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SUMMARY1. Coronary endothelial cells were isolated from adult guinea-pig hearts (Nees, Gerbes & Gerlach, 1981) and the electrical properties of primary cultures were studied using the tight-seal whole-cell recording mode of the patch clamp technique.2. On the third or fourth day in culture whole-cell clamp records from single coronary endothelial cells were obtained at 37 'C. The resting potential was -33 + 6 mV (n = 10). The membrane time constant determined with rectangular current pulses was 68 + 22 ms (n = 10).3. In voltage clamp experiments, no time-dependent membrane conductance changes were found in the range -80 to + 40 mV. The current-voltage relation was linear in normal physiological salt solution containing 5-4 mM-K+. The input resistance was 1P7 +0 4 GQ. When the external K+ concentration was increased to 116 mm the cells depolarized to about -3 mV and the clamp currents showed marked inward rectification.4. Between days four and seven in culture the endothelial cells formed confluent monolayers which showed the characteristic 'cobblestone' morphology. The input resistance of cells in a monolayer was 8 + 3 MQ, i.e. a factor of 200 lower than that found in single cells. It was concluded that the cells in the confluent monolayer are coupled electrically by gap junctions.5. Exposure of coronary endothelial cells to K+-free solution for 5 min produced a depolarization of about 8 mV. Upon readmission of normal external K+ the cells transiently hyperpolarized by up to 20 mV. This transient hyperpolarization decayed with a time constant of 1-9 + 0-3 min.6. The transient hyperpolarization could be abolished by application of 2 x 10-4 Mdihydro-ouabain (DHO). Application of DHO in the steady state produced a depolarization of 8+1 mV. From these findings it was concluded that coronary endothelial cells possess an electrogenic sodium pump which contributes about -8 mV to the resting potential.7. From the passive electrical properties of single cells and the morphological data t Present address:

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