Agonists stimulate divalent cation channels in the plasma membrane of human platelets

Abstract

Agonists such as thrombin, PAF (platelet-activating factor) and ADP are known to cause a larger elevation in [Ca²⁺]_i, in quin2-loaded platelets in the presence of extracellular Ca²⁺ than in its absence. The simplest interpretation of these observations is that in the presence of extracellular calcium there is an influx component across the cell surface. In the presence of Mn²⁺, a divalent cation which is known to avidly bind to quin2 and to quench its fluorescence, the agonists produce a small initial rise in quin2 fluorescence followed by a decrease in fluorescence to well below the resting level. The result indicates entry of Mn²⁺, presumably through some form of receptor-operated Ca²⁺ channel.