Cyclic nucleotides control a system which regulates Ca²⁺ sensitivity of platelet secretion

Abstract

Cellular responses to extracellular signals are mediated by changes in the intracellular concentrations of one or more second messengers¹. In platelets, inhibitory agonists increase intracellular cyclic-3′,5′-AMP ([cyclic AMP]_i (refs 2, 3)) whereas excitatory agonists increase [Ca²⁺]_i and/or [1,2-diacylglycerol]_i (refs 4–9), and in some cases decrease [cyclic AMP]_i (refs 10,11). Both activation and inhibition of platelet responses have been attributed to an increase in [cyclic-3′,5′-GMP]_i (refs 8, 12). The activity of protein kinase C, which is associated with the platelet secretory response, is increased by both 1,2-diacylglycerol and Ca²⁺ (refs 4, 7, 8). The role of cyclic AMP may involve either inhibition of Ca²⁺ mobilization to the cytosol¹³ or stimulation of intracellular Ca²⁺ uptake¹⁴, and in addition inhibition of 1,2-diacylglycerol formation^15,16. The relationship between cyclic-3′,5′-GMP (cyclic GMP) and other second messengers in platelet activation has not been defined. Using platelets made permeable by exposure to an intense electric field^17,18, we demonstrate here modulation of the Ca²⁺sensitivity of platelet secretion by thrombin, and by 12-O-tetradecanoylphorbol-13-acetate (TPA) and l-oleyl-2-acetylglycerol (OAG), both potent activators of protein kinase C. The effect of thrombin is selectively modified by cyclic GMP and cyclic AMP. The response to OAG and TPA is also modulated by cyclic AMP but to a much lesser extent.