The effect of the calcium channel agonist, Bay K-8644 on human vascular smooth muscle.

Bay K-8644, a calcium channel activator, caused a dose-dependent (ED50 = 12.8 nM) elevation of tone in the human umbilical artery. The response to Bay K-8644 was dependent upon extracellular calcium (Ca2+), competitively antagonized by the channel antagonist, nifedipine, and, in addition, the sensitivity to Bay K-8644 was increased by, but not dependent upon, elevating extracellular potassium. Bay K-8644 competitively displaced, IC50 = 1.8 nM, specific [3H]nitrendipine binding to a microsome fraction derived from the umbilical artery, suggesting that the Ca2+ channel activator and the structurally related dihydropyridine antagonists bind to a single site to affect Ca2+ channel function in human vascular smooth muscle.