Influence of vasoactive agents on cytoplasmic free calcium in vascular endothelial cells.

The regulation of cytoplasmic free calcium concentration [( Ca2+]i) in endothelial cells (EC) derived from human umbilical vein, aorta, and pulmonary artery, or from bovine pulmonary artery, was studied by means of the fluorescent Ca2+ indicator indo-1. Histamine and thrombin caused a rapid transient elevation in [Ca2+]i in the EC of all the human blood vessels tested. In aortic EC, [Ca2+]i also rose in response to ATP and bradykinin. It was shown that in bovine pulmonary artery EC [Ca2+]i rises in response to platelet-activating factor (PAF) and thrombin. For a more detailed investigation of the receptor-mediated mechanism of [Ca2+]i increase in EC we used histamine as a stimulating agent. Histamine effects were seen at concentrations ranging from 5 X 10(-7) to 10(-4) M [50% effective dose (ED50) approximately 2-4 microM)] and were mediated by H1-receptors. The histamine-induced increase in [Ca2+]i was not markedly diminished when the extracellular calcium was bound by excess ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The data obtained indicate that the histamine effect is best explained by Ca2+ mobilization from intracellular stores. The histamine-induced increase in [Ca2+]i was not influenced by elevating the intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) or cyclic guanylic acid (cGMP) by use of isobutylmethylxanthine and forskolin or by nitroprusside preincubation, respectively. However, the protein kinase C stimulator, phorbol myristate acetate (PMA), strongly inhibits [Ca2+]i elevation. It is assumed that a negative feedback mechanism that blocks receptor-mediated [Ca2+]i increase is triggered as a result of the activation of protein kinase C.