Cellular heterogeneity was examined in the hamster medullary thick ascending limb (MAL) perfused in vitro by electrophysiological measurements with an intracellular microelectrode. Random measurements of fractional resistance of basolateral membrane (RfB) revealed two cell populations, high basolateral conductance (HBC) cells having RfB of 0.05 +/- 0.01 (n = 24) and low basolateral conductance (LBC) cells having RfB of 0.80 +/- 0.03 (n = 32). Basolateral membrane potentials (VB) were not different between HBC cells and LBC cells (-72.6 +/- 1.2, n = 43 vs. -70.0 +/- 1.2, n = 35). Addition of 2 mmol/l Ba2+ to the bath depolarized the basolateral membrane in the HBC cells from -70.4 +/- 3.2 to -20.9 +/- 5.9 mV (n = 8) but not in the LBC cells (from -74.4 +/- 1.9 to -72.0 +/- 2.1 mV). Increasing K+ or decreasing Cl- in the bathing solution caused marked positive deflection of VB in the HBC cells but little or no change in VB in the LBC cells. Elimination of Cl- from the lumen or addition of furosemide to the lumen enhanced the potential response of the HBC cells to basolateral application of Ba2+. Accordingly, with Ba2+ present in the bath, the potential response of the HBC cells to a decrease in bath Cl- concentration was enhanced. These observations suggest that a K+ conductance exists in the basolateral membrane of HBC cells in parallel with a Cl- conductance. The basolateral cell membrane of LBC cells also contains a Cl- conductance.(ABSTRACT TRUNCATED AT 250 WORDS)