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Evidence against reciprocal regulation of Ca2+ entry by vasopressin in A7r5 rat aortic smooth-muscle cells.

Recent studies by Moneer and Taylor [(2002) Biochem. J. 362, 13-21] have proposed a reciprocal regulation of two Ca2+-entry pathways by AVP ([Arg8]-vasopressin) in A7r5 vascular smooth-amuscle cells. Their model proposes that AVP inhibits CCE (capacitative Ca2+ entry) and predicts a rebound of CCE after the removal of AVP. In the present study, we used whole-cell perforated patch-clamp techniques to measure ISOC (store-operated current) corresponding to CCE in A7r5 cells. When 100 nM AVP is present, it activates ISOC with no apparent rebound on removal of AVP. ISOC activated by thapsigargin or cyclopiazonic acid was not inhibited by 100 nM AVP. We also used fura 2 fluorescence techniques to re-examine the model of Moneer and Taylor, specifically focusing on the proposed inhibition of CCE by AVP. We find that 100 nM AVP activates capacitative Mn2+ entry and does not inhibit thapsigargin- or cyclopiazonic acid-activated Mn2+ entry. Moreover, Ca2+ entry after depletion of intracellular Ca2+ stores is enhanced by AVP and we detect no rebound of Ca2+ or Mn2+ entry after AVP removal. On the basis of these findings, we conclude that AVP does not inhibit CCE in A7r5 cells.