Identification of large conductance calcium activated potassium channel accessory beta4 subunit in rat and mouse bladder smooth muscle.

PURPOSE: The BK (large conductance voltage and Ca(2+) activated K(+)) channel is a key regulator of bladder smooth muscle contractility. To our knowledge in bladder smooth muscle the BK channel pore forming alpha subunit BKalpha associates in homotetramers with 4 regulatory smooth muscle specific beta1 subunits. We challenged this concept in identify whether other regulatory BKbeta subunits exist in mouse and rat bladder smooth muscle.
MATERIALS AND METHODS: We used a novel approach with single cell reverse transcriptase-polymerase chain reaction combined with immunocytochemical studies in freshly isolated mouse and rat bladder smooth muscle cells. Western blot was also performed.
RESULTS: Reverse transcriptase-polymerase chain reaction identified the mRNA expression of various BK channel subunits in freshly isolated bladder smooth muscle cells. Our data indicate that, in addition to BKalpha and BKbeta1, neuronal specific BKbeta4 is expressed in mouse and rat bladder smooth muscle cells. BKbeta4 expression was also revealed by Western blot. Immunocytochemistry was further applied to confirm the specific expression of BKbeta4 protein directly in freshly isolated mouse and rat bladder smooth muscle cells.
CONCLUSIONS: To our knowledge we performed the first comprehensive examination of the expression of BKalpha and BKbeta subunits in bladder smooth muscle. We identified that the bladder smooth muscle BK channel has a distinctive architecture involving pore forming BKalpha and regulatory BKbeta1/beta4. Further studies of the functional roles of BKalpha, BKbeta1 and BKbeta4 directly in human bladder smooth muscle may help the development of alternative therapeutic strategies to control bladder dysfunction. New drugs targeting specific BK channel subunits in human bladder smooth muscle may prove useful for overactive bladder.