Tris does not inhibit isolated vascular or intestinal smooth muscle contraction

There are conflicting reports on the effect of tris(hydroxymethyl)aminomethane (Tris) on smooth muscle contraction. Therefore, the effect of substitution of Tris for bicarbonate in physiological saline solution on smooth muscle contractility was investigated. Tris (25 mM) increased the amplitude without changing the frequency of spontaneous contractions in rat portal vein. Tris did not inhibit either norepinephrine-induced contractions in rat portal vein and aorta or histamine-induced contractions in taenia coli. When the pH of the Tris-buffered solution was lowered from the control value of 7.4 to 7.0, spontaneous contractions in portal vein as well as agonist-induced contractions in all of the above preparations were inhibited. Tris did not inhibit contractions induced by calcium in potassium-depolarized smooth muscle preparations if the Tris-buffered solution contained sufficient amounts of sodium ion (as did bicarbonate-buffered solution). These results suggest that Tris does not inhibit contractile responses of isolated vascular and intestinal smooth muscle preparations provided that the conditions other than the buffer system (especially pH of solution) are similar to those in bicarbonate-buffered solutions.