Comparative analysis of leucine transport in temperate fish liver in vivo.

The uptake of [14C]leucine in toadfish (Opsanus tau) liver in vivo at 10 degrees C has been studied by a single pulse injection technique. Transport parameters were determined on the basis of the distribution of the amino acid and of [3H]inulin, used as a marker for extracellular space, in liver free and protein-bound fractions and in venous blood draining from the liver. Saturation analysis by the Cornish-Bowden method yielded a maximal uptake of 0.26 mumole, which was similar on a concentration basis to that at 21 degrees C when circulation rate and dilution with blood are taken into account. Isoleucine and phenylalanine competed with leucine uptake at 10 degrees C as at 21 degrees C; additional competitors at 10 degrees C included histidine, methionine and valine. Fish acclimated to 10 degrees C for 2 weeks or more showed a restoration in maximal leucine uptake and disappearance of histidine inhibition. Methionine inhibition was retained. Three transport systems in this species are discussed: 20-20, operating in 20 degrees C-acclimated fish at 20 degrees C; 20-10, in 20 degrees C-acclimated fish at 10 degrees C; and 10-10, in 10 degrees C-acclimated fish at 10 degrees C. The properties of these systems are compared with the 0-0 system of Antarctic fish and with transport systems of mammalian cells. The latter are similar to our non-acclimated system, 20-10, suggesting that the mammalian cell may not be at a state of optimal temperature adaptation.