Increasing the medium osmolality, with a non‐ionic osmoticant, from control (289 mOsm) to 319 mOsm or 344 mOsm in the lumbrical muscle cell of the mouse, resulted in a depolarization of the membrane potential (Vm) of 5.9 mV and 10.9 mV, respectively. In control medium, the blockers of chloride related cotransport bumetanide and furosemide, induced a hyperpolarization of −3.6 and −3.0 mV and prevented the depolarization due to hypertonicity. When bumetanide was added in hypertonic media Vm fully repolarized to control values. In a medium of 266 mOsm, the hyperpolarization by bumetanide was absent. At 344 mOsm the half‐maximal effective concentration (IC50) was 0.5 μm for bumetanide and 21 μm for furosemide. In solutions containing 1.25 mm sodium the depolarization by hypertonicity was reduced to 2.3 mV. Reducing chloride permeability, by anthracene 9 carboxylic acid (9‐AC) in 289 mOsm, induced a small but significant hyperpolarization of −2.6 mV. Increasing medium osmolality to 344 mOsm enlarged this hyperpolarization significantly to −7.6 mV. In a solution of 344 mOsm containing 100 μm ouabain, the bumetanide‐induced hyperpolarization of Vm was absent. The results indicate that a Na‐K‐2Cl cotransporter is present in mouse lumbrical muscle fibre and that its contribution to Vm is dependent on medium osmolality. British Journal of Pharmacology (1997) 120, 39–44; doi:

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