INTESTINAL BASE EXCRETION IN THE SEAWATER-ADAPTED RAINBOW-TROUT - A ROLE IN ACID-BASE-BALANCE

A potential role for the intestine of seawater-adapted teleosts in aci d-base regulation was investigated following earlier reports of highly alkaline rectal fluids in the gulf toadfish Opsanus beta. Rectal samp les taken from starved seawater-adapted rainbow trout had a high fluid pH (8.90+/-0.03; mean S.E.M., N=13) and base (HCO3-+2CO(3)(2-)) conte nt of 157+/-26 mequiv kg(-1) (N=11). In trout fitted with rectal cathe ters, rectal fluid was voided at a rate of 0.47+/-0.11 ml kg(-1) h(-1) (N=8), giving a net base excretion rate of 114+/-15 mu equiv kg(-1) h (-1) (N=7). Drinking rates averaged 3.12+/-0.48 ml kg(-1) h(-1) (N=8), and accounted for only 6% of the base excreted via the intestine, ind icating substantial net transport of endogenously derived base into th e intestine. Rectally excreted base was approximately balanced by an e quivalent efflux of net acid from non-rectal sources (possibly as NH4 excretion via the gills).Samples taken from four sites along the inte stine revealed that the most anterior region (the pyloric intestine) w as responsible for the majority of HCO3-+2CO(3)(2-) accumulation. The pyloric intestine was subsequently perfused in situ to investigate pos sible mechanisms of base secretion. Net base fluxes were found to be d ependent on luminal Cl-, 76% stimulated by amiloride, 20 % inhibited b y 10(-4) mol l(-1) acetazolamide, but unaffected by either 10(-4) mol l(-1) SITS or 2x10(-5) mol l(-1) DIDS. This suggests that the mechanis m of base secretion within the pyloric intestine may involve a Cl-/HCO 3--ATPase. It is speculated that intestinal base secretion may play a role in facilitating osmoregulation of seawater-adapted teleosts.