Rat liver plasma membranes are enriched in a Ca 2+-dependent phosphodiesterase active on phosphatidylinositol 4,5-P2 and phosphatidylinositol 4-P, but not phosphatidylinositol. Inositol-Pg is the first product of the reaction, but is rapidly degraded. Micromolar concentrations of GTP and its nonhydrolyzable analogues stimulate the reaction, whereas GDP, GMP and other nucleoside triphosphates are inactive. GTP and its analogues decrease the requirement of the reaction for Cal+ and also increase its activity at saturating Ca2+. These results support the hypothesis that guanine nucleotides and a guanine nucleotide binding regulatory protein are involved in coupling the receptors for Ca 2+-mediated agonists to the breakdown of plasma membrane phosphatidylinositol 4,5-P2.
To better define their regulation,.tide and CSa chemoatacnt receptor cDNAs were t t expressed with high efficiency (...35-54%) in human kidney cefls. As
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