Ca2+ stores in smooth muscle cells: Ca2+ buffering and coupling to AVP-evoked inositol phosphate synthesis.
Cytosolic Ca2+ concentrations ([Ca2+]cyt) and [3H]inositol phosphates ([3H]InsP) were correlated while decreasing the Ca2+ content of sarcoplasmic reticulum (SR) stores in cultured A7r5 cells at rest and after activation with 8-arginine vasopressin (AVP). Decreasing Ca2+ influx by reducing extracellular Ca2+ or by treatment with verapamil had no effect on resting [Ca2+]cyt but significantly inhibited the AVP-evoked Ca2+ transients (delta Ca2+). Neither treatment affected basal [3H]InsP, but both treatments increased AVP-evoked synthesis of [3H]InsP. Likewise, basal [3H]InsP were unaffected by brief (10-30 s) exposures to thapsigargin (TG), while AVP-induced [3H]InsP synthesis was significantly augmented. Similar treatment with TG rapidly increased resting [Ca2+]cyt and decreased SR Ca2+ by 9-25% as manifested by decreased delta Ca2+. By contrast, ryanodine induced slow increases in [Ca2+]cyt that stabilized within 30 min; subsequent AVP-induced delta Ca2+ 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 inhibited AVP-evoked [3H]InsP formation. These observations provide further characterization of a communication pathway between the AVP-sensitive Ca2+ stores in the SR and the plasmalemmal enzyme system involved in the synthesis of inositol 1,4,5-trisphosphate. This pathway is manifested by an inverse relationship between the Ca2+ content of an AVP-sensitive, ryanodine-insensitive SR Ca2+ store and evoked [3H]InsP synthesis and may represent an important component in the tonic regulation of resting [Ca2+]cyt and vasoconstrictor- and hormone-evoked SR Ca2+ release.
Berman
D M
DM
Department of Physiology, University of Maryland School of Medicine, Baltimore.
Sugiyama
T
T
Goldman
W F
WF
D001127:Arginine Vasopressin / Q000494:pharmacology*; D002021:Buffers; D002110:Caffeine / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism; D007295:Inositol Phosphates / Q000096:biosynthesis*; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D012433:Ryanodine / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D002021:Buffers; D007295:Inositol Phosphates; D013729:Terpenes; D001127:Arginine Vasopressin; D012433:Ryanodine; D002110:Caffeine; D019284:Thapsigargin; D000242:Cyclic AMP