Transfected cGMP-dependent protein kinase suppresses calcium transients by inhibition of inositol 1,4,5-trisphosphate production.

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Abstract
CGMP is a key regulatory molecule in visual transduction, integration of neuronal response to excitatory neurotransmitters, relaxation of smooth muscle, intestinal secretion of water and salt, and reabsorption of sodium and water in the distal tubules of the nephron. Some of these cellular functions are associated with the activation of cGMP kinase and a decrease in cytosolic calcium levels ([Ca2+]i). The mechanism by which cGMP kinase lowers [Ca2+]i is controversial. We have used CHO cells stably transfected with cGMP kinase to test several of the proposed [Ca2+]i-lowering mechanisms. Thrombin induces a calcium transient in wild-type and cGMP kinase-expressing CHO cells by releasing calcium from intracellular stores. Preincubation of wild-type cells with 8-bromo-cGMP had no effect on the calcium transient, whereas 8-bromo-cGMP prevented the thrombin-stimulated calcium transient in cGMP kinase-expressing CHO cells. In both cell types 8-bromo-cGMP had no effect on [Ca2+]i transients induced by replacing extracellular sodium by tetramethylammonium, ruling out an effect of cGMP kinase on Ca(2+)-ATPases. However, cGMP kinase activation effectively suppressed thrombin-induced stimulation of inositol 1,4,5-trisphosphate production. These results show that cGMP kinase lowers [Ca2+]i by interfering with the inositol 1,4,5-trisphosphate synthesis.