Sensitivity of alpha-1 adrenoceptor-mediated pressor responses to inhibition by Ca++ entry blockers in the pithed rat: arguments against the role of receptor reserve.
In pithed rats, nifedipine (i.a., -15 min) maximally shifted the alpha-1 adrenoceptor-mediated log dose-pressor response curve to i.v. (-)-phenylephrine 5-fold to the right, doses of 1 and 3 mg/kg being equieffective. Phenoxybenzamine (3-1000 micrograms/kg i.v., -60 min) enhanced the potency of nifedipine, expressed as -log ID50, to inhibit the vasopressor response to (-)-phenylephrine. After treatment with 0.1, 0.3 or 1 mg/kg of phenoxybenzamine a dose of 1 mg/kg of nifedipine was sufficient to virtually eliminate the pressor responses. The potentiating effect of phenoxybenzamine was already present at a low dose of 3 micrograms/kg, which by itself had no influence on the log dose-pressor response curve to (-)-phenylephrine. It was also clearly limited, inasmuch as the -log ID50 values of nifedipine determined after treatment with 0.1, 0.3 and 1 mg/kg of phenoxybenzamine were identical and similar to the value reported for nifedipine against alpha-2 adrenoceptor-mediated vasoconstriction in pithed rats. Infusion of vasopressin to counteract the vasodilatory action of nifedipine did not affect its inhibitory potency compared to the effectiveness quantified under the conditions of reduced baseline diastolic pressure. The data support the conclusions that the resistance of the pressor response to (-)-phenylephrine (and other full alpha-1 adrenoceptor agonists) to inhibition by nifedipine (or other Ca++ channel blockers) is due to the dual nature of the Ca++ utilization in the vasoconstriction to these stimulants, i.e., transmembranous influx of extracellular Ca++ (sensitive to Ca++ channel blockers) and a mobilization of intracellular Ca++ (insensitive to Ca++ channel blockers).(ABSTRACT TRUNCATED AT 250 WORDS)
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D003655:Decerebrate State; D003971:Diastole / Q000187:drug effects; D008297:Male; D009543:Nifedipine / Q000494:pharmacology*; D010643:Phenoxybenzamine / Q000494:pharmacology*; D010656:Phenylephrine / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*
D011942:Receptors, Adrenergic, alpha; D010643:Phenoxybenzamine; D010656:Phenylephrine; D009543:Nifedipine