Evolutionary aspects of intestinal bicarbonate secretion in fish

Abstract

Experiments compared intestinal HCO 3 − secretion in the intestine of marine teleost Gulf toadfish, Opsanus beta, to representatives of early chondrostean and chondrichthyan fishes, the Siberian sturgeon, Acipenser baerii, and white-spotted bamboo shark, Chiloscyllium plagiosum, respectively. As seen in marine teleosts, luminal HCO 3 − concentrations were 10-fold plasma levels in all species when exposed to hyperosmotic conditions. While intestinal water absorption left Mg 2+ and SO 4 2− concentrated in intestinal fluids up to four-fold ambient seawater concentrations, HCO 3 − was concentrated up to 50 times ambient levels as a result of intestinal HCO 3 − secretion. Reduced luminal Cl − concentrations in the intestine of all species suggest that HCO 3 − secretion also occurs via Cl −/HCO 3 − exchange in chondrostean and chondrichthyan fishes. Sturgeon began precipitating carbonates from the gut after only 3 days at 14‰, a mechanism utilized by marine teleosts to reduce intestinal fluid osmolality and maintain calcium homeostasis. Analysis of published intestinal fluid composition in the cyclostome Lampetra fluviatilis reveals that this species likely also utilize intestinal HCO 3 − secretion for osmoregulation. Analysis of existing cyclostome data and our results indicate that intestinal Cl −/HCO 3 − exchange plays an integral role in maintaining hydromineral balance not only in teleosts, but in all fish (and perhaps other animals) with a need to drink seawater.