Spontaneous activity of mouse detrusor smooth muscle and the effects of the urothelium

Aims: To characterize the detrusor muscle of the mouse urinary bladder in order to understand more precisely spontaneous contractile behavior of this organ. This study examined the spontaneous electrical activity and Ca 2+ dynamics of the detrusor smooth muscle and investigated the role of the urothelium. Materials and Methods: Detrusor smooth muscle strips were isolated from mouse bladders. The urothelium was either kept intact or removed. Changes in membrane potential were recorded using sharp electrode intracellular recording. To image Ca2+ dynamics, tissue strips were exposed to 10 μM Oregon Green 488 BAPTA-1 AM for 70 min, and then image series were acquired with a laser-scanning confocal microscope. Results: (1) Mouse detrusor smooth muscle cells (SMCs) generate nifedipine-sensitive spontaneous action potentials (sAPs) at a low frequency (1.3 ± 0.9 min-1, n = 11) in preparations with intact urothelium. This frequency increased when the urothelium was removed (7 ± 8.3 min-1, n = 17) (P < 0.05, Student's t test). (2) Frequent ATP-mediated spontaneous depolarizations were recorded in all cells. (3) The frequency of whole cell Ca2+ flashes of detrusor smooth muscle cells was higher in preparations with the urothelium removed (median 1.2 min-1, n = 7) than in urothelium denuded preparations (median 0.6 min-1, n = 7) (P < 0.01, Mann-Whitney U-test). Conclusions: Spontaneous activity of the mouse detrusor smooth muscles was characterized enabling future comparative work on gene knock-out strains. Evidence suggesting release of an inhibitory factor by the urothelium was apparent.