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Section of Electrophysiology, National Institute on Alcohol Abuse and Alcoholism, Rockville, MD 20852.

1. Membrane currents, particularly the outward components, were studied in pineal cells acutely dissociated from adult rats using the whole-cell variant of the patch-clamp technique. 2. In current clamp, outward constant current elicited a transient graded depolarizing response. A sustained membrane rectification developed within 20 ms; this phenomenon was reduced in cells internally dialysed with 120 mM-CsCl. 3. Study of the membrane current revealed the existence of a transient and a delayed outward current. These currents were virtually eliminated when the cell was internally dialysed with CsCl. 4. The delayed outward current, isolated from a holding potential of -50 mV, activated at potentials near -20 mV, reached a steady-state current amplitude within 60 ms and had little or no decay during steps up to 400 ms in duration. This component was reduced by 80% or more with the addition of 5 mM-TEA. 5. From -100 mV, the transient outward current reached a peak within 15 ms and decayed with a single-exponential time course. The mean decay time constant was 66 +/- 10 ms (at -33 mV) and it showed little voltage sensitivity. This current, which activated at potentials positive to -60 mV and displayed half-inactivation at -76 +/- 8 mV, was reduced by 50% with the addition of 5 mM-4-AP (4-amino-pyridine). 6. In the presence of external Ca2+, the current-voltage relationship for the delayed current did not display a region of negative-slope conductance (N-shape). Increasing the intracellular ionized Ca2+ concentration by varying the Ca-EGTA buffer ratio did not alter the dependence of the current on the membrane potential. 7. Block of outward currents with internal Cs+ revealed a small (less than 90 pA) inward Ca2+ current when the external Ca2+ concentration was increased to 10 mM. From a holding potential of -50 mV, it had a threshold at -30 mV and peaked at +5 mV. Evidence for an inward Na+ current was not obtained. 8. We conclude that acutely dissociated pineal cells display two distinct K+ currents: (i) a slowly activating, sustained current similar to the delayed rectifier (IK); and (ii) a transient A-current (IA). At normal Ca2+ concentrations, no macroscopic Ca2+-activated outward current was observed.

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