Ionic mechanism for contractile response to hyposmotic challenge in canine basilar arteries

Department of Cellular and Molecular Pharmacology, Graduate School of Pharmaceutical Sciences, University of Shizuoka, Shizuoka, Japan Submitted 29 August 2003 ; accepted in final form 1 November 2004 A hyposmotic challenge elicited contraction of isolated canine basilar arteries. The contractile response was nearly abolished by the removal of extracellular Ca 2+ and by the voltage-dependent Ca 2+ channel (VDCC) blocker nicardipine, but it was unaffected by thapsigargin, which depletes intracellular Ca 2+ stores. The contraction was also inhibited by Gd 3+ and ruthenium red, cation channel blockers, and Cl – channel blockers DIDS and niflumic acid. The reduction of extracellular Cl – concentrations enhanced the hypotonically induced contraction. Patch-clamp analysis showed that a hyposmotic challenge activated outwardly rectifying whole cell currents in isolated canine basilar artery myocytes. The reversal potential of the current was shifted toward negative potentials by reductions in intracellular Cl – concentration, indicating that the currents were carried by Cl – . Moreover, the currents were abolished by 10 mM BAPTA in the pipette solution and by the removal of extracellular Ca 2+ . Taken together, these results suggest that a hyposmotic challenge activates cation channels, which presumably cause Ca 2+ influx, thereby activating Ca 2+ -activated Cl – channels. The subsequent membrane depolarization is likely to increase Ca 2+ influx through VDCC and elicit contraction. stretch-activated cation channels; Ca 2+ -activated Cl – channels; voltage-dependent Ca 2+ channels; large-conductance Ca 2+ -activated K + channels; gadolinium Address for reprint requests and other correspondence: T. Ishikawa, Dept. of Cellular and Molecular Pharmacology, Graduate School of Pharmaceutical Sciences, Univ. of Shizuoka, 52-1 Yada, Shizuoka City, Shizuoka 422-8526, Japan (E-mail: ishikat{at}u-shizuoka-ken.ac.jp )