The ionic basis of oscillatory responses of skate electroreceptors

When physiological conditions are simulated, skate electroreceptors produce small maintained oscillatory currents. Larger damped oscillations of similar time-course are observed in voltage clamp. Subtraction of leakage in voltage clamp data shows that the oscillations involve no net outward current across the lumenal surface of the epithelium. The oscillations are much faster than the late outward current generated by the lumenal membranes of the receptor cells. Treatment of the basal surface of the epithelium with tetraethyl ammonium (TEA), high K, Co, or EGTA reversibly blocks the oscillations in voltage clamp, but has little or no effect on the epithelial action potential in current clamp or on the current-voltage relation. The TEA sensitivity of the oscillations indicates that they involve a potassium conductance in the basal membranes of the receptor cells. Treatment of the basal membranes with TEA and high calcium, with strontium, or with barium causes these membranes to produce large regenerative responses. Direct stimulation of the basal membranes then elicits a lumen-positive action potential whereas stimulation of the lumenal membranes elicits a diphasic action potential. Excitability of the basal membranes is abolished by extracellular Co, Mn, or La. Modulation of the lumenal membrane calcium conductance by the basal membrane conductances probably gives rise to the oscillatory receptor currents evoked by small voltage stimuli. The slower calcium-activated late conductance in the lumenal membranes may be involved in sensory accommodation.