Calcium channels in thrombin-activated human platelet membrane.

Platelet-activating factor, 5-hydroxytryptamine, thromboxane A2, adenosine diphosphate and thrombin are known to activate platelets by stimulating calcium entry, but the nature of the entry pathways is unknown. We present the identification of single divalent cation channels from thrombin-activated human platelets. Membrane vesicles from unstimulated and thrombin-stimulated human platelets were incorporated in planar bilayers and unitary currents through single channels were measured. Divalent cation selective channels could only be demonstrated in thrombin-stimulated preparations. These channels share a number of properties in common with voltage-dependent calcium channels--a high degree of selectivity for divalent cations, a single channel conductance of about 10 pS (in 150 mM Ba2+) and sensitivity to blockade by inorganic calcium channel blockers such as Ni2+. In other respects, these channels are different as they are not voltage-dependent and are not blocked by 1,4-dihydropyridine calcium channel antagonists.