Interactions between the putative calcium entry promotor Bay k 8644 and pressor responses produced by alpha 1- and alpha 2-adrenoceptor agonists in the pithed normotensive rat

Role of different subtypes of adrenoceptors in pressor responses to catecholamines released from sympathetic nerve endings

The role of alpha 1- and alpha 2-adrenoceptors in the vascular effects of catecholamines, either released locally from sympathetic nerve endings (e.g., in vascular smooth muscle) or derived from the adrenal medulla or administered intravenously, was studied using selective antagonists of these adrenoceptors. The ganglionic stimulant dimethylphenyl-piperazinium-iodide (DMPP) exerted dual actions on blood pressure: a rapid and short-term pressor reaction (phase I) resulting from catecholamine release elicited by ganglion stimulation, followed by a more sustained blood pressure elevation (phase II) resulting from the circulating catecholamines released from the adrenal medulla. The selective alpha 2-adrenoceptor, but a not subtype selective, antagonist 7,8-(methylenedioxi)-14-alpha-alloberbane HCl (CH-38083) (50-100 micrograms/kg, IV) significantly (p < 0.05) inhibited the pressor effects of epinephrine and norepinephrine given intravenously and phase II of the DMPP-induced pressor reaction. Idazoxan exerted similar effects, but at higher doses (400-600 micrograms/kg, IV). WB-4101 (50-100 micrograms/kg, IV) and BRL-44408 (2-3 mg/kg, IV), two selective alpha 2A-adrenoceptor antagonists, had the same activity as CH-38083, except did not inhibit the pressor effect of intravenously administered norepinephrine. The alpha 2B-adrenoceptor selective antagonist, ARC-239 (150 micrograms/kg, IV) did not influence phase II of DMPP-induced pressor reaction. Prazosin (200 micrograms/kg, IV), an antagonist of alpha 1 and alpha 2B-adrenoceptors, reduced blood pressure, the pressor response to intravenously administered epinephrine, and phase I of the DMPP-induced pressor effect. In addition, it completely inhibited the pressor responses to DMPP remaining after administration of CH-38083. These results suggest that the postsynaptically located alpha 1- and alpha 2(A and B)-adrenoceptors are involved in pressor response to norepinephrine and epinephrine, and are sensitive and accessible to catecholamines released locally from the axon terminals, and from the circulation to a different extent. These results may have great therapeutical importance in hypertension, for which the involvement of both a high level of circulating and locally released catecholamines may be indicative of the usefullness of a combination (alpha 1- and alpha 2-adrenoceptors- and Ca-channel-blocking agents) therapy.

MPC-1304, another type of dihydropyridine, possessing highly potent vasodilating action

We investigated the vasodilating action of MPC-1304, one of the most potent dihydropyridines causing hypotension, in anesthetized dogs and compared this with its binding properties. After intraarterial injection, MPC-1304 was 3 times less potent than other dihydropyridines (nitrendipine, nifedipine, nicardipine and nisoldipine) in increasing femoral blood flow. After infusion of these drugs, however, MPC-1304 was the most potent in increasing femoral blood flow. The onset and recovery of the effect of MPC-1304 on femoral blood flow were slower than for nifedipine. Higher doses of Bay K 8644 were needed to antagonize the stimulating activity of MPC-1304 than for nifedipine. In a competition assay of [3H]nitrendipine binding, MPC-1304 and its metabolites bound to the dihydropyridine receptor with lower affinity than the other dihydropyridines. The binding affinity of [3H]MPC-1304 was lower than that of [3H]nitrendipine, consistent with the potency of this drug to increase femoral blood flow by bolus injection. The association and dissociation of [3H]MPC-1304 was slower than those of [3H]nitrendipine, which is consistent with the slow onset and long-lasting vasodilating effects of MPC-1304 on femoral blood flow. Moreover, diltiazem reduced a part of [3H]MPC-1304 binding in a competitive manner. In ex vivo binding assays with serum and aorta obtained after oral administration of the drug in spontaneously hypertensive rats, MPC-1304 inhibited [3H]nitrendipine binding to membrane preparations less potently than nifedipine. From these results, we conclude that MPC-1304 is a different type of dihydropyridine possessing the most potent vasodilating action of the representative dihydropyridines tested. Its activity cannot be explained solely by a slow interaction with voltage-dependent Ca2+ channels.

Alpha 2-adrenoceptor-mediated contractions in the rabbit aorta treated with BAY K 8644

We tested the effect of the selective alpha 2-adrenoceptor agonists B-HT 920 and B-HT 933 on the rabbit aorta. These drugs had weak contractile effects in the tissue, which were inhibited by the selective alpha 1-adrenoceptor antagonist prazosin (10(-5) M). Their contractile effects were potentiated by the Ca2+ channel facilitator BAY K 8644 (10(-6) M). The selective alpha 2-adrenoceptor antagonist yohimbine (10(-5) M) reduced the contractions elicited by B-HT 920 and B-HT 933 in the presence of BAY K 8644 (10(-6) M), but did not alter the control effect of these drugs. In the rabbit aorta, the contractile effect of B-HT 920 and B-HT 933 in the presence of BAY K 8644 (10(-6) M) was partly caused by alpha 1-adrenoceptor stimulation, because prazosin (10(-5) M) relaxed the contractions elicited under these conditions. In the aorta preincubated with BAY K 8644 (10(-6) M) and prazosin (10(-5) M), B-HT 920 (3 x 10(-4) M) elicited non-sustained phasic contractions (1-5 g), which were probably due to alpha 2-adrenoceptor stimulation, as they were inhibited by yohimbine (10(-5) M). In similar experiments B-HT 933 (3 x 10(-4) M) caused inconsistent and slight contractions (< 0.5 g developed tension).

The effects of nifedipine on alpha 2-adrenoceptor-mediated contractions in several isolated blood vessels from the rabbit

1. The effects of the dihydropyridine calcium channel blocker, nifedipine, on noradrenaline-induced contractile responses have been examined in several isolated blood vessels from the rabbit, with particular emphasis on responses mediated via postjunctional alpha 2-adrenoceptors. 2. In the isolated renal vein, ear vein, distal saphenous artery, saphenous vein and plantaris vein, 0.1 microM and 1 microM nifedipine reduced responses elicited by 54 mM KCl by more than 70%. The remaining responses were abolished by alpha-adrenoceptor blockade, suggesting the involvement of noradrenaline released from neurones activating a dihydropyridine-resistant mechanism. 3. In the renal vein (alpha 1-), ear vein (predominantly alpha 2-), distal saphenous artery (alpha 1- greater than alpha 2-), saphenous vein and plantaris vein (alpha 2- greater than alpha 1-), 0.01 microM and 0.1 microM nifedipine produced concentration-related reductions in the maximum response to noradrenaline. However, 1 microM nifedipine was no more effective than 0.1 microM nifedipine and the reduction in the maximum varied from 10-25% of the control response. Thus, a sizeable component of the alpha-adrenoceptor-mediated response in all blood vessels is resistant to dihydropyridine calcium channel blockers and this appears to be unrelated to the alpha-adrenoceptor subtype involved. 4. Following irreversible inactivation of alpha 1-adrenoceptors and isolation of functional alpha 2-adrenoceptors in the saphenous vein, plantaris vein and distal saphenous artery (the latter requiring the presence of angiotensin II), the effect of nifedipine on responses to noradrenaline was increased. However, a component of the alpha 2-adrenoceptor response in each preparation was present even after the concentration of nifedipine was increased to 1 microM. 5. In the saphenous vein, a preparation in which it has been demonstrated previously that alpha 2-adrenoceptor-mediated responses are highly dependent upon the presence of extracellular calcium ions, partial depolarization with 20mM KCl failed to increase the inhibitory effect of 0.1 microM nifedipine. This suggests the involvement of dihydropyridine-resistant Ca2+ channels. The possible relationship between these dihydropyridine-resistant Ca2+ channels, alpha-adrenoceptor subtypes and 'receptor-operated' Ca2 + channels is discussed.

Potentiation by treatment with reserpine of alpha 2-adrenoceptor-mediated contractions of rat tail artery

The effects of a treatment with reserpine on alpha-adrenoceptor-mediated contractile responses of rat tail arteries were investigated in vitro. The potency of norepinephrine was slightly increased in arteries obtained from rats treated with reserpine. There was no significant change in the sensitivity of the arteries to serotonin, KCl and selective alpha 1-adrenoceptor agonists (methoxamine and phenylephrine). However, the potency of clonidine and UK-14,304, both selective alpha 2-adrenoceptor agonists, was greatly increased. UK-14,304-induced contractions of the arteries from rats treated with reserpine were inhibited strongly by rauwolscine, a selective alpha 2-adrenoceptor antagonist, but only slightly by corynanthine, a selective alpha 1-adrenoceptor antagonist. The contractions caused by re-introduction of Ca2+ during exposure to UK-14,304 but not to methoxamine in a Ca2(+)-free medium were potentiated by treatment with reserpine. Bay K 8644, an agonist of Ca2+ channels, produced a concentration-dependent contraction only in the arteries from rats treated with reserpine. These results suggest that treatment with reserpine potentiates alpha 2- but not alpha 1-adrenoceptor-mediated responses in rat tail arteries and that the potentiation could be related to changes in mechanisms linked to Ca2+ influx.

Influence of Bay K 8644 on vascular responses mediated by alpha 1- and alpha 2-adrenoceptors

1. The calcium channel activator Bay K 8644 increased the potency of noradrenaline in cat middle cerebral (alpha 2-adrenoceptors) and mesenteric (atypical or mixed alpha 1- and alpha 2-adrenoceptor population) arteries, but not in rat middle cerebral and mesenteric arteries (alpha 1-adrenoceptors). 2. In cat arteries, exposure to 15 mM K+ solution shifted the noradrenaline concentration-response curve to the left in an almost identical manner as did Bay K 8644. 3. Bay K 8644 completely reversed the relaxation produced by nifedipine in K+-contracted cat middle cerebral arteries, whereas the relaxation induced by verapamil, diltiazem or flunarizine was only partially reversed. This suggests a specific interaction between Bay K 8644 and the dihydropyridine receptors on the calcium channels. 4. It is concluded that the degree to which noradrenaline promotes calcium influx through membrane channels is at least partly related to the alpha-adrenoceptor subtype mediating the response.

Theoretical and functional studies on alpha 1-and alpha 2-adrenoreceptors: an examination using the Schild plot

1 The influence on the shape of the Schild plot by a two-receptor system was studied in both functional and theoretical studies. In the functional studies, the alpha-adrenoreceptors in cat lingual arteries were studied since both alpha 1- and alpha 2-adrenoreceptors have been suggested to contribute to the noradrenaline-induced contractile response in this tissue. 2 The Schild plots constructed using noradrenaline (NA) as agonist and prazosin as antagonist gave a straight line with a slope close to unity. In contrast, the corresponding Schild plot constructed for rauwolscine appeared to be biphasic. 3 The results obtained in the functional study were discussed in view of Schild plots obtained from a theoretical model which was designed to take into consideration the presence of two distinct types of receptor, the percentile proportions of which could be altered. 4 The theoretical model indicates that the shape of the Schild plot can vary considerably depending on the relative contribution of each receptor subtype to the contractile response, the selectivity of the antagonist, and the range and number of antagonist concentrations used. 5 It is suggested that a response is predominantly mediated by alpha 1-adrenoreceptors and less by alpha 2-adrenoreceptors when the Schild plot for prazosin gives a slope close to unity and has a pA2-value representative for alpha 1-adrenoreceptors, and rauwolscine gives a slope less than unity and a significant shift of the NA concentration-response curve in concentrations around 10(-8) M. When the reverse is true (but with a significant shift caused by prazosin already at a concentration around 10(-9) M) the contraction elicited by NA is proposed to be predominantly mediated by alpha 2-adrenoreceptors and less by alpha 1-adrenoreceptors.

Equal potency of nifedipine to inhibit alpha 1-(dobutamine and BDF 6143) and alpha 2-adrenoceptor (B-HT 920) induced pressor responses in pithed rats; lack of effect of phenoxybenzamine

Intravenous (i.v.) dobutamine and BDF 6143 were partial agonists in increasing diastolic pressure in beta-adrenoceptor-blocked pithed rats. The log dose-pressor effect curves were not influenced by yohimbine (1 mg/kg i.v., -15 min) but were markedly shifted to the right by prazosin (0.1 mg/kg i.v., -15 min) indicating the exclusive involvement of alpha 1-adrenoceptors. Nifedipine (0.1-1 mg/kg i.a., -15 min) non-competitively inhibited the pressor effects of dobutamine and BDF 6143 as well as of the alpha 2-adrenoceptor agent B-HT 920 with equal potency. The -log ED50 values calculated for nifedipine amounted to 6.25 +/- 0.12, 6.16 +/- 0.14 and 6.20 +/- 0.10, respectively. Phenoxybenzamine (3 or 10 micrograms/kg i.v., -60 min) did not affect the effectiveness of nifedipine (0.1 mg/kg) to inhibit the pressor effects of dobutamine and BDF 6143. Following treatment with Bay k 8644 (1 mg/kg i.a., -15 min), the log dose-pressor effect curves for dobutamine and BDF 6143 were shifted to the left and the maximum responses were elevated. Our findings suggest that the alpha 1-adrenoceptor-induced pressor effects of dobutamine and BDF 6143 rely heavily on the influx of Ca2+, and are indistinguishable in this respect from the effects initiated by alpha 2-adrenoceptor stimulation. The data further support the view that the sensitivity of alpha-adrenoceptor-mediated pressor effects to inhibition by Ca2+ entry blockers depends on the extent to which Ca2+ influx contributes to the overall response and is not determined by the intrinsic activity or by the receptor reserve of the alpha-adrenoceptor agonist.

Inhibitory effect of calcium antagonist drugs on vasoconstriction induced by vascular alpha 2-adrenoceptor stimulation

A survey is given of the mechanisms of the antihypertensive effect of calcium entry blockers. The main background of the antihypertensive/hypotensive action is dilatation of precapillary arterioles (resistance vessels that cause a reduction in total peripheral resistance and, hence, a decrease in blood pressure). The vascular relaxation is caused by an inhibition of the transmembranous calcium influx and, probably less so, by interference with calmoduline. Calcium entry blockers significantly reduce the vasoconstriction induced by the excitation of vascular postsynaptic alpha 2 adrenoceptors. The inhibitory effect of calcium entry blockers is reversed by the calcium entry promoter Bay k 8644. The vasoconstriction induced by alpha 1-adrenoceptor stimulation is less generally influenced by calcium entry blockers than the alpha 2 effects. The interference with alpha 2-adrenoceptor-induced vasoconstriction may contribute to the vasodilator action of the calcium entry blockers, especially in hypertensive patients who show a hyperreactivity to pressor responses toward catecholamines.

The effects of BAY-K-8644 on the contraction of cat middle cerebral and femoral arteries

The effects of BAY-K-8644 on the reactivity of cylindrical segments of cat middle cerebral and femoral arteries were studied. BAY-K-8644 induced dose-dependent contractile responses in cerebral arteries up to 10(-6) M; higher concentrations tended to cause relaxation of the segments. The dihydropyridine elicited contractions in femoral arteries only when these vessels were previously exposed to 15 mM K+. Nifedipine (3 X 10(-7) M) produced a parallel shift to the right of the dose-response curve to BAY-K-8644, whereas 5 X 10(-6)M verapamil markedly reduced the responses evoked by all concentrations of this drug. The removal of Ca2+ from the medium abolished the response evoked by the Ca2+-channel activator at 10(-7) M in both kinds of arteries. Under these conditions Ca2+ addition induced vasoconstriction, which was blocked by nifedipine (3 X 10(-7) M). Preincubation of femoral arteries with 10(-7) M BAY-K-8644 potentiated the effects evoked by 25, 50, 75 and 125 mM K+, but did not modify those produced by 10(-5) M noradrenaline. Nifedipine (10(-7) M and 3 X 10(-7) M) blocked the potentiation caused by this drug in a dose-dependent manner. Both the increase of the response elicited by BAY-K-8644 and the inhibitory effects of nifedipine were greater at 25 mM K+ than at 125 mM. These results suggest that BAY-K-8644 facilitates Ca2+ influx into smooth muscle through Ca2+ channels that are possibly voltage sensitive and the voltage independence of the drug-induced contractions in cerebral arteries.

Bay K 8644, a dihydropyridine calcium agonist, augments vasoconstrictor responses to endogenous and exogenous noradrenaline in the peripheral vasculature of the dog

The effect of Bay K 8644 (a substance known to increase calcium influx through the voltage-dependent calcium channel) on vasoconstrictor responses of resistance vessels to endogenous and exogenous noradrenaline (NA) was investigated in pentobarbitone-anaesthetized dogs which had also undergone spinal anaesthesia and bilateral vagotomy and received atropine. In these dogs the saphenous arterial bed was perfused at fixed flow rates with autologous blood to give perfusion pressure close to the systemic blood pressure. Electrical stimulation (3-30 Hz) of the saphenous nerve and single intra-arterial (i.a.) injections of noradrenaline (NA, 0.03-3 micrograms) produced an increase in perfusion pressure (vasoconstriction) in a frequency- and a dose-dependent manner, respectively. Intra-arterial infusions of Bay K 8644 (3 and 10 micrograms min-1) per se produced no significant change in perfusion pressure. However, these infusions augmented vasoconstrictor responses to both saphenous nerve stimulation (endogenous NA) and i.a. NA (exogenous NA). These results suggest that augmentation by Bay K 8644 of vasoconstrictor responses of resistance vessels to endogenous and exogenous NA is probably due to promotion of the calcium influx through calcium channels closely associated with alpha-adrenoceptors in smooth muscle cells there.