This study was conducted to investigate the influence of extracellular Cl ([Cl]o) on the intracellular pH (pHi) regulation and the contractile state of the isolated aorta from Wistar Kyoto (WKY) rats. Isometric tension recording and fluorometry techniques were utilized to measure contractile response and pHi in isolated aortic strips. Decreasing extracellular pH (pHo) from 7.4 to 6.5 produced a marked contraction, which was 75.8 ± 5.6% of the 64.8 mmol/l KCl-induced contraction. The acidosis-induced contraction was significantly attenuated in low [Cl]o solution, the magnitude of which was 56.0 ± 3.0% of the 64.8 mmol/l KCl-induced contraction. Decreasing pHo of the normal solution to 6.5 rapidly decreased pHi in aortic smooth muscle cells and produced a corresponding contraction. When the pHo was decreased in low [Cl]o solution, a rapid fall in pHi followed by reversal of pHi changes, in a time-dependent manner was observed, despite low pHo. Omission of HCO3 from the low [Cl]o solution restored the contractile response to acidosis, which was comparable to that in normal solution. Similarly, following decrease in pHo to 6.5, no recovery of intracellular acidosis was observed. We conclude that low [Cl]o environment causes activation of extracellular HCO3-dependent pHi-regulating mechanism, that results in the rapid recovery of pHi following acidosis, and the attenuation of acidosis-induced contraction of WKY aorta.

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