Patch-clamp experiments in cell-attached (c/a) and inside-out (i/o) configurations were performed to directly observe ionic channels in lateral membranes of macula densa (MD) cells from rabbit kidney. In the presence of 140 mM KCl in the pipette and normal Ringer solution in the bath, we repeatedly observed in c/a and in i/o configurations a 20- to 23-pS channel with a linear current-voltage (I-V) relationship reversing near 0 mV. Ionic replacement in the bath solution clearly indicated a cationic selectivity but with equal permeability for Na+ and K+. Single-channel kinetics was characterized by higher open probability at positive membrane potentials. In i/o experiments, elimination of bath Ca2+ (<or=1 microM) abolished channel activity in a reversible manner. This MD nonselective cationic channel was found to display a certain Ca2+ permeability because single-channel events could be detected when the pipette potential was very negative (-60, -80, and -100 mV) in the presence of 73 mM CaCl2 in the bath solution. The similarities between this channel and some channels of the transient receptor potential family suggest a possible role for this MD basolateral channel in controlling membrane potential and regulating Ca2+ entry during MD cell signaling.