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-C a channels of circular smooth muscle of the rabbit colon. Transmural b iopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell con figuration, as well as in on-cell and excised inside-out patch re cording configurations, while K-Ca current and the activity of K-Ca ch annels, 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 reco rding configuration, NO (0.5% or 1%) in the bath increased NP0 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 NP0 of K-Ca channels. In the on-cell patch record ing configuration, methylene blue (1 mu M) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPo of K-Ca channels. NPo was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, an d zaprinast (100 mu M), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+ current by activa ting K-Ca channels through a cGMP pathway.