INTERACTION BETWEEN THE CALCIUM ENTRY BLOCKER NIFEDIPINE WITH THE ALPHA-1-ADRENORECEPTOR-MEDIATED INCREASE IN DIASTOLIC PRESSURE ELICITED BY CATECHOLAMINES

: In the pithed normotensive rat the effect of the calcium entry blocker nifedipine was studied on alpha 1-adrenoreceptor-mediated pressor responses of intravenously administered (-)-noradrenaline, dopamine and (-)-adrenaline as well as of (-)-noradrenaline released from sympathetic neurones by DMPP, tyramine, McN-A-343 and electrical stimulation of the spinal cord at the level Th5-L4. The alpha 1-adrenoreceptor-mediated pressor responses to both intravenously administered and neuronally released catecholamines were moderately antagonized by nifedipine. The alpha 1-adrenoreceptor-mediated increase in diastolic pressure elicited by DMPP was unique in its extreme insensitivity to blockade by nifedipine. The results support the hypothesis that phenylethylamine-like alpha 1-adrenoreceptor agonists induce an increase in diastolic pressure relatively independent of extracellular calcium in contrast to imidazolidine-like agonists. The data may be interpreted as indicating that each alpha 1-adrenoreceptor agonist activates the alpha 1-adrenoreceptor in a unique way.