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)     

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.     

Role of the alpha agonist activity of dobutamine in mediating cardiac output: effects of prolonged isoproterenol infusion

Hemodynamic activities of dobutamine enantiomers were studied in either control rats or those infused with isoproterenol (400 micrograms/kg/hr) for 4 days. In control animals prazosin attenuated the effects of (+/-)-dobutamine on cardiac output by approximately 50%; remaining activity was blocked by propranolol. After isoproterenol infusion (+/-)-dobutamine was less efficacious and the blocking effects of prazosin were greater than 90%. Isoproterenol infusion had no effect on (-)-dobutamine-mediated (alpha-1 adrenoceptor agonist) increases in cardiac output and these actions were blocked by prazosin. By contrast, compared to (+/-)- and (-)-dobutamine, effects of (+)-dobutamine (beta adrenoceptor agonist) on cardiac output were modest, not altered by prazosin and were blocked by propranolol; (+)-dobutamine was inactive after isoproterenol infusion. (-)-Dobutamine increased systemic vascular resistance in both control and isoproterenol infused rats, whereas (+)-dobutamine was inactive. (+/-)-Dobutamine increased systemic vascular resistance only in isoproterenol-infused rats. All increases in systemic vascular resistance were blocked by prazosin. Neither (+/-)- nor (+)-dobutamine significantly altered stroke volume. By contrast, (-)-dobutamine resulted in prazosin-sensitive increases in stroke volume in both control and isoproterenol-infused rats. In control animals, (+/-)-, (+)- and (-)-dobutamine increased heart rate in a dose-dependent manner; chronotropic effects of (-)-dobutamine were less than those of either (+/-)- or (+)-dobutamine. Chronotropic effects were not demonstrable in isoproterenol-infused animals. These data support the notion that in control rats cardiac output may be increased by either alpha or beta adrenoceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of (±) dobutamine and its (+) and (-) enantiomers in the isolated rat portal vein

Employing the isolated rat portal vein for testing (+/-) dobutamine, and its (+) and (-) enantiomers, we have confirmed the results obtained by Ruffolo et al (1981, 1983) using different methods. In the rat portal vein, (-) dobutamine acts as an alpha agonist, less potent than noradrenaline. The racemate acts as an alpha-agonist with about half the potency of (-) dobutamine. The (+) isomer shows no effects. In the presence of the alpha blocker BE 2254, all compounds, (+/-), (+) and (-) dobutamine, showed dose-dependent beta effects. These were surely the result of the action on beta 2 adrenoceptors since they could be antagonized by the potent and specific beta 2 antagonist ICI 118-551. We have found that (+) dobutamine is, as a beta 2 agonist, 355 to 1,480 times less potent than isoprenaline and 12-16 times more potent than (-) dobutamine. The pA2 of ICI 118-551 against (+) dobutamine is 9.36 (+/- 0.04). This high value is further proof that the beta receptor population of the rat portal vein is beta 2.     

Heterogeneity of smooth muscle alpha adrenoceptors in rat tail artery in vitro

Vascular smooth muscle alpha adrenoceptors in the proximal end of the rat isolated tail artery have been classified by determining pA2 values and -log KB values for the antagonists prazosin, corynanthine and idazoxan (RX 781094, a new synthetic alpha-2 adrenoceptor antagonist) against norepinephrine. The effects of the antagonists on responses to intramural sympathetic nerve stimulation were also assessed. Artery segments were perfused and superfused with Krebs' solution containing cocaine (4 microM) and propranolol (1 microM). Maximum responses to KCl, norepinephrine and nerve stimulation were not significantly different from one another. Corynanthine (0.1-100 microM), prazosin (10 nM-1 microM) and idazoxan (1-100 microM) caused competitive antagonism of norepinephrine responses with pA2 values consistent with the presence of alpha-1 adrenoceptors. However, idazoxan (10-100 nM) also caused parallel shifts in the concentration-response curves to norepinephrine with -log KB values higher than those consistent with the presence of alpha-1 adrenoceptors. The results have been interpreted to suggest a predominance of smooth muscle alpha-1 adrenoceptors in addition to a subpopulation of smooth muscle alpha-2 adrenoceptors which also contribute to vasoconstrictor responses to norepinephrine. In contrast to the results obtained with exogenous norepinephrine, responses to electrical stimulation were exquisitely sensitive to blockade by prazosin but resistant to idazoxan , suggesting an involvement of an alpha-1 adrenoceptor in these responses. It is concluded that idazoxan may be used to distinguish alpha-1 and alpha-2 adrenoceptors on vascular smooth muscle in vitro and that the results favor the existence of alpha-1 and alpha-2 adrenoceptors in terms of the existing subclassification scheme for alpha adrenoceptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adrenoceptor blocking properties of the stereoisomers of amosulalol (YM-09538) and the corresponding desoxy derivative (YM-11133)

The antagonist potencies of amosulalol (YM-09538), its stereoisomers and the corresponding desoxy derivative (YM-11133) have been compared at alpha-1, alpha-2, beta-1 and beta-2 adrenoceptors in isolated organs in vitro and in radioligand binding experiments. In isolated peripheral tissues, (+/-)-, (-)- and (+)-amosulalol and YM-11133 are selective alpha-1 adrenoceptor antagonists over alpha-2 adrenoceptors by two orders of magnitude and are nonselective beta adrenoceptor antagonists. (+)-Amosulalol and YM-11133 were 14 and 9.3 times more potent than (-)-amosulalol as alpha-1 adrenoceptor antagonists but approximately 50 and 40 times less potent antagonists at beta adrenoceptors than (-)-amosulalol, respectively. The adrenoceptor blocking profile of the racemate is approximately 2-fold less potent than that of the (+)-isomer at alpha and that of the (-)-isomer at beta adrenoceptors. The affinities of (+/)-, (-)- and (+)-amosulalol and YM-11133 obtained from radioligand binding experiments (pKi) using rat brain membrane correlated well with those obtained from in vitro experiments (pA2) at alpha-1 (r = 0.884), alpha-2 (r = 0.977), beta-1 (r = 0.993) and beta-2 (r = 0.971) adrenoceptors. These results indicate that the stereochemical requirements of alpha and beta adrenoceptors differ for the stereoisomers of amosulalol with the alpha adrenoceptor subtypes favoring the (+)-isomer and the desoxy form and the beta subtypes favoring the (-)-isomer.     

CH-38083, a selective, potent antagonist of alpha-2 adrenoceptors

The selectivity and specificity of CH-38083 [7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl], a berbane derivative for alpha adrenoceptors has been studied and compared with yohimbine and idazoxan in peripheral tissues and in the central nervous system. In isolated tissue experiments CH-38083 was a competitive antagonist at presynaptic alpha-2 adrenoceptors on the axon terminals of the rat vas deferens (pA2 against xylazine = 8.17 +/- 0.06) and of the longitudinal muscle strip of guinea pig ileum (pA2 against xylazine = 8.07 +/- 0.20). As far as its postsynaptic alpha-2 adrenoceptor antagonistic activity is concerned its affinity in rat vas deferens (pA2 = 4.95 +/- 0.11) against l-phenylephrine and in rabbit pulmonary artery (pA2 = 5.38 +/- 0.33 against l-norepinephrine) was markedly less than that displayed for presynaptic sites. From pA2 values obtained in rat vas deferens the calculated alpha-1/alpha-2 adrenoceptor selectivity ratios for yohimbine, idazoxan and CH-38083 were 4.7, 117.5 and 1659, respectively. CH-38083 failed to show any affinity for histamine and muscarinic receptors and it even potentiated the effect of serotonin on atropinized longitudinal muscle strip of guinea pig ileum. It enhanced the release of [3H]norepinephrine from electrically stimulated mouse vas deferens loaded previously with labeled [3H]norepinephrine. In binding studies carried out in rat brain membrane preparations using [3H]prazosin and [3H]idazoxan, the selectivity ratios (Ki alpha-1/Ki alpha-2) proved to be 32.5, 289.5 and 1368 for yohimbine, idazoxan and CH-38083, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of the beta adrenoceptor subtype(s) mediating the positive inotropic effects of epinine, dopamine, dobutamine, denopamine and xamoterol in isolated human right atrium.

In patients with chronic heart failure cardiac beta-1 adrenoceptors are reduced, whereas beta-2 adrenoceptor changes vary depending on the etiology of the disease. Beta Adrenoceptor agonists can be used for short-term inotropic support in chronic heart failure; their clinical efficacy might depend on which beta adrenoceptor subtype(s) mediates their positive inotropic effect. Thus, the beta adrenoceptor subtype(s) involved in the positive inotropic effects of clinically used beta adrenoceptor agonists was characterized on isolated electrically driven human right atria by the use of the selective beta-1 adrenoceptor antagonist CGP 20712 A (300 nmol/l) and/or the selective beta-2 adrenoceptor antagonist ICI 118,551 (30 nmol/l). Epinine evoked positive inotropic effects through stimulation of beta-1 and beta-2 adrenoceptors to about the same degree, whereas dobutamine acted mainly at beta-1 adrenoceptors but had a significant beta-2 adrenoceptor component. Both agonists were full agonists causing the same maximum increase in contractile force (Emax) as did isoprenaline or Ca++ (Emax = 1.0). In contrast, denopamine was a partial selective beta-1 adrenoceptor agonist (Emax = 0.75-0.85). Dopamine was in the presence of uptake1-blockade (by 5 mumol/l phenoxybenzamine) a partial agonist (Emax = 0.60-0.70) acting selectively at beta-1 adrenoceptors; in the absence of uptake1-blockade, however, dopamine was a full agonist, indicating that part of its positive inotropic effect is indirect via the release of endogenous noradrenaline. Xamoterol did not exert positive inotropic effects, but concentration-dependently slightly decreased basal force of contraction.     

Antagonist characterization of atypical beta adrenoceptors in guinea pig ileum: blockade by alprenolol and dihydroalprenolol

The present study was undertaken to further characterize the atypical beta adrenoceptor in guinea pig ileum. Tension was developed in isolated segments of ileum using transmural electrical stimulation of enteric cholingeric nerves. The ability of isoproterenol to relax the ileum, via beta-1 adrenoceptor and atypical beta adrenoceptor agonism, was measured. Propranolol (5 x 10(-6) M) and bromoacetylaprenololmetane blocked beta-1 adrenoceptors but, at the concentrations tested, were without affinity at atypical beta adrenoceptors. (-)-Alprenolol and (-)-dihydroalprenolol, however, acted as competitive antagonists at both sites (pA2 values of 8.2 and 8.81 at beta-1 adrenoceptors and 6.47 and 6.43 at atypical beta adrenoceptors, respectively). (-)-Alprenolol also exerted agonistic activity at the atypical beta adrenoceptor. [3H](-)-Dihydroproalprenolol failed to identify beta-1 adrenoceptors or atypical beta adrenoceptors but, instead, bound to a putative lipophilic site unrelated to ileal adrenoceptors. Before this study, nadolol (pA2 = 4.7) was the only documented antagonist at the atypical beta adrenoceptor in guinea pig ileum. Thus, the present results detail two additional pharmacological probes which exhibit about a 100-fold greater affinity than nadolol for the atypical site.     

An in vitro quantitative analysis of the alpha adrenoceptor partial agonist activity of dobutamine and its relevance to inotropic selectivity

In rat anococcygeus muscle, dobutamine produced concentration-related submaximal contractions which were antagonized competitively by phentolamine (pKB = 8.3) and dobutamine antagonized norepinephrine-induced contractions in a competitive manner with an equilibrium dissociation constant for the alpha adrenoceptor of 20 nM (pKB = 7.7). Therefore, dobutamine satisfied criteria for a partial agonist of alpha adrenoceptors having an affinity for alpha adrenoceptors 25 times that of norepinephrine (pKA = 6.3) in this tissue. An estimate of the relative efficacy of dobutamine showed one-fortieth the the efficacy of norepinephrine at the alpha adrenoceptors. Dobutamine contracted rabbit aorta and produced concentration-related relaxations at 1000 times greater concentrations after alkylation of alpha adrenoceptors by phenoxybenzamine. In noncontracted canine saphenous vein, dobutamine had no visible agonist activity but did produce contractions after propranolol. In partially contracted saphenous vein, dobutamine produced a small contraction which was converted to a propranolol-sensitive relaxation of tone after phentolamine. Dobutamine was a full beta adrenoceptor agonist in guinea-pig trachea under spontaneous tone but a partial agonist after strong contraction by bethanechol. This allowed measurement of the pKB of dobutamine at beta adrenoceptors (pKB = 5.35) and estimation of efficacy at beta adrenoceptors relative to isoproterenol (eDob/eIso = 1/20). No evidence for beta adrenoceptor selectivity was found in studies of potency ratios and relative efficacy using isoproterenol for comparison. Dobutamine showed a slight (2-fold) selectivity for inotropy in vitro when compared to isoproterenol in guinea-pig right and left atria. This selectivity was removed by phentolamine suggesting a cardiac alpha-like adrenoceptor effect; this finding was confirmed in propranolol-treated guinea-pig left atria. These results are discussed in terms of the in vivo effects of dobutamine and its use as a tool for classification of beta adrenoceptors, particularly the putative presynaptic beta adrenoceptor.     

Prazosin-sensitive component of the inotropic response to norepinephrine in rabbit heart

Earlier experiments have usually failed to demonstrate a competitively displaceable alpha adrenoceptor blocker-sensitive component in the dose-dependent inotropic response to norepinephrine in mammalian hearts. We reinvestigated if it was possible to reveal this phenomenon by carefully choosing a concentration of the alpha adrenoceptor blocker prazosin that would give a significant displacement while it still was possible to completely surmount the blockade by reasonable concentrations of norepinephrine. Both inotropic and lusitropic dose-dependent responses to norepinephrine in rabbit heart papillary muscles were recorded. In the presence of 3 X 10(-9) M prazosin there was a significant rightward shift of a component corresponding to about 20% of the total inotropic response to norepinephrine. The prazosin-sensitive component was shifted significantly more to the left by 3 X 10(-5) M cocaine than the nonsensitive component. The maximal inotropic response to norepinephrine was increased at lower prazosin-concentrations (3 X 10(-9) M), whereas at 10(-7) M prazosin the maximal response was unchanged compared to the absence of prazosin. The maximal lusitropic response to norepinephrine was increased monophasically and dose-dependently by prazosin. Thus, by carefully considering the relative potencies of the agonist and the antagonist it was possible to demonstrate an alpha adrenoceptor blocker sensitive component in the inotropic response to norepinephrine in rabbit heart. The effect of cocaine upon the prazosin-sensitive component would indicate that the alpha adrenoceptor population in rabbit myocardium is located more closely to the sympathetic nerve endings than the beta adrenoceptor population.(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)     

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.     

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)     

Postjunctional alpha-1 and alpha-2 adrenoceptors in the coronaries of the perfused guinea-pig heart

Prazosin and yohimbine were used to differentiate postjunctional alpha adrenoceptors in the coronaries of the perfused guinea-pig heart. Two postjunctional alpha adrenoceptor subtypes were distinguished by the affinities of the receptor for yohimbine and prazosin. The pA2 for yohimbine were 8.74 against alpha-methylnorepinephrine and 8.98 against BHT-920, and the pA2 for prazosin was 9.84 against phenylephrine. Yohimbine was not very active against the alpha-1 selective agonist as was prazosin against the alpha-2 selective agonists. Alpha-1 and alpha-2 postjunctional adrenoceptors mediate vasoconstriction in the whole coronary bed of the perfused guinea-pig heart.     

Pharmacological differentiation of postsynaptic alpha adrenoceptors in the dog saphenous vein

The dog saphenous vein has postsynaptic subpopulations of both alpha-1 and alpha-2 adrenoceptors which are easily demonstrable using selective agonists and antagonists. Specific alpha-1 (methoxamine and SK&F 89748) or mixed alpha-1, alpha-2 (l-norepinephrine and M7) agonists as well as the specific alpha-2 agonist, BHT 920, cause concentration-related contraction of this tissue. However, maximum contractions produced by alpha-2 activation are significantly less than maximum contractions produced by alpha-1 or combined alpha-1, alpha-2 adrenoceptor activation. SK&F 89748-induced contractions are competitively inhibited by prazosin (pA2 = 7.74) and rauwolscine (pA2 = 6.63); BHT 920-induced contractions are unaffected by prazosin but inhibited by rauwolscine (pA2 = 8.93). Contractile responses to l-norepinephrine are inhibited by prazosin, rauwolscine or phenoxybenzamine in a manner that suggests that norepinephrine interacts with two subtypes of alpha adrenoceptors in this tissue. These data indicate that the dog saphenous vein strip is a suitable in vitro preparation for study of drug action at both postsynaptic adrenoceptors inasmuch as either subpopulation of alpha adrenoceptor can be studied independently using specific agonists or antagonists.     

Cardiac inotropic beta adrenoceptors are fully active at low temperature.

The purpose of this study was to test the adrenoceptor interconversion hypothesis of Kunos and Nickerson (Brit. J. Pharmacol. 59: 603--614, 1977) which claims that lowering temperature from 31 to 17 degrees C converts inotropic beta adrenoceptors in rat atria to alpha adrenoceptors. If lowering temperature transforms the adrenoceptor from a beta type to an alpha type, one should expect that the sympathomimetic drug phenylephrine would be more potent at the low than at the high temperature, that the reverse would be true for the sympathomimetic drug isoproterenol, and that the blocking ability of the beta adrenoceptor blocking drug propranolol might be reduced and that of the alpha adrenoceptor blocking drug phentolamine increased by lowering temperature. This was not observed. It was impossible to obtain satisfactory inotropic effects at 17 degrees C and the calcium concentration in the incubation medium had to be reduced, which lowered contractility and permitted strong inotropic effects. At 17 degrees C the sympathomimetic drug effects were inhibited only by propranolol and not by phentolamine, and there was no evidence of "adrenoceptor interconversion."     

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.     

Evaluation of the alpha and beta adrenoceptor-mediated activities of the novel, orally active inotropic agent, ibopamine, in the cardiovascular system of the pithed rat: comparison with epinine and dopamine

The alpha and beta adrenoceptor-mediated effects of the novel, orally active inotropic prodrug, ibopamine, have been studied in the pithed rat and compared with those effects mediated by dopamine and the active form of ibopamine, epinine. All three agents produced alpha adrenoceptor-mediated pressor responses in pithed rats, and the vasopressor effects of ibopamine and epinine, but not dopamine, were potentiated by beta adrenoceptor blockade with propranolol (3 mg/kg i.v.). Catecholamine depletion with reserpine (5 mg/kg i.p.) did not affect the vasoconstrictor response elicited by any of these agents, indicating a direct effect in the vasculature. Epinine was 10 times more potent than ibopamine or dopamine. The pressor response to all three agents was antagonized by the alpha-1 adrenoceptor antagonist, prazosin (0.1 mg/kg i.v.) and the alpha-2 adrenoceptor antagonist, rauwolscine (0.5 mg/kg i.v.), suggesting the involvement of both alpha adrenoceptor subtypes in the vasopressor responses elicited by these compounds. After complete blockade of alpha adrenoceptors using a combination of phenoxybenzamine (3 mg/kg i.v.), prazosin (0.1 mg/kg i.v.) and rauwolscine (1 mg/kg i.v.), higher doses of ibopamine, epinine and dopamine produced a propranolol-sensitive, beta-1 adrenoceptor-mediated positive chronotropic response that was significantly reduced in reserpine-pretreated rats, indicating a significant indirect component in the activity of these compounds at the level of the myocardium. Epinine and dopamine were equipotent and were 10 times more potent than ibopamine as directly acting beta-1 adrenoceptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for heterogeneity between pre- and postjunctional alpha-2 adrenoceptors using 9-substituted 3-benzazepines

A series of alpha adrenoceptor antagonists, including both reference compounds and the novel benzazepine antagonists, SK&F 86466 (6-chloro-2,3,4,5-tetrahydro-3-methyl-1H-3-benzazepine) and two of its 9-substituted derivatives, SK&F 101253 and SK&F 104078, were tested in vitro for affinity at central and peripheral alpha adrenoceptor subtypes. Peripheral alpha-1 adrenoceptor antagonist potency of these agents, as assessed by the receptor dissociation constant (KB) against norepinephrine-induced contraction in the rabbit aorta, correlated with the Ki value for inhibition of [3H]prazosin binding to central alpha-1 adrenoceptors in rat brain homogenates. Central alpha-2 adrenoceptor affinity, measured as the Ki for inhibition of [3H]rauwolscine binding to rat brain homogenates, correlated well with antagonist activity at peripheral postjunctional alpha-2 adrenoceptors as reflected by the KB against B-HT 920-induced contraction in canine saphenous vein. The 9-substituted benzazepines, SK&F 101253 and SK&F 104078, produce preferential blockade of postjunctional vs. prejunctional alpha-2 adrenoceptors in peripheral models. The high affinity of SK&F 104078 for postjunctional alpha-2 adrenoceptors in the canine saphenous vein was confirmed by its ability to inhibit [3H]rauwolscine binding to postjunctional alpha-2 adrenoceptors in this tissue. The observation that the Ki values for these antagonists against [3H] rauwolscine binding correlate with their KB values at the postjunctional alpha-2 adrenoceptors, rather than those at the prejunctional neuroinhibitory alpha-2 adrenoceptor, suggests a pharmacologic similarity between the postjunctional vascular alpha-2 adrenoceptors and the central [3H]rauwolscine binding site.(ABSTRACT TRUNCATED AT 250 WORDS)