Calcium efflux from an intracellular pool activated by GTP hydrolysis in cultured gastric smooth muscle

Abstract

In these studies, we have characterized calcium movement due to guanosine triphosphate (GTP) hydrolysis from an ATP-sequestered intracellular calcium pool in cultured gastric smooth muscle. GTP (1-100 microM), when added to an ATP-regenerating medium, resulted in a concentration-dependent and irreversible efflux of calcium from an organellar calcium pool. GTP-induced calcium efflux was not affected by variation of the ATP/ADP ratio (8.5-155.0), indicating that GTP did not act by inhibiting calcium influx via calcium adenosinetriphosphatase. To assess whether the calcium increase was necessarily associated with GTP hydrolysis, experiments were performed with the nonhydrolyzable guanine nucleotide analogues guanosine 5'-[beta-thio]diphosphate (GDP beta S), 5'-guanylyl imidodiphosphate guanosine (GppNHp), and 5'-O-(3-thiotriphosphate) (GTP gamma S). Administration of GDP beta S and GppNHp resulted in no significant calcium efflux. GTP gamma S caused a small steady-state calcium increase (20% of that induced by the hydrolyzable nucleotide) but irreversibly inhibited all subsequent calcium increase due to GTP. The possibility that GTP may either modify the concentration of mobilizable calcium in inositol trisphosphate (IP3)-sensitive calcium stores or the responsivity of IP3-associated calcium channels was assessed by two experiments: 1) prior administration of GTP at concentrations < or = 100 microM had no effect on IP3-induced calcium release, and 2) heparin, which competitively inhibits IP3 binding to its receptor on the endoplasmic reticulum, did not affect GTP-associated calcium increase. These results demonstrate that, in gastric smooth muscle, GTP causes calcium efflux from an intracellular pool that is functionally independent from that pool sensitive to IP3.(ABSTRACT TRUNCATED AT 250 WORDS)