Glycyl-L-sarcosine transport by ATP-depleted isolated enterocytes from chicks

Normally energized and ATP-depleted isolated chicken enterocytes have been used to investigate the energetics of intestinal glycyl-L-sarcosine (Gly-Sar) transport, and the results were compared with those obtained for 3-O-methyl-D-glucose (3-OMG) active transport, which is known to be energized by the electrochemical Na+ gradient. The results show that even though Gly-Sar had no effect on Na+ efflux from labeled (22Na) enterocytes, 20 mM L-leucine and 20 mM 3-OMG increased the rate constant of Na+ efflux by 34 and 76%, respectively. In ATP-depleted cells 3-OMG was accumulated in response to experimentally imposed inward Na+ gradients; however, Gly-Sar was not accumulated. In the absence of Na+, an inward proton gradient slightly increased Gly-Sar uptake, although no transient accumulation was observed. An increase in membrane potential by replacement of Cl- by NO3- or by imposition of an outward K+ gradient in the presence of valinomycin stimulated both 3-OMG and Gly-Sar uptake, even in the absence of either H+ or Na+ gradient. When in addition to a K+ diffusion potential, a Na+ gradient was present, 3-OMG transient accumulation was further increased (approximately 2-fold over equilibrium value), and Gly-Sar uptake showed the same pattern as that observed when only membrane potential was present. However, in the presence of an outward K+ diffusion potential and an inward H+ gradient Gly-Sar was stimulated approximately threefold, clearly showing a significant overshoot, whereas 3-OMG was not transiently accumulated.