Two populations of smooth muscle cells in the guinea-pig gastric antrum.

K+ outward currents (I[K]) expressed by guinea-pig antral smooth muscle cells were studied using the whole-cell voltage-clamp technique. In about 88% of cells depolarization steps applied from Vh = -70 mV activated a fast transient component (I[K(to)]) with voltage-dependent characteristics, and a noninactivating component with slow activation kinetics (I[K(sl)]). Both components were carried by K+ ions. Apamin (10 nM to 1 microM) selectively depressed I(K[to]) in a concentration-dependent manner. I(K(sl)) was blocked by 1 mM tetraethylammonium or 0.1 microM charybdotoxin. 10 mM tetraethylammonium abolished both components of I(K). Nicardipine (1 microM) did not affect the voltage- and time-dependent characteristics of the net I(K), but reduced the current density of I(K[sl]) from 22.36+/-1.38 microA/cm2 to 13.06+/-0.92 microA/cm2 at +40 mV. In about 12% of the cells depolarization-evoked I(K) could be separated as two pharmacologically distinguishable components: a glipizide-sensitive current (forming about 70% of the net I[K]) and a charybdotoxin-sensitive current (30% of the net I[K]). Nicardipine (1 microM) affected neither the amplitude nor the time-course of I(K) of this cell population. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM) or ryanodine (1 microM) led to a 50-200% increase of I(K[sl]) in the majority of cells and to an about 30% increase of the net I(K) in 12% of cells. The data obtained suggest the existence of at least two populations of cells in guinea-pig antral smooth muscle. Twelve percent of cells seem to be responsible for the generation of slow wave potentials, while 88% of cells most probably respond passively to the electrotonically spread depolarization.