THE EFFECT OF CROMAKALIM ON THE ELECTRICAL-PROPERTIES OF AND [RB-86+]EFFLUX FROM NORMAL AND HYPERTROPHIED RAT BLADDER

1. The activity of smooth muscle strips from normal and hypertrophied rat bladders was compared. The hypertrophied bladders were produced by partially obstructing the urethra 8-13 weeks previously. 2. Spontaneo us mechanical activity was more frequent and smaller in amplitude in s trips from normal than hypertrophied bladders and was less sensitive t o cromakalim, being reversibly abolished by cromakalim at 10(-6) mol/L compared with 10(-7) mol/L for hypertrophied bladder. 3. The mean res ting membrane potentials of smooth muscle cells from normal and hypert rophied rat bladders were -47.2 and -47.6 mV, respectively. Bursts of spontaneous action potentials, corresponding to the mechanical activit y, were seen in some cells. 4. Nifedipine at 10(-6) mol/L had no signi ficant effect on the resting membrane potential. Occasional single spi kes occurred with increased duration and the afterhyperpolarization wa s abolished. Cromakalim at 10(-5) mol/L produced hyperpolarization of 3-9 mV and, in the continued presence of the drug, occasional singe sp ikes could be recorded from both normal and hypertrophied bladders. 5. Nifedipine at 10(-6) mol/L abolished movement but did not significant ly alter [Rb-86+] efflux from strips from either normal or hypertrophi ed bladders. Addition of cromakalim at 5 X 10(-6) or 5 X 10(-5) mol/L in the presence or absence of nifedipine increased efflux from the nor mal bladder by 30-40%. In the hypertrophied bladder the efflux increas ed by about 14% and 28% in the presence of 5 X 10(-6) and 5 X 10(-5) m ol/L cromakalim, respectively. 6. No significant differences between e lectrical properties of normal and hypertrophied bladders were therefo re found and we are unable to explain why the mechanical activity of h ypertrophied rat bladders was more sensitive to cromakalim than normal bladders.