Interaction between the calcium entry blocker nifedipine with the alpha 1-adrenoreceptor-mediated increase in diastolic pressure elicited by catecholamines

Studies on the anti-vasoconstrictor activity of BRL 34915 in spontaneously hypertensive rats; a comparison with nifedipine

1. The blood pressure lowering and anti-vasoconstrictor effects of BRL 34915 and nifedipine were compared in female spontaneously hypertensive rats (SHR). 2. In conscious SHR, intravenous injection of BRL 34915 (0.1, 0.3 mg kg-1) produced rapid, dose-related falls in mean arterial pressure of greater than 3 h duration. Nifedipine, at the same intravenous dose levels, also evoked rapid anti-hypertensive effects, though these responses were of lesser magnitude and duration than those observed for BRL 34915. 3. In anaesthetized, ganglion-blocked SHR, BRL 34915 (0.1, 0.3 mg kg-1 i.v.) dose-dependently antagonized the pressor responses to incremental intravenous infusions of noradrenaline (3.8-28.5 ng min-1) or phenylephrine (120-907 ng min-1) but did not inhibit pressor responses to incremental infusions of methoxamine (0.47-3.63 micrograms min-1), angiotensin II (7.0-52.9 ng min-1) or vasopressin (0.27-2.0 mu min-1). 4. In anaesthetized, ganglion-blocked SHR, nifedipine (0.1, 0.3 mgkg-1 i.v.) antagonized the pressor responses to each of the infused vasoconstrictor agents, being most effective against responses to noradrenaline or angiotensin II. 5. In pithed SHR, both BRL 34915 and nifedipine (each at 0.3 mg kg-1 i.v.) reduced the basal blood pressure level and produced marked inhibition of frequency-dependent pressor responses evoked by electrical stimulation of the spinal cord sympathetic outflow (0.25-4.0 Hz). Restoration of the basal diastolic blood pressure to within the control range, using a continuous intravenous infusion of vasopressin (0.98 mu min-1), prevented the inhibitory effect of BRL 34915. In the case of nifedipine, however, even raising the basal blood pressure to a level exceeding that recorded in control rats (with vasopressin, 2.0 mu min-1), did not reverse the inhibitory effect of the drug on frequency-dependent pressor responses. 6. It is concluded that the anti-hypertensive properties of BRL 34915 in SHR are probably unrelated to an anti-vasoconstrictor action. In contrast, it is suggested that the broadly-based anti-vasoconstrictor properties of nifedipine may contribute substantially to the anti-hypertensive properties of this drug.

Characterization of alpha-adrenoreceptor mediated reduction of flow in a saline-perfused constant pressure model of rat perfused hindquarters

An analysis of vasopressor responses elicited by alpha-adrenoreceptor agonists and KCl in a constant pressure saline perfused model of rat perfused hindquarters is presented. Using methoxamine (alpha 1), cirazoline (alpha 1), B-HT 920 (alpha 2) and St 587 (alpha 1) as alpha-adrenoreceptor agonists, and rauwolscine and prazosin as selective alpha 2- and alpha 1-adrenoreceptor antagonists, respectively, it is concluded that alpha-adrenoreceptor mediated vasoconstriction in this model is mediated by the alpha 1-subtype. Pretreatment of the animals with reserpine (i.p., at various doses and dose-regimens) or addition of angiotensin II (10(-9)-10(-6) M), PGF2 alpha(10(-8) & 5 X 10(-7) M) or KCl (2 & 4 X 10(-3) M), or changing [Mg2+] did not significantly affect the potency or intrinsic activity of the alpha-adrenoreceptor agonists studied. Experiments with the calcium entry blocking drugs gallopamil and nifedipine, or deletion of CaCl2 from the perfusion solution, did not reveal a contribution from the influx of extracellular calcium towards alpha 1-adrenoreceptor-mediated reduction of flow in RPH. KCl-induced vasoconstriction in RPH was found to be sensitive to the blockade of calcium entry by calcium entry blocking drugs, but not to alpha-adrenoreceptor-blocking drugs and calmodulin antagonist. It is concluded that the model described here is of potential value in the characterization of alpha-adrenoreceptor antagonists and calcium entry blocking drugs under in vitro conditions in resistance vessels of the rat.

Influence of respiratory acidosis or alkalosis on pressor responses mediated by alpha 1- and alpha 2-adrenoceptors in pithed normotensive rats

The effect of respiratory acidosis and alkalosis on the vasoconstriction to alpha 1- and alpha 2-adrenoceptor stimulation was studied in pithed normotensive rats. The selective alpha 1-adrenoceptor agonists (-)amidephrine, cirazoline, (+/-)erythro methoxamine, (-)phenylephrine, Sgd 101/75 and St 587 were used, as well as the selective alpha 2-adrenoceptor agonists B-HT 920, B-HT 933, DP-6,7-ADTN, M-7 and UK 14,304. The non-selective alpha-adrenoceptor agonists xylazine, noradrenaline and adrenaline were included as well. The latter two were also studied under selective doses of the antagonists rauwolscine and prazosin, thus yielding the respective alpha 1- and alpha 2-adrenoceptor components of the vasoconstriction to these agonists. The effect of acid-base balance disturbances on presynaptically released noradrenaline elicited by electrical stimulation of preganglionic nerves was studied as well. Dose response curves for the agonists were generated under various conditions of ventilation, yielding either alkalotic, normal or acidotic values of arterial blood pH. Pressor responses to all agonists were maximally affected by changes in acid-base status at the low doses of the agonists. Acidosis was found to inhibit increases in diastolic pressure mediated by the alpha 1-as well as the alpha 2-adrenoceptor agonists studied, although not to the same extent. Alkalosis exerted either an obvious potentiation or did not significantly influence alpha 1-adrenoceptor mediated pressor responses. On the basis of acid-base sensitivity the following groups of agonists were distinguished: Cirazoline, phenylephrine, methoxyamine, electrically released noradrenaline from presynaptic sites, of which pressor responses are obviously potentiated and attenuated by alkalosis and acidosis, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)