Mechanism of inositol trisphosphate-induced Ca²⁺ release in saponin-permeabilized human platelets

Abstract

Inositol 1, 4, 5-trisphosphate (IP₃) is formed in response to receptor stimulation by phospholipase C-mediated hydrolysis of phosphatidylinositol 4, 5-bisphosphate, and is thought to be responsible for the release of Ca²⁺ from intracellular stores. Cyclic AMP (cAMP) also serves as a second messenger that mediates the inhibition of platelet functions but the mechanism underlying the inhibitory action is not clear. The effect of cAMP on IP₃-induced Ca²⁺ release in human platelets permeabilized with saponin were studied. Ca²⁺ was sequestered by intracellular organelles in the presence of ATP (2mM) and IP₃ (2μM) released 40 to 50% of the sequestered Ca²⁺ The addition of cAMP (20μM) to permeabilized platelets did not alter Ca²⁺ uptake but depressed IP₃-induced Ca²⁺ release. When permeabilized platelets were incubated with [γ-³²P] ATP, cAMP stimulated phosphorylation of 20k-, 22k-, 24k- and 50k-Da proteins. These results indicate that cAMP may modulate IP₃-induced Ca²⁺ release in human platelets through protein phosphorylation.