Electrogenic Na+ transport in a crustacean coxal receptor

1. The response of the coxal receptors of the crab Scylla serrata to step stretches consisted of a partial action potential, Va, followed by a steady-state depolarization, Vs. The input resistance of the fibre was reduced during Vs. 2. In the absence of stimulation, the dendrites of the receptors depolarized when external Na+ was substituted with choline or Li+, and when the external K+ concentration was increased or decreased. The dendrites also depolarized when ouabain was added to the saline. 3. The amplitude of both Va and Vs was dependent on external Na+. In cells which were depolarized by ouabain, the amplitude of Vs increased when the K+ concentration of the saline was reduced. 4. Vs was followed by a small, but long-lasting, after-potential which was depolarizing when the membrane potential was between -70 and -60 mV. In cells depolarized by ouabain or by low K+ saline, the after-potential became hyperpolarizing. 5. When trains of brief stretches (each 5 ms in duration) were used as stimuli, the cells responded with trains of Va responses. During this tetanic stimulation the cells hyperpolarized; cessation of the stimulus train was followed by a long-lasting hyperpolarization (PTH). 6. PTH was abolished in Li+ saline, in low K+ saline, and in the presence of ouabain. In control or in low K+ saline, PTH was not accompanied by a decrease in the input resistance of the fibres. 7. It is concluded that an electrogenic Na+ pump (or equivalent process) contributes a substantial fraction of the membrane potential of the unstimulated coxal receptors. Pump activity could be increased by Na+-loading the distal part of the cells with trains of Va responses. By contrast, during the steady-state response to stretch, the pump was not activated.