Effects of pH on contractility of rainbow trout (Oncorhynchus mykiss) intestinal muscle in vitro

Extreme acid-base imbalances in salmonids have been shown to impact on gastrointestinal motility. Changes in pH are known to affect cardiac and skeletal muscle in fish and mammalian intestinal smooth muscle preparations. This study was conducted to determine if the contractility of rainbow trout intestinal muscle was sensitive to pH fluctuations and to begin the investigation into the mechanisms by which pH affected 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 CO₂ aerating the solution and with HCl or lactic acid. Contractility was determined as response to the administration of 5-hydroxytryptamine (5-HT, serotonin), KCl, and transmural stimulation 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. Decreasing the pH with CO₂ resulted in a greater inhibition of smooth muscle contractility than when the pH was decreased in 100% O₂ with either HCl or lactic acid, particularly when the tissues are stimulated electrically. This effect was ameliorated as the fish attained sexual maturity, although the mechanisms involved were not clear. The Cl^-/HCO ₃ ^- exchanger appeared to be involved in recovery from acidotic stress, particularly at pH levels below 6.5. The role played by the Na⁺/H⁺ exchanger is still unclear. Although contractility was inhibited in the presence of the amiloride analog EIPA (5-(N-ethyl-N-isopropyl)amiloride), whether the inhibition was due to blockage of ion transport or to cytosolic effects is yet to be determined.