Strain-specific effects of acidic pH on contractile state of aortas from Wistar and Wistar Kyoto rats.

The effects of acidosis were investigated on the resting and precontracted aortas from Wistar and Wistar Kyoto (WKY) rats. Decrease in pH from 7.4 to 6.5, having no effect on the resting tension of Wistar aorta, induced a marked contraction of WKY aorta. Acidic pH markedly relaxed the contraction to 300 nM phenylephrine in Wistar aorta, whereas in WKY aorta, it produced a biphasic response, an initial relaxation followed by potentiation of the contraction. In aortas loaded with fura 2-AM, phenylephrine caused an increase in intracellular Ca2+ ([Ca2+]i) and a contraction in both Wistar and WKY rats. pH 6.5 produced a decrease in [Ca2+]i to a near-basal level and almost abolished the phenylephrine-induced contraction in Wistar rat aorta. However, in WKY aorta, a biphasic response, an initial decline and later a recovery of [Ca2+]i level, was observed. Interestingly, at similar sustained [Ca2+]i, the contractile response to phenylephrine in WKY aorta was potentiated under acidic pH conditions. Acidic pH-induced inhibition of the contraction to phenylephrine was unaffected by iberiotoxin, 4-aminopyridine, and glibenclamide (Ca2+-activated, delayed rectifier and ATP-sensitive K+ channel inhibitors, respectively), in aortas from both Wistar and WKY. Decrease in extracellular pH was associated with a rapid fall in intracellular pH (pHi) and the intracellular acidification profile was not different in both strains. All these results show that acidic pH induces strain-specific inhibitory and excitatory effects on the contractile state of aortas from Wistar and WKY rats, respectively. The sustained and transient relaxant responses to acidic pH in Wistar and WKY aortas, respectively, are due to decrease in [Ca2+]i levels, but this decrease in [Ca2+]i is independent of the activation of K+ channels.