Objective: To investigate the changes in spontaneous bladder smooth muscle contractions that occur during detrusor instability (DI), and to test the possibility that altered function or expression of ryanodine receptors (RyRs) could account for the increased bladder contractions.
Materials and methods: After 8 weeks of partial bladder outlet obstruction, DI was confirmed in female experimental rats by filling cystometry. Muscle strips were dissected from freshly isolated bladders, and isometric tension recorded in strips from DI and normal bladders. The contractions were recorded during electrical stimulation or exposure to various agents. Western blot analysis was used to determine RyR expression in DI and normal bladder muscle.
Results: In DI bladder muscle, spontaneous contractile activity persisted in the presence of blockers for known neurotransmitter receptors in the bladder wall. The RyR blocker ryanodine significantly increased the spontaneous contractile frequency in normal bladder strips, but failed to affect spontaneous contractions in DI muscle. Caffeine inhibited spontaneous contractile activity in both the DI and normal strips. After administering the l-type Ca(2+) channel antagonist nimodipine, the myogenic contractile activity was abolished in normal strips; in contrast, in DI strips, the amplitude of contractions was reduced but the frequency of contractions was unchanged. Western blot analysis showed that RyR expression was lower in DI muscle than in normal bladder muscle.
Conclusion: These results provide the first characterization of a loss of regulation of spontaneous contractile activity by RyRs in DI muscle associated with a significant decrease in RyR expression. RyRs in normal detrusor muscle act as negative-feedback regulators of spontaneous contractile activity, presumably by releasing Ca(2+) that activates Ca(2+)-dependent K(+) channels to decrease contractility. This mechanism might be weakened in DI muscle, resulting in spontaneous contractile overactivity.