Dietary fat assimilation and bile salt absorption in the killifish intestine

Radiolabeled taurocholate (TC) and triolein were used to study fat assimilation and bile salt absorption in the stomachless saltwater killifish, Fundulus heteroclitus. Fat absorption occurred primarily in the proximal intestine with approximately 87% of a single dose (9 mg fat/8 g fish) absorbed in 2 h. Luminal triolein hydrolysis and enterocyte triolein resynthesis were tightly coupled. Killifish gallbladder bile contains taurocholate and cholate in an equal molar ratio at a combined concentration of 237 +/- 25 mM (n = 10) in 24-h-fasted fish. During fat assimilation luminal bile salt and fatty acid concentrations ranged between 10 and 30 mM. Between and during meals the total concentration of bile salts in the intestinal tissue remained roughly constant (4-6 mM) with the proximal one-third of the intestine containing 40% of the total and the remainder equally distributed between the mid and distal regions. All three regions of the intestine rapidly incorporated ingested TC in vivo, with the amount incorporated proportional to the pool size. In contrast, in vitro at low TC concentrations (60 nM), the distal one-third of the intestine incorporated 10 times as much TC in 2-min uptake experiments as the proximal and mid regions. Although there are many similarities between fat and bile salt assimilation in killifish and mammals, overall the processes are much simpler in killifish.