Comparative Studies on the Relaxing Action of Several Adenosine 5'-Triphosphate-Sensitive K⁺ Channel Openers in Pig Urethra

Abstract

The relaxing effects of the adenosine 5'-triphosphate (ATP)-sensitive K⁺ channel openers (K_ATP openers ; diazoxide, minoxidil, pinacidil, (±)-cromakalim, (+)-cromakalim and (-)-cromakalim) were investigated on the resting tone of pig proximal urethra. In addition, patch-clamp techniques were utilized for recording cromakalim-induced ionic currents in cells dispersed from the same urethral region. The (-)-cromakalim-induced relaxation of urethral muscle strips was stable, reversible and reproducible. The rank order of potency regarding of K_ATP openers in lowering the resting urethral tone was (-)-cromakalim > pinacidil> diazoxide > minoxidil. K_ATP opener-induced urethral relaxation was suppressed by subsequent application of glibenclamide (1μM).(+)-Cromakalim (>10μM) did not relax the urethra nor antagonize the (-)-cromakalim-induced urethral relaxation.However, at higher concentrations, (+)-cromakalim (>30μM) caused a small but significant urethral relaxation. In accordance with these observations, the relaxation induced by 5μM (-)-cromakalim was identical to that induced by 10μM (±)-cromakalim, as expected from a theoretical half potency for (±)-cromakalim. In whole-cell recording, (-)-cromakalim and (+)-cromakalim (100μM) activated a glibenclamide-sensitive outward current which was due to the activation of the glibenclamide-sensitive 43 pS K⁺ channel (K_GS-43 pS). The potency of (+)-cromakalim to activate K_GS-43 pS was much weaker than that of (-)-cromakalim. These results indicate that the ability of K_ATP openers to relax pig urethral smooth muscle can be accounted for by activation of glibenclamide-sensitive K⁺ channels.