Selective action of diltiazem on cerebral vascular smooth muscle in the rabbit: antagonism of extrinsic but not intrinsic maintained tone.

Diltiazem selectively reduces the second equilibrium phase of contraction of several arteries and veins in the rabbit to a number of agonists, including norepinephrine, serotonin, histamine and potassium. Its effect is consistent with blockage of the entry of activator calcium from the extracellular space into the vascular smooth muscle cell. It has a preferential action on responses of the basilar artery in comparison with those of the ear and mesenteric arteries and saphenous vein. Although the spontaneously developing, maintained intrinsic tone of the basilar artery and facial vein are very resistant to diltiazem, the spontaneous rhythmic activity of the portal vein is sensitive to its antagonistic action. Findings reported here are consistent with the possibility that diltiazem preferentially blocks receptor-operated and potential-sensitive slow calcium channels of the cerebral artery compared with other systemic vessels tested but has little effect on the mechanisms responsible for maintained intrinsic tone development.