Properties of a tonically active, sodium-permeable current in mouse urinary bladder smooth muscle

Urinary bladder smooth muscle (UBSM) elicits depolarizing action potentials, which underlie contractile events of the urinary bladder. The resting membrane potential of UBSM is approximately -40 mV and is critical for action potential generation, with hyperpolarization reducing action potential frequency. We hypothesized that a tonic, depolarizing conductance was present in UBSM, functioning to maintain the membrane potential significantly positive to the equilibrium potential for K(+) (E(K); -85 mV) and thereby facilitate action potentials. Under conditions eliminating the contribution of K(+) and voltage-dependent Ca(2+) channels, and with a clear separation of cation- and Cl(-)-selective conductances, we identified a novel background conductance (I(cat)) in mouse UBSM cells. I(cat) was mediated predominantly by the influx of Na(+), although a small inward Ca(2+) current was detectable with Ca(2+) as the sole cation in the bathing solution. Extracellular Ca(2+), Mg(2+), and Gd(3+) blocked I(cat) in a voltage-dependent manner, with K(i) values at -40 mV of 115, 133, and 1.3 microM, respectively. Although UBSM I(cat) is extensively blocked by physiological extracellular Ca(2+) and Mg(2+), a tonic, depolarizing I(cat) was detected at -40 mV. In addition, inhibition of I(cat) demonstrated a hyperpolarization of the UBSM membrane potential and decreased the amplitude of phasic contractions of isolated UBSM strips. We suggest that I(cat) contributes tonically to the depolarization of the UBSM resting membrane potential, facilitating action potential generation and thereby a maintenance of urinary bladder tone.