Effect of nitric oxide on calcium-activated potassium channels in colonic smooth muscle of rabbits

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates K(Ca) channels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while K(Ca) current and the activity of Kca channels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from -59 to -73 mV (n = 8 cells, P < 0.01). In the on-cell patch recording configuration, NO (0.5% or 1%) in the bath increased NP(o) of K(Ca) channels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NP(o) of K(Ca), channels. In the on- cell patch recording configuration, methylene blue (1 μM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NP(o) of K(Ca) channels. NP(o) was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 μM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K⁺ current by activating K(Ca) channels through a cGMP pathway.