AIMS: To investigate the activity of the T-type calcium channel (TCC) and the expression of its subtypes in overactive detrusor (OD) myocytes in rats after partial bladder outflow obstruction (PBOO). METHODS: Thirteen male Wistar rats with OD after PBOO (OD group) and eight sham-operated rats (control group) were studied. The two groups were compared regarding the expression of TCC subtype genes by reverse transcription-polymerase chain reaction (RT-PCR) and the TCC kinetics and cell action potential by whole-cell patch-clamp. RESULTS: The time course and density of the current were significantly higher in the OD cells than those in the control detrusor. Whole-cell patch-clamp analysis showed that the activation of TCCs in detrusor myocytes in the OD group was faster than the control group, but inactivation was almost the same in both groups, suggesting a significant enhancement of the Ca(2+) "window" current in the OD group. Patch-clamp recording of action potentials in the OD cells indicated an increase in excitability and a decrease in the repolarization interval. RT-PCR assay showed an abnormal expression of alpha1G subtype in the OD cells. CONCLUSIONS: TCCs could be one of the crucial factors for the abnormal excitation in OD cells. The development of OD after PBOO presumably relates to the increase in TCC current in the bladder cells, the enhancement of the Ca(2+) "window" current for Ca(2+) inflow, the prolongation of the intracellular calcium oscillations, and the acceleration of the cell depolarization. (c) 2007 Wiley-Liss, Inc.