The effect of cromakalim on the smooth muscle of the guinea‐pig urinary bladder

1 The actions of cromakalim were studied on the detrusor muscle from guinea‐pig urinary bladder. Cromakalim reduced the frequency and amplitude of spontaneous contractile activity of the smooth muscle of the guinea‐pig urinary bladder at 5 × 10−8 m and abolished the activity at concentrations above 5 × 10−7 m. 2 Electrophysiological experiments demonstrated that cromakalim increased membrane conductance, caused a dose‐dependent hyperpolarization of the cell membrane and loss of spike activity. These events are consistent with the opening of K+ channels. 3 The effects of 10−6 m and 10−5 m cromakalim on the contractile responses to carbachol, potassium and transmural nerve stimulation were studied. Cromakalim did not prevent the detrusor from responding to these agents, although it significantly reduced the contractile response to K+ at concentrations below 70 mm. 4 Uptake and efflux experiments using 86Rb+ were unable to demonstrate any significant effect on transmembrane movement produced by cromakalim (10−5 m). 5 43K+ efflux showed a dose‐dependent increase in the rate constant on addition of cromakalim. The difference in the selectivity for K+ over Rb+ was confirmed in dual label uptake experiments. 6 Substitution experiments in which the K+ ions in the tissue were gradually replaced by Rb+ demonstrated that cromakalim had a progressively decreasing effect on spontaneous activity as internal K+ was lowered. When all the K+ was replaced by Rb+, cromakalim no longer inhibited spontaneous activity, confirming that the channel opened by cromakalim appears relatively impermeant to Rb+.