IONIC CURRENTS IN SINGLE SMOOTH-MUSCLE CELLS OF THE CANINE RENAL-ARTERY

Membrane currents from single smooth muscle cells enzymatically isolated from canine renal artery were recorded using the patch-clamp technique in the whole-cell and cell-attached configurations. These cells exhibited a mean resting potential, input resistance, membrane time constant, and cell capacitance of -51.8+/-2.1 mV, 5.2+/-0.98 G-OMEGA, 116.2+/-16.4 msec, and 29.1+/-2.0 pF, respectively. Inward current, when elicited from a holding potential of -80 mV, activated near -50 mV, reached a maximum near 0 mV and was sensitive to the dihydropyridine agonist Bay K 8644 and dihydropyridine antagonist nisoldipine. Two components of macroscopic outward current were identified from voltage-step and ramp depolarizations. The predominant charge carrier of the net outward current was identified as K+ by tail-current experiments (reversal potential, -61.0+/-O.8 mV in 10.8 mM [K+]o/140 mM [K+]i). The first component was a small, low-noise, voltage- and time-dependent current that activated between -40 and -30 mV (I(K(dr))), and the second component was a larger, noisier, voltage- and time-dependent current that activated at potentials positive to +10 MV (I(K(Ca))). Both I(K(dr)) and I(K(ca)) displayed little inactivation during long (4-second) voltage steps. I(K(ca)) and I(K(dr)) could be pharmacologically separated by using various Ca2+ and K+ channel blockers. I(K(Ca)) was substantially inhibited by external NiCl2 (500-mu-M), CdCl2 (300-mu-M), EGTA (5 mM), tetraethylammonium (K(i) at +60 mV, 307-mu-M), and charybdotoxin (100 nM) but was insensitive to 4-aminopyridine (0.1-10 mM). I(K(dr)) was inhibited by 4-aminopyridine (K(i) at +10 mV, 723-mu-M) and tetraethylammonium (K(i) at +10 mV, 908-mu-M) but was insensitive to external NiCl2 (500-mu-M), CdCl2 (300-mu-M), EGTA (5 mM), and charybdotoxin (100 nM). Two types of single K+ channels were identified in cell-attached patches. The most abundant K+ channel that was recorded exhibited voltage-dependent activation, was blocked by external tetraethylammonium (250-mu-M), and had a large single-channel conductance (232+/-12 pS with 150 mM K+ in the patch pipette, 130+/-17 pS with 5.4 mM K+ in the patch pipette). The second channel was also voltage dependent, was blocked by 4-aminopyridine (5 mM), and exhibited a smaller single-channel conductance (104+/-8 pS with 150 mM K+ in the patch pipette, 57+/-6 pS with 5.4 mM K+ in the patch pipette). These results suggest that depolarization of canine renal artery cells opens dihydropyridine-sensitive Ca2+ channels and at least two K+ channels. The two time-dependent K+ currents (I(K(ca)) and I(K(dr))) reflect the behavior of two distinct K+ channels. According to the pharmacology of the whole-cell and single-channel experiments, I(K(Ca)) is primarily carried by the large conductance Ca2+-activated K+ channel, and I(K(dr)) is carried by the smaller conductance delayed rectifier K+ channel.