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Effects of substitution for NaHCO3 (and 5% CO2) in the physiological solution with equimolar N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) or morpholinopropane sulfonic acid (MOPS) (and 100% O2) at various pH levels on the contractility of smooth muscle were examined. At pH 7.4, spontaneous contraction in rat portal vein was not inhibited by the artificial buffer solutions, as compared to findings in the case of bicarbonate buffer solution. Norepinephrine-dose response curve in the case of rat portal vein and the histamine-dose response curve in cases of guinea pig taenia coli remained unchanged in the artificial buffer solutions. At pH 7.2 and 7.0, the spontaneous contraction in portal vein was reversibly inhibited either in artificial or in bicarbonate buffer solutions. The norepinephrine- and histamine-dose response curve shifted downwards and/or to the right in these low pH solutions. Thus, HEPES and MOPS had no inhibitory effect on the smooth muscle contractility. Since low pH strongly inhibited the contractility, attention should be directed to the temperature-dependent decreases in pKa of the artificial buffers (i.e., if pH of the solution is adjusted at room temperature and then warmed, pH decreases).

Artificial buffers do not inhibit contractile responses in the smooth muscle of rat portal vein and guinea pig taenia coli.