Active sugar transport in renal cortex cells: The electrolyte requirement

Abstract

The electrolyte requirement for the active transport of α-methyl- d-glucoside, d-galactose, 2-deoxy- d-glucose and 2-deoxy-α-methyl- d-galactose was investigated using slices of rabbit kidney cortex: 1. 1. As compared with values obtained at [ Na +] 0 = 0 , the presence of external Na + (128 mM) increased the apparent v max of d-galactose transport without affecting the transport K m . 2. 2. The Na + requirement for the transport of α-methyl- d-glucoside and d-galactose was not saturated at 128 mM Na +. Within the range of [Na +] 0 25–128 mM, indications of a single Na + entering a rate-limiting event in α-methyl- d-glucoside transport were found. 3. 3. A significant fraction of the d-galactose transport was found to be Na + independent and insensitive to 2 mM ouabain. 4. 4. As compared with values obtained at [ K +] 0 = 0 , the presence of 0.5–7 mM K + activated the transport of α-methyl- d-glucoside and d-galactose; this K + stimulation was saturated at [K +] 0 1 mM and affected the v max , rather than the K m , of sugar transport. No such effect of [K +] 0 on the transport of both 2-deoxyhexoses was observed. 5. 5. The absence of external Cl − had no effect on the transport of the four sugars tested. 6. 6. The absence of saline Ca 2+ markedly depressed both the influx and the steady-state accumulation level of all four sugars tested; the efflux of d-galactose and 2-deoxy- d-galactose was accelerated. This Ca 2+ stimulation of the sugar transport was saturated at [Ca 2+] 0 0.5 mM and affected the v max rather than the K m of sugar transport. The Ca 2+ effect was found to be fairly specific for this ion: whereas Sr 2+, Ba 2+ and, to some extent, Mn 2+ could replace Ca 2+ in its effect on the sugar transport, Mg 2+ and La 3+ wer ineffective. 7. 7. The above results are discussed in relation to hypotheses put forward to explain the mechanism of the Na +-dependent nonelectrolyte transport in some animal cells. It is concluded that: (a) In kidney cortex cells, the presence of Na + (external and intracellular) is not mandatory to bring about an active transport of 2-deoxyhexoses and also a fraction of galactose; (b) the K + stimulation of sugar transport is related to the Na + requirement for sugar transport; (c) Ca 2+ stimulates a common step in the transport of all sugars tested.