Inhibitory effects of NS-21, a novel drug for urinary incontinence, and its active metabolite, RCC-36, on L-type calcium currents in isolated guinea pig detrusor smooth muscle cells
1. By means of two patch-pipettes, isolated urinary bladder myocytes were longitudinally stretched up to 20% beyond slack length (delta L = 20%). 2. Experiments were conducted using both voltage and current clamp configurations. In current clamped cells at 23 degrees C, delta L depolarized the membrane from -50 to ca -15 mV, the amplitude of depolarization increasing with the extent of delta L. At 36 degrees C, delta L induced action potentials or increased the frequency of spontaneous action potentials. 3. In voltage clamped cells at a holding potential of -50 mV, stretch induced an inward current (Iin) and increased the input conductance. Both effects increased with delta L. They were blocked by 40 microM gadolinium, suggesting stretch activation of non-selective cation channels (SACs) as the underlying mechanism. 4. Stretch-induced difference currents rectified outwardly and reversed at a reversal potential (Erev) of -28 +/- 10 mV. Twenty millimolar [TEA]o suppressed the rectification and shifted Erev to 0 +/- 1 mV. The result suggests that stretch can activate not only SACs but also TEA-sensitive K+ channels. 5. Stretch changed the net current due to clamp steps to 0 mV as though it increased the potassium current (IK) and reduced the calcium current (ICa). While 20 mM intracellular BAPTA did not modify the stretch-induced whole-cell inward current (Iin) at -50 mV, it suppressed the stretch effects on IK and ICa as if these effects were mediated by an increase in the subsarcolemmal Ca2+ concentration. 6. The results support the hypothesis that longitudinal stretch can activate SACs and Ca2+ influx through them. In non-clamped cells, stretch can also modulate Ca2+ influx through L-type Ca2+ channels via changes in membrane potential.