Inhibition of guinea pig detrusor contraction by NS-1619 is associated with activation of BKCa and inhibition of calcium currents

The effects of NS-1619 on bladder contractile function and on transmembrane currents were evaluated in vitro on isolated guinea pig detrusor strips and isolated detrusor myocytes, respectively. In the isolated bladder strip, NS-1619 inhibited KCl-induced contractions in a concentration-dependent manner (IC50 = 12.2 +/- 3.2 mu M). Isolated detrusor myocytes were quiescent and had resting membrane potentials that averaged -45.3 +/- 2.7 mV. With patch-clamp techniques we demonstrated that exposure to 10 to 100 mu M NS-1619 increased an iberiotoxin-sensitive current consistent with the activation of the large conductance calcium-dependent potassium channel (BKCa). Single-channel analysis confirmed that NS-1619 increased the open probability of BKCa channels. NS-1619 also appeared to decrease inward calcium current (I-Ca). After exposure to 30 mu M NS-1619, peak current amplitude significantly decreased by approximately 50%. Analysis of the current voltage relationship revealed a significant decrease in maximal conductance from 10.5 +/- 4 to 6.2 +/- 3 nS. The voltage dependence of calcium current activation and inactivation was well fit by a Boltzmann relationship. Besides the decrease in conductance, there was a small, but significant shift in the half-inactivation voltage, which suggests that NS-1619 preferentially blocks the open state of the channel. Steady-state (window) calcium current was also decreased. Analysis of the theoretical window current revealed a 71% decrease in this noninactivating current. These data indicate that NS-1619 inhibits detrusor smooth muscle contraction in a concentration-dependent manner and that the underlying mechanism of action for this effect involves inhibition of calcium current, and may also include activation of the BKCa channel. Compounds with this profile may be useful in the treatment of bladder instability.