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

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Abstract
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. Spontaneous mechanical activity was more frequent and smaller in amplitude in strips from normal than hypertrophied bladders and was less sensitive to cromakalim, being reversibly abolished by cromakalim at 10(-6) mol/L compared with 10(-7) mol/lL for hypertrophied bladder. 3. The mean resting membrane potentials of smooth muscle cells from normal and hypertrophied rat bladders were -47.2 and -47.6 mV, respectively. Bursts of spontaneous action potentials, corresponding to the mechanical activity, were seen in some cells. 4. Nifedipine at 10(-6) mol/L had no significant effect on the resting membrane potential. Occasional single spikes occurred with increased duration and the after hyperpolarization was abolished. Cromakalim at 10(-5) mol/L produced hyperpolarization of 3-9 mV and, in the continued presence of the drug, occasional singe spikes could be recorded from both normal and hypertrophied bladders. 5. Nifedipine at 10(-6) mol/L abolished movement but did not significantly alter [86Rb+] efflux from strips from either normal or hypertrophied 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 normal bladder by 30-40%. In the hypertrophied bladder the efflux increased by about 14% and 28% in the presence of 5 x 10(-6) and 5 x 10(-5) mol/L cromakalim, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)