Effects of pH manipulation on contractility of rainbow trout intestinal smooth muscle in vitro

This study was conducted to determine if the contractility of rainbow trout intestinal muscle is sensitive to pH alterations and to begin the investigation into the mechanisms by which pH affects the contractility. Isolated duplicate or triplicate segments of rainbow trout intestine were suspended in organ baths containing modified Krebs-Henseleit solution. The pH was adjusted by varying the concentration of CO$\sb2$ aerating the solution or with HCl. Contractility was determined as response to the administration of 5-hydrozyamine (5-HT), KCl, and transmural stimulation (TS) within the pH range of 6.3 to 8.5. Optimum pH for proximal segments was 7.85 while the range was much wider for distal segments. When associated with decreasing pH, CO$\sb2$ causes a greater inhibition than decreasing the pH in 100% O$\sb2,$ particularly when the tissues are stimulated electrically. This effect is ameliorated as the fish attain sexual maturity, although the mechanisms involved are not clear. The Cl$\sp-$/HCO$\sp{3-}$ exchanger--inhibited by 4-acetamido-4$\sp\prime$-isothiocyanto-stilbene-2-2$\ sp\prime$-disulfonic acid (SITS)-appears to be involved in recovery from acidotic stress, particularly at pH levels below 6.5. The role played by the Na$\sp+$/H$\sp+$ exchanger inhibited by 5-(N-ethyl-N-isopropryl) amiloride (EIPA) is still unclear. Although contractility is inhibited in the presence of EIPA, whether the inhibition is due to blockage of ion transport, or to cytosolic effects is unknown. Lactic acid does not inhibit contractility induced by 5-HT or KCl; however, at physiological concentrations, an inhibition of electrically induced contractions occurred. It can be concluded that the effects of pH on rainbow trout intestinal smooth muscle contractility are exerted primarily on the nerves innervating the gut rather than on the muscle itself. (Abstract shortened by UMI.).