Possible role of a cAMP-dependent phosphorylation in the calcium release mediated by inositol 1,4,5-trisphosphate in human platelet membrane vesicles

Abstract

The addition of inositol 1,4,5-trisphosphate (IP 3) to a 45Ca-preloaded human platelet membrane fraction (dense tubular system) induced a transient release of Ca 2+. When the vesicle fraction was loaded with 45Ca 2+ to isotopic equilibrium in the presence of the catalytic subunit of the cAMP-dependent protein kinase, the level of Ca 2+ uptake was increased and the subsequent IP 3-induced Ca 2+ release was enhanced. The stimulation was observed regardless of the IP 3 concentration used, and was maximal with an enzyme concentration of 5 μg/ml. The addition of the protein kinase inhibitor prevented the stimulatory effect of the catalytic subunit on IP 3-induced calcium release, and also abolished the calcium release detected in the absence of added enzyme. It is concluded that a cAMP-dependent protein phosphorylation may be involved in the regulation of the IP 3-induced Ca 2+ release in human platelets.