Electrophysiological characteristics of cultured human umbilical vein endothelial cells (HUVEC) were determined using the patch-clamp technique in the whole cell configuration. In isolated cells, membrane potential, capacitance, and input resistance were (Mean +/- SD) - 16.3 +/- 12.7 mV, 53.9 +/- 26 pF, and 2.3 +/- 1.3 G omega, respectively (N = 26); and in confluent cells - 23.6 +/- 5.5 mV, 127 +/- 59 pF, and 0.254 +/- 0.077 G omega, respectively (N = 6). The almost 10 times higher input resistance, and smaller capacitance of isolated versus confluent cells, indicated that the latter were in electrical communication, presumably through open gap junctions, which was confirmed by intercellular diffusion of Lucifer Yellow. Whole-cell currents of isolated cells were made up of at least three components: First, two outward currents, an early transient one with activation-inactivation kinetics and a small delayed sustained component with 6.75 +/- 4.8 and 0.73 +/- 0.089 nS conductance, respectively. Second, an inward component which was rectified and had 1.58 +/-1.2 nS conductance. In contrast to a reported lack of voltage-gated channels in HUVEC, the above currents were voltage dependent. Inhibition of the whole-cell currents by external Ba2, internal Cs, and other K+ blockers indicates that the three observed currents are carried by K+. This was confirmed by changes of outside K+ concentrations shifting the I-V curve intercept in the direction expected for K(+)-selective channels. Voltage-gated Ca2+ currents were not apparent in the whole-cell current records. HUVEC membrane potential was as low as that of microvascular cells, while inward current rectification at normal external K+ was like that in arterial endothelial cells. This mixed phenotypic expression and multipotential behavior suggests that the electrical features of HUVEC may be primarily determined by embryonic origin and the local effect of the microenvironment rather than strictly by vessel size.