Previous studies in isolated, in vitro perfused rectal gland tubules (RGT) have revealed that the basolateral membrane possesses a K+ conductive pathway. In the present study, we have utilized the patch clamp technique in RGT segments to characterize this pathway. The basolateral membrane was approached with patch pipettes at the open end of in vitro perfused segments. Recordings were obtained in cell-attached as well as in excised inside-out patches. In cell-attached patches with the pipette filled with a KCl solution (274 mmol/l) and the bath containing NaCl shark Ringer (275 mmol/l), inward K+ currents (from pipette into cell) with a mean slope conductance of 123 +/- 26 pS (n = 3) were observed. We were unable to generate outward K+ currents at high depolarizing (cell more positive) clamp voltages. This indicates inward rectification of this channel. To examine the rectification properties further, excised (inside out) patches were exposed to K+ concentration gradients, directed out of, as well as into the pipette. With NaCl in the pipette and KCl in the bath, K+ outward currents were observed. The current-voltage (IV) relation revealed Goldman-type rectification, with a mean single channel conductance of 185 +/- 28 pS (n = 7) at high positive voltages (linear range of the IV curve). The single-channel permeability coefficient for K+ was 0.26 +/- 0.04 X 10(-12) cm3/s (n = 7). In the reversed experiment (pipette KCl, bath NaCl), inward currents of similar kinetics and amplitude were obtained. The single channel conductance was 146 +/- 21 pS (n = 7) at high negative voltages (linear range of the IV curve).(ABSTRACT TRUNCATED AT 250 WORDS)