Invariable susceptibility to blockade by nifedipine of vasoconstriction to various α2-adrenoceptor agonists in pithed rats

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Abstract
The sensitivity of the increase in diastolic pressure brought about by the selective agonists of α2-adrenoceptors, B-HT 920, B-HT 933, xylazine, UK-14,304, M-7, TL-99 and DP-6, 7-ADTN in pithed normotensive rats to blockade by the calcium entry inhibitor nifedipine has been investigated. To exclude any participation of vascular α1- and β2-adrenoceptors, as well as cardiac β1-adrenoceptors, in the pressor responses, the study was made after treatment of the pithed rats with prazosin (0·1 mg kg−1) and (-)-propranol (1 mg kg−1). Without exception, the preferential agonists of α2-adrenoceptors elicited vasoconstrictor responses which were susceptible to inhibition by nifedipine (0·03–1 mg kg−1) in a dose-dependent manner regardless of the differences in intrinsic activity of the compounds. The pressor activity was almost completely abolished after 1 mg kg−1 of nifedipine. The results show that vasoconstriction induced in pithed rats by various selective stimulating agents of postjunctional vascular α2-adrenoceptors is invariably and equally sensitive to attenuation by nifedipine. This susceptibility of α2-adrenoceptor-mediated vasoconstriction to impairment by blockade of calcium entry is not dependent on the nature, the potency or the efficacy of the agonist.

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