Identification of Large Conductance Calcium Activated Potassium Channel Accessory β4 Subunit in Rat and Mouse Bladder Smooth Muscle

Purpose: The BK (large conductance voltage and Ca2+ activated K+) channel is a key regulator of bladder smooth muscle contractility. To our knowledge in bladder smooth muscle the BK channel pore forming a subunit BK alpha associates in homotetramers with 4 regulatory smooth muscle specific beta 1 subunits. We challenged this concept in identify whether other regulatory BK beta 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 BK alpha and BK beta 1, neuronal specific BK beta 4 is expressed in mouse and rat bladder smooth muscle cells. BK beta 4 expression was also revealed by Western blot. Immunocytochemistry was further applied to confirm the specific expression of BK beta 4 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 BK alpha and BK beta subunits in bladder smooth muscle. We identified that the bladder smooth muscle BK channel has a distinctive architecture involving pore forming BKa and regulatory BK beta 1/beta 4. Further studies of the functional roles of BK alpha, BK beta 1 and BK beta 4 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.