IDENTIFICATION OF A STRETCH-ACTIVATED MONOVALENT CATION CHANNEL FROM TELEOST URINARY-BLADDER CELLS

The urinary bladder of euryhaline teleosts is an important osmoregulatory organ which absorbs Na+, Cl-, and water from urine. Using patch clamp technique, single stretch-activated channels, which were permeable to K+ and Na+ (P(Na)/P(K) almost-equal-to 0.75) and had conductances of 55 and 116 pS, were studied. In excised, inside-out patches which were voltage-clamped in the physiological range of membrane potential, the single-channel open probability (P(o)) was low (almost-equal-to 0.02), and increased to a maximum of 0.9 with applied pipette suction. Single-channel conductance also increased with suction. The channels showed adaptation to applied suction and relaxed to a steady-state activity about 20 seconds after application of suction. The P(o) increased up to 0.9 with strong membrane depolarization (V(m) = 0 to +80 mV); however, there was little dependence of P(o) on membrane potential in the physiological range. The kinetic data suggest that there is one conducting state and at least two non-conducting states of the channel. The open-time constant increased with suction but remained unchanged with membrane potential (V(m) = -70 to +60 mV). The mean closed-time of the channel decreased with suction and membrane depolarization. These results demonstrate the presence of an non-selective monovalent cation channel which may be involved in cell volume regulation in the goby urinary bladder. Additionally, this channel may function as an enhancer of Na+ influx and K+ efflux across the bladder cell as part of transepithelial ion transport if it is located in apical membrane.