CALCIUM EFFLUX FROM AN INTRACELLULAR POOL ACTIVATED BY GTP HYDROLYSISIN CULTURED GASTRIC SMOOTH-MUSCLE

Calcium efflux from an intracellular pool activated by GTP hydrolysis in cultured gastric smooth muscle. Am. J. Physiol. 266 (Gastrointest. Liver Physiol. 29): G388-G394, 1994. - In these studies, we have chara cterized calcium movement due to guanosine triphosphate (GTP) hydrolys is from an ATP-sequestered intracellular calcium pool in cultured gast ric smooth muscle. GTP (1-100 mu M), 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 efflu x was not affected by variation of the ATP/ADP ratio (8.5-155.0), indi cating that GTP did not act by inhibiting calcium influx via calcium a denosinetriphosphatase. To assess whether the calcium increase was nec essarily associated with GTP hydrolysis, experiments were performed wi th the nonhydrolyzable guanine nucleotide analogues guanosine 5' -[bet a-thioldiphosphate (GDP beta S), 5'-guanylyl imidodiphosphate guanosin e (GppNHp), and 5'-O-(3-thiotriphosphate) (GTP gamma S). Administratio n 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 tri sphosphate (IP3)-sensitive calcium stores or the responsivity of IP3-a ssociated calcium channels was assessed by two experiments: 1) prior a dministration of GTP at concentrations less than or equal to 100 mu M had no effect on IP3-induced calcium release, and 2) heparin, which co mpetitively inhibits IP3 binding to its receptor on the endoplasmic re ticulum, 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. GTP-induced calcium movement may comprise a me chanism by which steady-state calcium translocations occur between int racellular pools or between intracellular pools and the cytosol.