1. Mammalian platelets were freshly isolated from human, rabbit, or rat blood. The whole‐cell and cell‐attached voltage‐clamp variations of the patch‐clamp technique were employed to study the passive electrical properties and ion channels of unstimulated platelets. 2. The input capacitance of a platelet measured by the phase‐sensitive detection method was about 128 fF, the input resistance of a platelet was about 59 G omega and the resting membrane potential was about ‐50 mV which was directly measured by a whole‐cell recording in the current‐clamp mode. 3. The predominant ion channel was a voltage‐gated K+ channel resembling the delayed rectifier K+ channel of nerve, muscle and T‐lymphocyte. There was no indication of any inward current in the platelet membrane. The activation of the K+ current could be fitted by n4 kinetics, and was half‐maximal at about ‐35 mV. 4. The time constant of K+ current inactivation was virtually independent of voltage and varied from cell to cell. Recovery from inactivation was slow and dependent on the size and duration of the preceding conditional voltage step. Steady‐state inactivation was half‐maximal at about ‐50 mV and was complete at positive potentials. 5. The predominant single‐K+‐channel conductance was 9 pS and the estimated number of K+ channels per platelet was about 325, corresponding to a density of 25/micron2 apparent membrane area.
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