Differential megakaryocytic desensitization to platelet agonists

Platelets are released into the peripheral circulation from the bone marrow where they arise as fragments of megakaryocyte cytoplasm. To characterize the effects of platelet agonists on megakaryocytes, we examined calcium signaling and desensitization to thrombin, the thromboxane A₂ (TxA₂) mimetic (15S)-hydroxy-11α,9α-(epoxymethano)prosta-5Z,13E-dienoic acid (U46619), and platelet-activating factor (PAF) in cultured CHRF-288-11 megakaryocytic cells. Initially, we compared agonist-stimulated calcium transients in fura-2-loaded CHRF-288-11 cells and human platelets. The 50% effective concentration values for the agonists to increase free cytosolic calcium were as follows: thrombin (0.11 ± 0.02 U/ml in CHRF, 0.19 ± 0.03 U/ml in platelets), U46619 (147 ± 33 nM in CHRF, 157 ± 5 nM in platelets), and PAF [15 ± 2 nM in CHRF, 16 ± 2 nM in platelets (n = 4 each)]. CHRF-288- 11 thrombin, TxA₂, and PAF receptors were demonstrated to be coupled to phospholipase C because each of the agonists stimulated phosphatidylinositol hydrolysis in myo-[³H]inositol-loaded CHRF-288-11 cells and pharmacological inhibition of phospholipase C-blunted agonist-stimulated calcium signaling. CHRF-288-11 cells exposed to the three agonists for 1 h showed different patterns and extent of homologous and heterologous desensitization. Protein kinase C activation appeared to be necessary but not sufficient for desensitization because 1) activation of protein kinase C with phorbol 12- myristate 13-acetate inhibited the calcium responses to all three agonists, 2) inhibition of protein kinase C with staurosporine attenuated subsequent desensitization to each agonist, and 3) each agonist increased protein kinase C activity in CHRF-288-11 cell homogenates.