Adaptive responses to Na(+)-coupled solute transport and osmotic cell swelling in salamander proximal tubule.

Measurements of basolateral membrane potential and relative K+ conductance were performed in isolated perfused proximal tubules from Ambystoma. To investigate adaptive increases in basolateral membrane K+ conductance (gK) associated with Na(+)-solute cotransport, measurements were made comparing transport of glucose and alanine, with changes caused by hypotonicity- and solute-induced cell swelling. Luminal perfusion with alanine produced results consistent with an adaptive increase in gK; perfusion with glucose failed to show this response. Hypotonic peritubular solutions also produced results consistent with an adaptive increase in gK, but isosmotic increases of peritubular glucose sufficient to swell the cells failed to produce this. No changes in the responses to luminal perfusion with alanine or glucose were induced by hypotonic peritubular solutions. With a high concentration of glucose in isosmotic peritubular solutions, perfusion of the lumen with glucose now produced results consistent with an adaptive increase in gK. Isosmotic peritubular solutions containing urea produced adaptive changes similar to those observed using hypotonic peritubular solutions, but when glucose was subsequently added to the lumen, no further adaptive response occurred. We conclude that cell swelling alone is insufficient to explain the mechanisms involved in the adaptive responses of gK occurring during Na(+)-solute cotransport in the salamander proximal tubule.