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 pre parations. This study was conducted to determine if the contractility of rainbow trout intestinal muscle was sensitive to pH fluctuations an d to begin the investigation into the mechanisms by which pH affected contractility. Isolated duplicate or triplicate segments of rainbow tr out intestine were suspended in organ baths containing modified Krebs- Henseleit solution. The pH was adjusted by varying the concentration o f CO2 aerating the solution and with HCl or lactic acid. Contractility was determined as response to the administration of 5-hydroxytryptami ne (5-HT, serotonin), KCl, and transmural stimulation within the pH ra nge 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 w hen the pH was decreased in 100% O-2 with either HCl or lactic acid, p articularly when the tissues are stimulated electrically. This effect was ameliorated as the fish attained sexual maturity, although the mec hanisms involved were not clear. The Cl-/HCO3- exchanger appeared to b e 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 t o be determined.