The present paper examines the effects of reduced transepithelial Na transport (J(na)) on membrane electrophysiological parameters in proximal convoluted tubules and the possible role of cytosolic calcium concentration ([Ca](i)) in the regulation of basolateral membrane K conductance (G(K)). When J(Na) was reduced by elimination of glucose and alanine and replacement of 100 mM sodium with N-methyl-D-glucamine from the luminal perfusate, basolateral membrane potential (V(BL)) hyperpolarized transiently by 12.6 mV and the ratio of apical to basolateral membrane resistance (R(A)/R(BL)) doubled. The apparent transference number for K at the basolateral membrane (G(K)/G(cell)) decreased from 0.13 to 0.08 in the first 4 min following reductions in J(Na). The elimination of Na-alanine and Na-glucose cotransport was responsible for the initial hyperpolarization and increase in R(A)/R(BL), whereas the resultant decrease in the cellular concentrations of glucose and alanine, together with the reductions in G(K), could elicit the secondary V(BL) depolarization. Measurement of [Ca](i) with the fluorescent probe fura-2 during reductions in J(Na) revealed that [Ca](i) increased by an average of 12%, a value very similar to the average reduction in cellular volume (13%) measured using morphometric techniques. The observation that [Ca](i) increased while G(K) was decreasing is inconsistent with the effect of [Ca](i) on putative basolateral Ca-activated K channel. We believe that [Ca](i) changes passively (at least in the first few minutes) in response to a decrease in cell volume occurring as a consequence of reduction in J(Na) and that some as yet unidentified volume-sensitive mechanism is responsible for the regulation of G(K).