Vol. 283, Issue 3, 1193-1200, 1997

Inhibition of Guinea Pig Detrusor Contraction by NS-1619 Is Associated with Activation of BK_Ca and Inhibition of Calcium Currents

Jeffrey H. Sheldon, N. Wesley Norton and Thomas M. Argentieri

Cardiovascular and Metabolic Disorders, Wyeth-Ayerst Research, Princeton, New Jersey

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 µM). 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 µM NS-1619 increased an iberiotoxin-sensitive current consistent with the activation of the large conductance calcium-dependent potassium channel (BK_Ca). Single-channel analysis confirmed that NS-1619 increased the open probability of BK_Ca channels. NS-1619 also appeared to decrease inward calcium current (I_Ca). After exposure to 30 µM 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 BK_Ca channel. Compounds with this profile may be useful in the treatment of bladder instability.