CA2-MUSCLE CELLS - CA2+ BUFFERING AND COUPLING TO AVP-EVOKED INOSITOLPHOSPHATE SYNTHESIS( STORES IN SMOOTH)

Cytosolic Ca2+ concentrations ([Ca2+](cyt)) and [H-3]inositol phosphat es ([H-3]InsP) were correlated while decreasing the Ca2+ content of sa rcoplasmic reticulum (SR) stores in cultured A7r5 cells at rest and af ter activation with 8-arginine vasopressin (AVP). Decreasing Ca2+ infl ux by reducing extracellular Ca2+ or by treatment with verapamil had n o effect on resting [Ca2+](cyt) but significantly inhibited the AVP-ev oked Ca2+ transients (Delta Ca2+). Neither treatment affected basal [H -3]InsP, but both treatments increased AVP-evoked synthesis of [H-3]In sP. Likewise, basal [H-3]InsP were unaffected by brief (10-30 s) expos ures to thapsigargin (TG), while AVP-induced [3H]InsP synthesis was si gnificantly augmented. Similar treatment with TG rapidly increased res ting [Ca2+](cyt) and decreased SR Ca2+ by 9-25% as manifested by decre ased Delta Ca2+. By contrast, ryanodine induced slow increases in [Ca2 +](cyt) that stabilized within 30 min; subsequent AVP-induced Delta Ca 2+ were attenuated by 50%. Ryanodine had no effect on either basal or stimulated [3H]InsP levels. Agents that elevate adenosine 3',5'-cyclic monophosphate (cAMP) such as caffeine, 8-bromo-cAMP, and forskolin in hibited AVP-evoked [3H]InsP formation. These observations provide furt her characterization of a communication pathway between the AVP-sensit ive Ca2+ stores in the SR and the plasmalemmal enzyme system involved in the synthesis of inositol 1,4,5-trisphosphate. This pathway is mani fested by an inverse relationship between the Ca2+ content of an AVP-s ensitive, ryanodine-insensitive SR Ca2+ store and evoked [H-3]InsP syn thesis and may represent an important component in the tonic regulatio n of resting [Ca2+](cyt) and vasoconstrictor- and hormone-evoked SR Ca 2+ release.