We have recently described cytokeratin-positive (CK(+)) and cytokeratin-negative (CK(-)) microvascular endothelial cells (MVECs) in the bovine corpus luteum. The two phenotypes show a different expression and release of adhesion molecules and cytokines. Since secretion of mediators is specifically regulated by the electrophysiological membrane parameters, this report will compare voltage-dependent potassium (K(+)) channels in the two cell types cultured under the same conditions. CK(+) and CK(-) MVECs derived from the microvascular bed of one organ differ in their K(+)-channel characteristics. In CK(-) MVECs, an inwardly rectifying K(+) current was discovered, showing the characteristics of the Kir2.1. CK(+) MVECs displayed a voltage-dependent K(+) current that activates rapidly on depolarization and inactivates very slowly, and is associated with a member of the Kv family. The mRNA for Kir2.1 was identified by RT-PCR in CK(-) and CK(+) MVECs, but there was no evidence of Kv1.4 mRNA in either of them. The function of Kir2.1 in CK(-) MVECs might be induced during cultivation, whereas CK(+) MVECs appear to be more resistant to environmental conditions and do not express an inward current.