Role of cyclic nucleotides in store-mediated external Ca2+ entry in human platelets

This study explores the role of cyclic nucleotides (i.e. cyclic AMP and cyclic GMP) in store-regulated external Ca2+ entry in human platelets. To stimulate store-regulated external Ca2+ entry, thapsigargin was used to deplete Ca2+ from the dense tubules, and sodium nitroprusside and iloprost respectively were used to stimulate endogenous cyclic GMP and cyclic AMP formation. Pretreatment with sodium nitroprusside and iloprost (a) attenuated the thapsigargin-evoked external Ca2+ entry and (b) reduced the rate of Ca2+ release from the dense tubules. The effects on external Ca2+ entry and Ca2+ release from the dense tubules were exerted independently and were apparently mediated through activation of the respective cyclic nucleotide-dependent protein kinases. Both sodium nitroprusside and iloprost reduced tyrosine kinase phosphorylation of a number of proteins, particularly a 72 kDa protein band. Both agents also attenuated the thapsigargin-evoked tyrosine kinase phosphorylation of the 72 kDa band. Intracellular Ca2+ depletion resulted in a reduction in tyrosine kinase-mediated phosphorylation of a number of protein bands, including the 72 kDa band and the further attenuation of thapsigargin-mediated tyrosine phosphorylation of this band. The effects of the cyclic nucleotides on cellular Ca2+ homoeostasis in thapsigargin-treated platelets were not exerted via acceleration of Ca2+ extrusion or Ca2+ sequestration into the mitochondria. We conclude that cyclic nucleotides participate in store-regulated control of external Ca2+ entry by slowing down the rate of external Ca2+ entry and Ca2+ release from intracellular Ca2+ stores. These effects are apparently mediated via cyclic nucleotide-dependent protein kinases and the attenuation of protein phosphorylation by tyrosine kinases.