Voltage-gated K(+) channels sensitive to stromatoxin-1 regulate myogenic and neurogenic contractions of rat urinary bladder smooth muscle

Members of the voltage-gated K(+) (K(V)) channel family are suggested to control the resting membrane potential and the repolarization phase of the action potential in urinary bladder smooth muscle (UBSM). Recent studies report that stromatoxin-1, a peptide isolated from tarantulas, selectively inhibits K(V)2.1, K(V)2.2, K(V)4.2, and K(V)2.1/9.3 channels. The objective of this study was to investigate whether K(V) channels sensitive to stromatoxin-1 participate in the regulation of rat UBSM contractility and to identify their molecular fingerprints. Stromatoxin-1 (100 nM) increased the spontaneous phasic contraction amplitude, muscle force, and tone in isolated UBSM strips. However, stromatoxin-1 (100 nM) had no effect on the UBSM contractions induced by depolarizing agents such as KCl (20 mM) or carbachol (1 microM). This indicates that, under conditions of sustained membrane depolarization, the K(V) channels sensitive to stromatoxin-1 have no further contribution to the membrane excitability and contractility. Stromatoxin-1 (100 nM) increased the amplitude of the electrical field stimulation-induced contractions, suggesting also a role for these channels in neurogenic contractions. RT-PCR experiments on freshly isolated UBSM cells showed mRNA expression of K(V)2.1, K(V)2.2, and K(V)9.3, but not K(V)4.2 channel subunits. Protein expression of K(V)2.1 and K(V)2.2 channels was detected using Western blot and was further confirmed by immunocytochemical detection in freshly isolated UBSM cells. These novel findings indicate that K(V)2.1 and K(V)2.2, but not K(V)4.2, channel subunits are expressed in rat UBSM and play a key role in opposing both myogenic and neurogenic UBSM contractions.