Capacitative Ca2ϩ entry (CCE) in vascular smooth muscle cells contributes to vasoconstrictor and mitogenic effects of vasoactive hormones. In A7r5 rat aortic smooth muscle cells, measurements of cytosolic free Ca 2ϩ concentration ([Ca 2ϩ ] i ) have demonstrated that depletion of intracellular Ca 2ϩ stores activates CCE. However, there is disagreement in published studies regarding the regulation of this mechanism by the vasoconstrictor hormone [Arg 8 ]-vasopressin (AVP). We have employed electrophysiological methods to characterize the membrane currents activated by store depletion [store-operated current (I SOC )]. Because of different recording conditions, it has not been previously determined whether I SOC corresponds to CCE measured using fura-2; nor has the channel protein responsible for CCE been identified. In the present study, the pharmacological characteristics of I SOC , including its sensitivity to blockade by 2-aminoethoxydiphenylborane, diethylstilbestrol, or micromolar Gd 3ϩ , were found to parallel the effects of these drugs on thapsigargin-or AVP-activated CCE measured under identical external ionic conditions using fura-2. Thapsigargin-stimulated I SOC was also measured in freshly isolated rat mesenteric artery smooth muscle cells (MASMC). Members of the transient receptor potential (TRP) family of nonselective cation channels, TRPC1, TRPC4, and TRPC6, were detected by reverse transcription-polymerase chain reaction and Western blot in both A7r5 cells and MASMC. TRPC1 expression was reduced in a stable A7r5 cell line expressing a small interfering RNA (siRNA) or by infection of A7r5 cells with an adenovirus expressing a TRPC1 antisense nucleotide sequence. Thapsigargin-stimulated I SOC was reduced in both the TRPC1 siRNAand TRPC1 antisense-expressing cells, suggesting that the TRPC1 channel contributes to the I SOC /CCE pathway. Article, publication date, and citation information can be found at

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.