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 ≈ 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 (P₀) was low (≈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 P₀ increased up to 0.9 with strong membrane depolarization (V_m = 0 to + 80 mV); however, there was little dependence of P₀ 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 a 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.