepinephrine-induced contractions which was apparent 21 hr after drug washout. The inhibition, which was dose and time dependent, was characterized by progressive shift to the right in the norepinephrine dose-response curve. The ED50 for norepinephrine was shifted from 10(-7) M to 8 X 10(-7), 3 X 10(-6), 1 X 10(-5) and 5 X 10(-4) M after incubation (30 min) and washout of increasing concentrations of SZL-49. Surprisingly, SZL-49, irrespective of the dose or incubation time, did not decrease the maximal response of aortic rings to norepinephrine. This resulted in a norepinephrine dose-response curve after SZL-treatment that is parallel to the control. SZL-49 had no effect on the spasmogenic actions of histamine, serotonin, KCl or CaCl2. In contrast to the inhibitory pattern seen with SZL-49, incubation with 10(-7) M phenoxybenzamine shifted the norepinephrine dose-response curve to the right in a nonparallel manner and significantly depressed the maximal response obtainable with norepinephrine. Incubation with 10(-6) M phenoxybenzamine for 30 min virtually abolished the response to norepinephrine. Phenoxybenzamine (10(-7) M) was without effect on aortic rings treated with a maximally effective dose of SZL-49. Prazosin weakly antagonized the contractile actions of norepinephrine observed after SZL-49 treatment, whereas yohimbine was without effect on these norepinephrine-induced contractions. In control binding studies [3H]prazosin bound to two classes of sites in both aorta and brain preparations. Affinities and densities for these sites were K1 = 67.5 pM, K2 = 309 pM; R1 = 38.2 fmol/mg, R2 = 46.47 fmol/mg in aorta and K1 = 29.6 pM, K2 = 182 pM; R1 = 6.6 fmol/mg and R2 = 30.4 fmol/mg in brain. Treatment with increasing amounts of SZL-49 (10(-10) to 10(-8) M) progressively reduced the number of [3H]prazosin sites without altering the affinity of the sites remaining. At 10(-7) M, SZL-49 eliminated completely all specific [3H]prazosin binding. Our results indicate that the site mediating norepinephrine contraction after treatment with SZL-49 does not possess the characteristics of an alpha-1 receptor and supports the hypothesis that a low affinity site for norepinephrine and prazosin exists in vascular smooth muscle.(ABSTRACT TRUNCATED AT 400 WORDS)     

Responses to norepinephrine resistant to inhibition by alpha and beta adrenoceptor antagonists

The mode of action of clonidine and norepinephrine (NE) has been investigated in the isolated transmurally stimulated guinea-pig ileum preparation using rauwolscine, idazoxan and benextramine as antagonists. Both clonidine and NE produced concentration-dependent inhibition of the cholinergically induced twitch response and were antagonized by rauwolscine and idazoxan, but only clonidine was antagonized in a truly competitive fashion. Benextramine, an irreversible alpha adrenoceptor antagonist, in a concentration of 10(-5) M, inhibited the effect of clonidine completely but only partially antagonized the inhibitory action of NE. The antagonism of NE by rauwolscine and benextramine was most pronounced after reserpine pretreatment and blockade of neuronal and extraneuronal uptake. Under these conditions, the concentration-effect curve to NE remaining after treatment with benextramine showed an IC50 of about 3 X 10(-7) M and an intrinsic activity of 0.6. This curve was resistant to further inhibition by clonidine (10(-5) M), phentolamine (3 X 10(-6) M), rauwolscine (3 X 10(-6) M), prazosin (10(-6) M) and propranolol (1.2 X 10(-5) M). Thus, alpha and beta adrenergic receptors do not appear to be involved. It is postulated that NE-induced inhibition of the guinea-pig ileum twitch response is mediated by two distinct sites: one is the classical alpha-2 adrenoceptor (the site of action of clonidine and the alpha adrenoceptor antagonists) whereas the other is a site seemingly unrelated to the alpha and beta subtypes.     

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.     

The beta adrenoceptor blocker tertatolol causes vasodilatation in the isolated perfused vasoconstricted rat kidney

The activity of the beta adrenoceptor antagonist tertatolol on renal vasoconstrictions was investigated. Infusion of increasing concentrations of tertatolol (10(-8) to 10(-5) M) progressively inhibited the constrictor responses to bolus injections of norepinephrine and to electrical stimulation in isolated perfused kidneys of both normotensive and spontaneously hypertensive rats. Also, in kidneys of normotensive rats the vasoconstrictions caused by serotonin and barium chloride were inhibited by tertatolol. During sustained vasoconstrictions induced by infusion of norepinephrine (6 X 10(-7) M) increasing doses of tertatolol (2.5 X 10(-7) g to 2 X 10(-5) g) caused rapid, reversible dilatations in the rat kidneys. The inhibitory responses caused by tertatolol were not antagonized by propranolol, atropine, hexamethonium, SCH23390, metoclopramide, mepyramine, cimetidine, naloxone, cocaine or indomethacin. During constrictions caused by norepinephrine, methylene blue significantly inhibited the renal vasodilatations caused by tertatolol, acetylcholine, papaverine and nitroglycerin but not those caused by atrial natriuretic factor. Unlike the other vasodilators, tertatolol did not inhibit the constrictions induced by prostaglandin F2 alpha (5 X 10(-6) M) in the rat kidneys. In canine renal arteries with endothelium, tertatolol (10(-9) to 10(-5) M) did not cause relaxations during contractions induced by norepinephrine, electrical stimulation or prostaglandin F2 alpha. Our data illustrate that tertatolol has potent vasodilator properties in the isolated perfused vasoconstricted rat kidney. The dilator response to the beta blocker cannot be inhibited by a variety of classical receptor blockers but ultimately seems to depend on the formation of cyclic GMP.     

Demonstration of an alpha adrenoceptor-mediated inotropic effect of norepinephrine in rabbit papillary muscle

Other investigators have claimed that norepinephrine does not evoke a significant alpha adrenergic inotropic effect in rabbit ventricular myocardium in contrast to some other mammalian species, indicating an important functional limitation of the cardiac alpha adrenoceptors. We therefore characterized the inotropic effects of norepinephrine in isometrically contracting rabbit papillary muscles. We studied both contraction and relaxation by measuring developed tension and its first and second derivatives. Both the influence of propranolol and prazosin on concentration-effect curves of norepinephrine and the qualitative characteristics of the responses revealed that norepinephrine evoked both alpha and beta adrenergic inotropic effects. The alpha adrenergic response to norepinephrine was qualitatively markedly different from the beta adrenergic effect and qualitatively similar to the alpha adrenergic effect of phenylephrine which was also characterized for comparison. Although the alpha adrenergic response to norepinephrine was marked, the beta adrenergic effect was the dominating one when norepinephrine was administered alone. Thus, the beta adrenergic effect had to be extensively blocked to reveal the prazosin-sensitive alpha-1 adrenergic response. It is concluded that also in rabbit papillary muscles, norepinephrine evokes inotropic effects through both alpha and beta adrenoceptors.     

Relation between density (maximum binding) of alpha adrenoceptor binding sites and contractile response in four canine vascular tissues

In microsomal fractions from dog aorta, saphenous veins, mesenteric arteries and veins, both [3H]prazosin and [3H]rauwolscine displayed monophasic saturation in binding. The Kd for [3H] rauwolscine binding was similar for all these blood vessels, but the maximum number of [3H]rauwolscine binding sites was 3 to 7 times higher in veins compared to arteries. The Kd for [3H] prazosin was higher in saphenous vein than that in the arteries. The maximum number of binding sites for [3H]prazosin was similar, except for that in aorta, which was 3 times greater. Phenylephrine (alpha-1 adrenoceptor selective agonist) or norepinephrine (nonselective adrenoceptor agonist) produced similar maximal responses in all vessels. The alpha-2 adrenoceptor selective agonist, B-HT 920 (2-amino-6-allyl-3,4,7,8-tetrahydro-6H-thiazolo[5,4-d]-azepine)-induced contraction in veins but not in arteries. Prazosin (10(-6) M) inhibited completely the contractions to norepinephrine (3 x 10(-6) M) in mesenteric arteries and to phenylephrine (3 x 10(-6) M) in arteries and veins. Contractile responses of mesenteric artery were unaffected by rauwolscine. Rauwolscine (10(-7) M) caused a greater parallel rightward shift of the concentration-response curve to norepinephrine than did prazosin (10(-7) M) in saphenous veins, and a further rightward shift of responses to norepinephrine after 10(-7) M prazosin in mesenteric vein and saphenous vein and abolished B-HT 920-induced responses at alpha-2 adrenoceptors. The tissues responding to B-HT-920 correspond to those having the highest alpha-2 receptor density as measured by [3H]rauwolscine binding. The density of such sites required for contraction to be initiated in veins was much higher than with alpha-1 adrenoceptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

Alpha adrenergic and histaminergic effects of tolazoline-like imidazolines.

For eliciting contraction of rabbit aorta, the relative potency of agonists in terms of negative log molar ED50 were: oxymetazoline (8.4) greater than naphazoline (7.95) greater than phenylephrine (7.31) greater than tetrahydrozoline (6.5) greater than tolazoline (5.80). None of the imidazolines, however, produced maximal effects equal to that of phenylephrine. All agonists were directly acting agents. The interactions between oxymetazoline and phentolamine or tolazoline and phentolamine were competitive with pA2 values of 8.1 and 8.0, respectively. Phentolamine with tetrahydrozoline, naphazoline or phenylephrine produced nearly equal KB values. Thus, all the agonists and alpha adrenoceptor blockers must act at a common site in rabbit aorta. As expected, the contraction of rabbit aorta produced by an imidazole histamine was competitively antagonized by the histamine 1 antagonist, chlorpheniramine (K B 7.6 X 10(-9) M). The antagonist failed to block the contraction produced by the imidazolines studied. On guinea-pig aorta, the relative potency of the agonists varied greatly. On guinea-pig atria, tetrahydrozoline and tolazoline produced positive chronotropic effects which were not influenced by reserpine and cocaine treatment, or treatment with a beta adrenoceptor blocker, propranolol. The histamine 2 receptor antagonist, metiamide, however, selectively blocked the cardiac effects. It is concluded that oxymetazoline and naphazoline do not activate histamine 1 or histamine 2 receptors or beta adrenoceptors. Thus, the drugs are highly specific alpha adrenoceptor stimulants. On the other hand, tetrahydrozoline and tolazoline interact with histamine 2 receptors and with alpha adrenoceptors, but not with histamine 1 receptors or with beta adrenoceptors.     

Evaluation of alpha-1 and alpha-2 adrenoceptor-mediated vasoconstriction in the in situ, autoperfused, pulmonary circulation of the anesthetized dog

Alpha-1 and alpha-2 adrenoceptor-mediated vasoconstriction was studied in the in situ, autoperfused pulmonary circulation of the open-chest anesthetized dog utilizing selective alpha adrenoceptor agonists and antagonists. Animals were pretreated with propranolol (1 mg/kg i.v.) to eliminate beta adrenoceptor-mediated effects in the pulmonary circulation. Blood was withdrawn from the right femoral artery and transferred, via a peristaltic pump, to the pulmonary arterial branch supplying the left diaphragmatic lobe of the lung. The flow rate of the pump was set so that the perfusion pressure in the lobe was equal to resting diastolic pulmonary artery pressure (10 +/- 1 mm Hg). Under conditions of constant left atrial pressure and pulmonary blood flow, intralobar administration of alpha adrenoceptor agonists elicited increases in perfusion pressure of the lobe, reflecting changes in pulmonary vascular resistance. Intralobar administration of the selective alpha-1 adrenoceptor agonist methoxamine and the selective alpha-2 adrenoceptor agonist B-HT 933 elicited dose-dependent increases in lobar perfusion pressure, as did the nonselective alpha adrenoceptor agonist norepinephrine. Prazosin (100 micrograms/kg i.v.), a selective alpha-1 adrenoceptor antagonist, inhibited pulmonary vasopressor responses to methoxamine and norepinephrine without altering significantly the response to B-HT 933. Rauwolscine (100 micrograms/kg i.v.), a selective alpha-2 adrenoceptor antagonist, inhibited the response to B-HT 933 and norepinephrine with little effect on methoxamine. Intralobar administration of tyramine to evoke the release of endogenous norepinephrine resulted in dose-dependent increases in lobar perfusion pressure. The response to tyramine was inhibited selectively by prazosin with little effect of rauwolscine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased negative inotropic effect of calcium-channel blockers in hypertrophied and failing rabbit heart.

The effects on ventricular function of calcium channel blockers and isoproterenol were studied in isovolumically beating perfused control rabbit hearts and in hearts subjected to a double pressure plus volume overload studied at the early phase of heart failure. In control hearts, isoproterenol produced an increase of systolic ventricular function and relaxation that was maximal at 10(-7) M. In failing hearts, inotropic state increase in response to isoproterenol was significantly smaller (P less than .01) with no observed lusitropic effect. In control hearts, verapamil and diltiazem produced dose-dependent decreases of ventricular function which were larger with verapamil than with diltiazem (median drug concentration50 of developed pressure was, respectively, 1163 +/- 131 nM and 4524 +/- 451 nM, P less than .001). In failing hearts, contractility decrease was larger than in control hearts (median drug concentration50 of developed pressure was 604 +/- 69 nM and 2691 +/- 580 nM with verapamil and diltiazem, respectively). In contrast, Ro 40-5967, a new calcium-channel blocker, did not produce reductions of inotropic state with concentrations up to 10(-5) M. All three calcium-channel blockers produced a 2-fold increase of coronary flow at 10(-6) M. We conclude that the deleterious effect of verapamil and diltiazem in heart failure is due, at least in part, to a direct depressant effect of these drugs on contractility, which is larger than in control hearts. Additionally, the in vivo sympathetic compensation is probably reduced, as indicated by the decreased ventricular responsiveness to isoproterenol.     

Pathophysiological and pharmacological mechanisms of acute cocaine toxicity in conscious rats.

In conscious rats, continuous i.v. infusion of cocaine (2 mg/kg/min) produced a marked increase in blood pressure, an initial moderate increase followed by a decrease in heart rate, tonic-clonic convulsions and, finally, a lethal episode of status epilepticus. No change in rectal temperature was observed. Infusion of cocaine methiodide (2 mg/kg/min), a quaternary derivative of cocaine, also produced a lethal episode of status epilepticus, but it was 6 times less potent than cocaine on a molar basis. In pentobarbital-anesthetized, spontaneously breathing rats, cocaine produced death by respiratory failure. Artificial ventilation of pentobarbital-anesthetized rats elevated the lethal dose of cocaine by 15-fold and these animals died of marked hypotension. In conscious rats, pretreatment with dl-, d- or l-propranolol or the alpha 2-selective adrenoceptor antagonist yohimbine enhanced the convulsive and lethal effects of cocaine. In contrast, the alpha 2-selective adrenoceptor agonist clonidine or the alpha 1-selective adrenoceptor antagonist prazosin attenuated these effects. Yohimbine antagonized the protective effect of clonidine. The nonselective alpha adrenoceptor antagonist phentolamine, the autonomic ganglionic blocker chlorisondamine and various calcium channel blockers had no effect on the convulsive or lethal doses of cocaine. The pressor response to cocaine was attenuated by calcium channel blockers, clonidine, phentolamine and dl- or l-propranolol, but not by d-propranolol. The pressor response to cocaine was abolished by chlorisondamine, reversed to a depressor response by prazosin and enhanced by yohimbine. The initial tachycardiac response to cocaine was reversed to bradycardia by dl- and l-propranolol, prazosin, yohimbine or high doses of the calcium channel blockers, but was unaffected by phentolamine, d-propranolol, clonidine or chlorisondamine. These results indicate that in spontaneously breathing animals, acute i.v. infusions of lethal doses of cocaine produce death primarily by central effects, namely by status epilepticus in conscious rats and by respiratory arrest in pentobarbital-anesthetized rats. In artificially ventilated, pentobarbital-anesthetized rats, however, cocaine produces death by effects on the cardiovascular system. In conscious rats, endogenous alpha 1 adrenoceptors exert a deleterious influence on cocaine-induced convulsive and lethal effects, whereas alpha 2 adrenoceptors provide protective influence. Propranolol appears to enhance cocaine-induced acute lethality through a mechanism independent of beta adrenoceptors. Calcium channel blockers appear ineffective in antagonizing cocaine's lethality.     

The cardiac pharmacology of tolbutamide.

Recent clinical studies have suggested an association of tolbutamide therapy with an increased incidence of cardiovascular deaths. Due to the paucity of information concerning the acute cardiac actions of tolbutamide, the effects of this agent upon cardiac contractility and automaticity were examined under in vivo and in vitro conditions in rabbit, cat and dog heart muscle preparations. Tolbutamide (10(-6) to 3 x 10(-3) M) produced a biphasic inotropic response with a peak positive inotropic response at 3 X 10(-3) M which was 13.7 +/- 5.1% of the maximal obtainable increase in tension. Similar studies in cat papillary muscle resulted in a response that averaged 19% of the maximal increase in contractile force. In contrast, canine papillary muscles as well as the intact canine heart failed to develop a positive inotropic response to tolbutamide. Responses of rabbit atrial strips to isoproterenol were not potentiated by previous exposure to tolbutamide. Exposure of rabbit atria to theophylline, 2.5 X 10(-4) M, did not potentiate the inotropic effects of tolbutamide. Stidies in spontaneously beating rabbit right atria and cat papillary muscle-Purkinje fiber preparations demonstrated that tolbutamide does not have the potential to augment automaticity in these tissues. In intact dog heart, the intracoronary administration of tolbutamide did not lead to disturbances in cardiac rhythm, providing additional evidence that tolbutamide does not increase ventricular automaticity. It is concluded that tolbutamide possess a species-specific positive inotropic effect in rabbit and cat but not in the dog. The inotropic effect is small when compared to the maximum inotropic response and is observed only in vitro. Tolbutamide lacks the ability to enhance cardiac pacemaker activity. These data do not support the conclusions of previous investigatirs concerning the possible deleterious cardiac effects of tolbutamide.     

Prazosin selectively antagonizes norepinephrine contractions at low-affinity, non-alpha adrenoceptor sites (extraceptors) in arterial muscle

The characteristics of a low-affinity site activated by norepinephrine (NE), which is demonstrable in arterial smooth muscle pretreated with phenoxybenzamine (PBZ) to alkylate most alpha adrenoceptors were studied. Treatment with PBZ resulted in biphasic dose-response curves to NE. The initial phase associated with concentrations of NE up to 10(-5) M was due to its action in tissues where alpha adrenoceptors have been reduced to a number that would not support a maximum contraction. The second phase occurred with higher doses of NE. We postulate that this latter PBZ-resistant phase or contraction, which can represent more than 60% of the control maximal response, occurs at low-affinity sites for NE "extraceptors" distinct from alpha adrenoceptors in arterial smooth muscle. These residual responses to NE after PBZ treatment were inhibited selectively by prazosin and protected by prazosin from further inhibition by additional doses of PBZ. Effects due to prazosin at low-affinity sites were observed only with doses in excess of those required to competitively inhibit responses through alpha adrenoceptors in smooth muscle. The response to NE occurring through PBZ-resistant adrenoceptors was not dependent on the presence of the endothelium. This low-affinity site for NE was also evident after alkylation of alpha adrenoceptors with other irreversibly acting antagonists (e.g., benextramine and dibenamine). These observations are discussed in relation to the suggestion for a new adrenoceptor, the gamma-adrenoceptor. Differences and similarities of the extraceptor with this proposed adrenoceptor are discussed. Both sites have a high threshold to NE, and both may be influenced by prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of cocaine on excitation-contraction coupling of aortic smooth muscle from the ferret.

The mechanism by which cocaine alters vascular tone is not fully understood. We determined the effects of cocaine on excitation-contraction coupling of isolated ferret aorta. Cocaine in concentrations less than or equal to 10(-4) M caused a contractile response in a dose-dependent manner. The response of control muscle was significantly larger than that in muscle from ferrets pretreated with reserpine. Cocaine-induced contraction was not affected by endothelial factors, but was significantly inhibited by prazosin 10(-7) M pretreatment. The intracellular calcium [( Ca++]i), as measured with aequorin, rose in conjunction with cocaine-induced contraction. The degree of contraction generated by 10(-4) M cocaine decreased after higher concentrations of cocaine greater than or equal to 10(-3) M, while aequorin luminescence remained elevated above the levels before 10(-6) M cocaine. The dose-response relationships of norepinephrine and sympathetic nerve stimulation were enhanced by 10(-6) M cocaine in control muscles; this did not occur in muscles from reserpine pretreated ferrets. In conclusion, (a) cocaine in concentrations less than or equal to 10(-4) M caused vascular contraction presumably by its presynaptic action with consequent alpha-1 adrenoceptor activation and consequent [Ca++]i rise; (b) high concentrations of cocaine greater than or equal to 10(-3) M reduced muscle tone by decreasing the Ca++ sensitivity of the contractile proteins; and (c) supersensitivity to norepinephrine was mediated by cocaine's action on adrenergic nerve endings.     

Differential inhibition of alpha-1 vs. alpha-2 adrenoceptor-mediated responses in canine saphenous vein by nitroglycerin

In the present investigation, the subtype of alpha adrenoceptor that is preferentially antagonized by the noncalcium entry blocker, nitroglycerin (GTN), was studied in vitro using rings prepared from isolated canine saphenous vein (CSV). The calcium entry blocker, diltiazem (DZ), was used for purposes of comparison. Contractions were produced by the following alpha adrenergic agonists: norepinephrine (NE), a nonselective alpha adrenoceptor agonist, phenylephrine (PE), a selective alpha-1 adrenoceptor agonist and B-HT 920, a selective alpha-2 adrenoceptor agonist. The inhibitory effects of GTN on contractions produced by submaximal concentrations (EC65 to EC75) of NE, PE and B-HT 920 were determined. GTN was found to inhibit preferentially the postsynaptic alpha-2 adrenoceptor-mediated responses to B-HT 920 while producing minimal effects on those produced by NE or PE. Similar results were found with DZ. However, when a portion of the alpha-1 adrenoceptor pool was inactivated by phenoxybenzamine (5 X 10(-8) to 1 X 10(-7) M), GTN and DZ both produced a significant depression of responses to PE, a full alpha-1 adrenoceptor agonist. In addition, contractions produced by l-dobutamine, a selective partial alpha-1 adrenoceptor agonist (no alpha receptor reserve), were highly sensitive to inhibition by GTN. These results suggest that the selective inhibition of contractions mediated by postjunctional alpha-2 vs. alpha-1 adrenoceptors in CSV by GTN and DZ may be due partially to the presence of a large alpha-1 adrenoceptor reserve that conceals an underlying functional antagonism to alpha-1 adrenoceptor-mediated responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monoamine mediation of cocaine-induced hypothalamo-pituitary-adrenal activation.

The acute administration of cocaine (5-20 mg/kg) to rats produced a dose-dependent elevation in both serum corticosterone and plasma adrenocorticotrophic hormone (ACTH). These elevations were maximal at 30 min and returned to basal values by 60 min. The dopamine (DA) uptake blockers GBR12909 and nomifensine, the norepinephrine uptake blocker desipramine, as well as the serotonin (5-HT) uptake blocker fluoxetine, also stimulated hypothalamo-pituitary-adrenal (HPA) axis activity, whereas the local anesthetic procaine did not. Pretreatment with haloperidol (0.2 mg/kg) significantly attenuated the elevations in corticosterone and ACTH elicited by cocaine, as well as the elevation in ACTH produced by GBR12909. Higher doses of haloperidol (1 or 3 mg/kg) also attenuated the HPA response to cocaine and GBR12909. Pretreatment with the D1 antagonist SCH23390, the D2 antagonist sulpiride, the D1/D2 antagonist fluphenazine, or the 5-HT2 antagonist ketanserin significantly decreased the ACTH elevations after cocaine. In contrast, neither the 5-HT antagonist cyproheptadine, the alpha-1 antagonist prazosin nor the beta adrenergic antagonist propranolol attenuated the ACTH response to cocaine. The present results suggest an important stimulatory role for DA in regulation of HPA activity, and a role for both DA and 5-HT in the adrenocortical stimulation by cocaine. Both D1 and D2 receptors appear to be involved in the dopaminergic component of this response.