Demonstration of alpha-adrenoceptors in the rabbit heart by [3H]-dihydroergocryptine binding

Adrenaline reveals the torsadogenic effect of combined blockade of potassium channels in anaesthetized guinea pigs

BACKGROUND AND PURPOSE

Torsade de pointes (TdP) can be induced in several species by a reduction in cardiac repolarizing capacity. The aim of this study was to assess whether combined I(Kr) and I(Ks) blockade could induce TdP in anaesthetized guinea pigs and whether short-term variability (STV) or triangulation of action potentials could predict TdP.

EXPERIMENTAL APPROACH

Experiments were performed in open-chest, pentobarbital-anaesthetized, adrenaline-stimulated male Dunkin Hartley guinea pigs, which received three consecutive i.v. infusions of either vehicle, the I(Kr) blocker E-4031 (3, 10 and 30 nmol kg(-1) min(-1)), the I(Ks) blocker HMR1556 (75, 250, 750 nmol kg(-1) min(-1)) or E-4031 and HMR1556 combined. Phenylephrine-stimulated guinea pigs were also treated with the K(+) channel blockers in combination. Arterial blood pressure, ECGs and epicardial monophasic action potential (MAP) were recorded.

KEY RESULTS

TdP was observed in 75% of adrenaline-stimulated guinea pigs given the K(+) channel blockers in combination, but was not observed in guinea pigs treated with either I(K) blocker alone, or in phenylephrine-stimulated guinea pigs. Salvos and ventricular tachycardia occurred with adrenaline but not with phenylephrine. No changes in STV or triangulation of the MAP signals were observed before TdP.

CONCLUSIONS AND IMPLICATIONS

Combined blockade of both I(Kr) and I(Ks) plus the addition of adrenaline were required to induce TdP in anaesthetized guinea pigs. This suggests that there must be sufficient depletion of repolarization reserve and an appropriate trigger for TdP to occur.

alpha 2-Adrenergic receptor binding in canine Purkinje fibers

Autoradiograms were developed from free running strands of Purkinje fibers and from left ventricular myocardium of dog hearts exposed to [7-methoxy-3H]prazosin or [O-methyl-3H]yohimbine in the presence or absence of excessive non-radioactive ligands. Both Purkinje fiber and ventricular myocardium show high density, tissue-specific binding to [3H]prazosin. In contrast, high density, tissue-specific binding to [3H]yohimbine was present in Purkinje fibers but not in ventricular myocardium. Membrane fractions showed high affinity, saturable, and displaceable binding with [3H]yohimbine in preparations from canine cardiac Purkinje fibers but not those from canine cardiac ventricular myocardium. Scatchard analysis of the canine Purkinje membrane alpha 2-adrenergic receptor binding showed a Bmax of 54.9 fmol/mg protein with a Kd of 6.25 nM. These results confirm the electrophysiological findings that post-junctional alpha 2-adrenergic receptors are present in Purkinje fibers.

Inotropic effect of phenylephrine and myocardial alpha-adrenergic receptor in newborn and adult animals

Developmental changes in the myocardial alpha-receptor density were studied using rabbit, rat and dog hearts. In all species studied, alpha-receptor density in the newborn was greater than in the adult. The inotropic effect of phenylephrine was measured using the isolated arterially perfused heart preparation of rabbit and rat. The heart was stimulated electrically at 40/min. In the presence of propranolol, phenylephrine caused a significant positive inotropic effect which was significantly less in the newborn than in the adult. Since alpha-adrenergic stimulation activates protein kinase C, the inotropic effect of protein kinase C activation was studied in the rabbit and rat using phorbol myristate acetate (PMA). PMA caused a negative inotropic effect and the decrease in contractile function in the newborn was greater than in the adult. These data suggest that myocardial alpha-receptor density decreases and the positive inotropic effect of alpha-agonist increases with development. The reasons for this discrepancy remain unclear but there may be developmental differences in the signal transduction processes of alpha-stimulation. The greater negative inotropy of protein kinase C activation in the premature heart may be one of the mechanisms of the reduced inotropy of alpha-agonist in this age group.

Do both adrenaline and noradrenaline stimulate cardiac alpha-adrenoceptors to induce positive inotropy of rat atria?

1. The positive inotropic responses of rat paced left atria to adrenaline and noradrenaline were recorded. Desmethylimipramine (DMI, 1 microM) and metanephrine (10 microM) were initially present throughout. 2. The positive chronotropic responses of spontaneously beating right atria to adrenaline were used as a reference. In these, pindolol, in increasing concentrations, caused progressive shift of the concentration-response curves to the right, which yielded a pA2 value (8.15) compatible with antagonism of beta-adrenoceptors. 3. The left atrial tension responses to adrenaline showed an initial progressive displacement by pindolol (up to 3 microM) which gave an unexpectedly low pA2 value (6.48). However, with further increases in pindolol concentration there was no additional shift of the curve. In the presence of pindolol (3 microM), prazosin (0.1 microM) displaced the curve to the right but the pA2 value derived from this shift (7.75) was less than expected for alpha 1-adrenoceptor antagonism. 4. When the experiments in the presence of pindolol (3 microM) were repeated in the absence of DMI, prazosin displaced the concentration-response curves for adrenaline-induced left atrial tension to a greater extent and the pA2 value (8.76) was now compatible with adrenaline stimulating typical alpha 1-adrenoceptors. 5. The concentration-response curves for noradrenaline-induced left atrial tension were also progressively displaced to the right by pindolol (0.1, 0.3 and 1.0 microM). These concentrations yielded a Schild plot of unity slope and a pA2 value of 7.94 +/- 0.04. This was not significantly different from the pA2 value of 8.02 +/- 0.07 determined for pindolol against isoprenaline in the left atria, which indicates a normal interaction of noradrenaline with beta-adrenoceptors in the absence and presence of low concentrations of pindolol. 6. A further increase in the concentration of pindolol to 3 microM failed to induce an additional shift of the noradrenaline curves, whether a 'before and after' antagonist or a 'naïve tissue' design was adopted. Similarly, the rightwards shift of the concentration-response curves by timolol reached a limit as the concentration was increased. In all cases the limit of shift occurred at a noradrenaline EC50 value of 5-10 microM. 7. At the limit of beta-adrenoceptor antagonism, prazosin and dibenamine did not displace the noradrenaline curves further. The residual inotropic response to noradrenaline therefore appeared to be mediated via neither alpha- nor beta-adrenoceptors. 8. DMI, in the absence of beta-blockade, produced the potentiation of adrenaline and noradrenaline expected of a neuronal uptake inhibitor. However, in the presence of pindolol, there was no potentiation of the right atrial rate response to adrenaline while its left atrial tension responses were antagonized. This suggested that DMI was acting as an alpha-adrenoceptor antagonist. It also explained the less-than-expected shift by prazosin of the adrenaline responses in the presence of both pindolol and DMI, the latter drug already exerting some alpha-blocking activity. In contrast, the left atrial tension responses to noradrenaline in the presence of pindolol (1 microM) were neither potentiated nor antagonized by DMI. 9. When the effects of prazosin upon left atrial tension responses to noradrenaline in the presence of pindolol (10 microM) were examined in the presence of a lower concentration of DMI (O.1 microM) or cocaine (1O microM), again there was no further shift of the curve. However, when the effect of prazosin) The Macmillan Press Ltd 1989 598 K.L. WILLIAMSON & K.J. BROADLEY was examined in the absence of DMI, but in the presence of pindolol (1 and 1O microM) or timolol (3 microM), there was a small shift of the curves by prazosin (0.1 microM). This yielded pA2 values of 7.19, 7.34 +/- 0.1 and 7.66 +/- 0.09, which were at least one order of magnitude less than literature values and that obtained with adrenaline (8.76 +/- 0.18), and are not consistent with noradrenaline stimulating an alpha 1-adrenoreceptor in the presence of beta-adrenoceptor blockade, the increase in left atrial tension by noradrenaline does not appear to be mediated by beta l- or typical alpha-adrenoceptors. This is in contrast to adrenaline which in these conditions stimulates typical alpha 1-adrenoceptors.

Preponderance of beta- over alpha-adrenoceptors in mediating the positive inotropic effect of phenylephrine in the ferret ventricular myocardium

[3H]prazosin bound to the membrane fraction derived from the ferret ventricular muscle with high affinity in a saturable manner (Kd = 0.25 nmol/l and Bmax = 27 fmol/mg protein in the right ventricle). [3H]CGP-12177, a beta-adrenoceptor ligand, bound to the membrane fraction with a Kd value of 0.29 nmol/l and a Bmax of 42 fmol/mg protein. In the isolated ferret papillary muscle driven at 1 Hz at 37 degrees C, phenylephrine elicited a concentration-dependent positive inotropic effect. The maximal effect of phenylephrine was comparable to that of isoprenaline. Prazosin (0.3 mumol/l) shifted the concentration-response curve for phenylephrine slightly but significantly to the right, the maximal response being unaffected. In contrast, bupranolol (0.3 mumol/l) shifted the curve for phenylephrine markedly downwards: the maximal response was depressed significantly to 40% and the curve became less steep. In the presence of prazosin and bupranolol the curve was shifted to the right, being essentially parallel to the control curve. These results indicate that in the ferret ventricular myocardium both alpha- and beta-adrenoceptors mediate the positive inotropic effect of phenylephrine. The extent of contribution of the two classes of adrenoceptor is quite different from that in other mammalian species. In the ferret heart, beta-adrenoceptors predominate over alpha-adrenoceptors in mediating the positive inotropic effect of phenylephrine, although the number of beta-adrenoceptors is not especially high when compared with other species.

Further characterization of the myocardial alpha-adrenoceptors mediating positive inotropic effects in the rabbit myocardium

[3H]Prazosin bound with high affinity to the membrane fraction derived from the rabbit ventricular myocardium. Oxymetazoline displaced [3H]prazosin from its binding site, did not elicit a positive inotropic effect but antagonized the positive inotropic effect of phenylephrine mediated by alpha-adrenoceptors in the presence of a beta-antagonist. Naphazoline was more potent in displacing [3H]prazosin and behaved as a weak partial agonist. YM-12617 (5-[2-[[2-(2-ethoxyphenoxy)ethyl]amino]propyl]-2- methoxybenzenesulfonamide HCl), a potent selective alpha 1-antagonist, displaced [3H]prazosin and antagonized the alpha-mediated positive inotropic effect with equal potency. Thus, a good correlation was found between the potency of alpha-antagonists to displace [3H]prazosin and their ability to antagonize the alpha-mediated positive inotropic effect. On the other hand, there was no significant correlation between the Ki and the pD2 value of the alpha-agonists (norepinephrine, epinephrine, phenylephrine and naphazoline), indicating that there is a non-linear relationship between agonist binding to myocardial alpha 1-adrenoceptors and subsequent functional changes. Myocardial alpha 1-adrenoceptors showed some pharmacological characteristics which appear to be different from those in smooth muscle tissues.

An evidence for presynaptic excitatory alpha 2-adrenergic receptors in frog myocardium

This study was undertaken to determine the effects of clonidine on sympathetic neurotransmission in frog myocardium. In the electrically driven ventricular strips of frog heart, clonidine was found to be ineffective on contractility. However, clonidine increased the positive inotropic responses to transient additional stimulations. This effect of clonidine was antagonized by yohimbine, an alpha 2-adrenergic receptor antagonist. Clonidine did not change the positive inotropic effects of exogenously applied noradrenaline. These results suggest that clonidine facilitates sympathetic neurotransmission in frog myocardium via an action on alpha 2-adrenergic receptors located on sympathetic nerve terminals.

The pharmacology of dobutamine

Dobutamine is a sympathomimetic amine that was designed as an inotropic agent for use in congestive heart failure. Clinically, dobutamine increases cardiac output by selectively augmenting stroke volume, and this is associated with a decrease in total peripheral vascular resistance that is mediated, in part, by reflex withdrawal of sympathetic tone to the vasculature. This hemodynamic profile of dobutamine makes the drug of value in the management of low output cardiac failure. The inotropic activity of dobutamine has previously been attributed to selective stimulation of myocardial beta 1-adrenoceptors. However, recent studies from a number of laboratories indicate that the mechanism of action of dobutamine is substantially more complex. Dobutamine has the capacity to stimulate beta 1-, beta 2-, and alpha 1-adrenoceptors in the cardiovascular system at doses that approximate those used clinically. It has recently been suggested that the inotropic activity of dobutamine results from combined beta 1- and alpha 1-adrenoceptor stimulation in the myocardium, and that this activity could explain, at least in part, the inotropic selectivity of the compound. Furthermore, in the vasculature, the beta 2-adrenoceptor-mediated vasodilatory effect of dobutamine is exactly offset by the alpha 1-adrenoceptor-mediated vasoconstrictor activity, such that net changes in blood pressure are minimal following the administration of dobutamine. It is concluded, therefore, that the hemodynamic profile of dobutamine in patients with congestive heart failure is derived from a unique and complex series of interactions with alpha- and beta-adrenoceptors in the cardiovascular system.

Distribution and function of peripheral alpha-adrenoceptors in the cardiovascular system

alpha-Adrenoceptors may be subdivided based on their anatomical distribution within the synapse. Presynaptic alpha-adrenoceptors are generally of the alpha 2-subtype and modulate neurotransmitter liberation via a negative feedback mechanism. Postsynaptic alpha-adrenoceptors are usually of the alpha 1-subtype and mediate the response of the effector organ. Although this "anatomical" subclassification is generally applicable, many exceptions exist. A more useful classification of alpha-adrenoceptor subtypes is based on a pharmacological characterization in which selective agonists and antagonists are used. Peripheral alpha-adrenoceptors are critical in the regulation of the cardiovascular system. Postsynaptic alpha-adrenoceptors in arteries and veins represent a mixed population of alpha 1/alpha 2-adrenoceptors, with both subtypes mediating vasoconstriction. In the peripheral arterial circulation, postsynaptic vascular alpha 1-adrenoceptors are found in the adrenergic neuroeffector junction, whereas postsynaptic vascular alpha 2-adrenoceptors are located extrajunctionally. In the venous circulation, it appears that alpha 2-adrenoceptors may be predominantly junctional, whereas alpha 1-adrenoceptors may be predominantly extrajunctional. It has been proposed that junctional alpha-adrenoceptors will respond predominantly to norepinephrine liberated from sympathetic neurons, whereas extrajunctional alpha-adrenoceptors likely respond to circulating catecholamines. The functional role of extrajunctional alpha-adrenoceptors may be more important in disease states such as hypertension and congestive heart failure where circulating levels of catecholamines may be high and contribute to the maintenance of elevated vascular resistance. alpha 2-Adrenoceptors are also associated with the intima and may play a role in the release of an endogenous relaxing factor from the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemic hemodynamic effects of dopamine, (+/-)-dobutamine and the (+)-and (-)-enantiomers of dobutamine in anesthetized normotensive rats

The hemodynamic effects of dopamine, (+/-)-dobutamine (racemic mixture) and the (+)- and (-)-enantiomers of dobutamine were evaluated in anesthetized normotensive rats. Dopamine and (+/-)-dobutamine produced hemodynamic effects in anesthetized rat that were qualitatively similar to those reported for these compounds in man. The increase in cardiac output produced by dopamine and (+/-)-dobutamine was due mainly to an increase in stroke volume, with heart rate being only minimally affected. Dopamine produced a large increase in mean arterial pressure and slightly increased total peripheral vascular resistance, whereas (+/-)-dobutamine only modestly increased blood pressure and significantly reduced total peripheral resistance. The (-)-enantiomer of dobutamine, which possesses mainly alpha 1-adrenoceptor agonist activity, produced marked increases in cardiac output, stroke volume, total peripheral resistance and mean arterial pressure, but did not significantly increase heart rate. 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 entirely to increased heart rate since stroke volume was not increased. Total peripheral vascular resistance and mean arterial blood pressure were both reduced by (+)-dobutamine, characteristic of a beta-adrenoceptor agonist. The increase in cardiac output and blood pressure produced by (+/-)-dobutamine, but not the positive chronotropic effect, were significantly inhibited by alpha 1-adrenoceptor blockade with prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptors in the guinea-pig heart. A binding study

The binding of dopaminergic agonists and antagonists to guinea-pig myocardial membrane preparations was studied using 3H-dopamine and 3H-spiperone as radioligand. 3H-Dopamine bound specifically to heart membranes while 3H-spiperone did not. A Scatchard analysis of 3H-dopamine binding showed a curvilinear plot indicating the presence of two dopamine receptor populations that we have termed high- (Kd = 1.2 nM, Bmx = 52.9 fmol/mg prot.) and low- (Kd = 11.8 nM, Bmx = 267.3 fmol/mg prot.) affinity binding sites, respectively. The characterisation of the high-affinity component of 3H-dopamine binding indicated that the binding is rapid, saturable, stereospecific, pH- and temperature-dependent, and displaced by dopaminergic agonists and antagonists known to act similarly in vivo. The finding that pretreatment with dibenamine (which has been described as an alpha-adrenoceptor irreversible blocker) did not affect the binding of dopamine to cardiac membrane preparations suggests that alpha-adrenoceptors and dopamine receptors have separate recognition sites in the heart. We conclude that 3H-dopamine binds to specific dopamine receptors in the heart of guinea-pigs.

Interactions of three inotropic agents, ASL-7022, dobutamine and dopamine, with alpha- and beta-adrenoceptors in vitro

Three inotropic agents, ASL-7022, dobutamine and dopamine, were evaluated for their alpha- and beta-adrenoceptor mediated effects in vitro in a variety of isolated organs and in radioligand binding studies. All compounds were alpha 1-adrenoceptor agonists in rat and guinea pig aortae, but the rank orders of potency were exactly opposite in these two tissues. Only the rank potency order of dobutamine greater than ASL-7022 greater than dopamine obtained in rat aorta was consistent with the results obtained in radioligand binding studies to alpha 1-adrenoceptors in rat cerebral cortex and to previous results obtained in vivo in the pithed rat. The results obtained in guinea pig aorta did not parallel the radioligand binding studies in rat brain or our previous results in pithed rat, and suggests that species differences exist between postsynaptic vascular alpha 1-adrenoceptors in rat and guinea pig aorta, consistent with previous conclusions. ASL-7022 was found to be a potent alpha 2-adrenoceptor agonist in field-stimulated guinea pig ileum, and was approximately 10-fold more potent than dobutamine in this respect, which was also confirmed by radioligand binding studies to alpha 2-adrenoceptors in rat cerebral cortex. The beta 1-adrenoceptor mediated effects of these compounds were evaluated in guinea pig atria, where the rank order of potency was dobutamine greater than ASL-7022 greater than dopamine. An identical rank order of affinity was established for these compounds by displacement of 3H-dihydroalprenolol from beta 1-adrenoceptors in rat cerebral cortex. The beta 1-adrenoceptor mediated effects of dobutamine and ASL-7022 in guinea pig atria were completely direct in nature and not secondary to the release of endogenous catecholamines. In contrast, a major component of the beta 1-adrenoceptor mediated tachycardia produced by dopamine in guinea pig atria was indirect in nature as evidenced by the marked attenuation in potency that occurred following catecholamine depletion with reserpine. All three compounds elicited beta 2-adrenoceptor mediated inhibition of tone in rat uterus, with the rank order of potency being ASL-7022 greater than dobutamine greater than dopamine. Again, this rank order of beta 2-adrenoceptor potency was also reflected in beta 2-adrenoceptor affinity as assessed by displacement of 3H-dihydroalprenolol from beta 2-adrenoceptors in rat cerebellum. Based on these results, it may be concluded that for alpha-adrenoceptors, dobutamine is a selective alpha 1-adrenoceptor agonist, ASL-7022 is a selective alpha 2-adrenoceptor agonist, and dopamine is a nonselective alpha-adrenoceptor agonist.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective additive effect of phenylephrine to the inotropic action of isoproterenol on rabbit left atria

The chronotropic and inotropic effects of isoproterenol, adrenaline and phenylephrine were examined in isolated right and left rabbit atria. 1. Spontaneous right atrial rate (RAR) and left atrial contractile force (LACF) (electrically stimulated at 2 Hz) were monitored during separate concentration-response curves to all 3 agonists. The order of potency for both parameters was isoproterenol greater than adrenaline greater than phenylephrine. 2. At maximum agonist effect, adrenaline produced the greatest LACF increase, followed in order by isoproterenol and phenylephrine. 3. When phenylephrine (6.0 x 10(-5) M) was added to the left atria maximally stimulated with isoproterenol, an additional 32% increase in LACF was recorded, but an additive effect was not noted when phenylephrine was added to atria maximally stimulated with adrenaline. The LACF achieved by a combination of isoproterenol and phenylephrine approximated the maximum LACF induced by adrenaline alone. 4. At maximum agonist effect, isoproterenol and adrenaline produced a greater RAR increase than phenylephrine. Further, phenylephrine did not increase the maximum RAR effect of isoproterenol or adrenaline. 5. Adrenergically mediated chronotropic increases appear to be mediated by a beta adrenoceptor and the maximum response to agonists of full intrinsic activity can not be further increased by agonist combinations. However inotropic increases appear to be mediated by 2 adrenoceptor types which may be simultaneously stimulated to produce additive effects. The two adrenoceptors mediating inotropic change are probably alpha and beta.