SEQUENCE AND FUNCTIONAL EXPRESSION IN XENOPUS OOCYTES OF A HUMAN INSULINOMA AND ISLET POTASSIUM CHANNEL

Regulation of insulin secretion involves the coordinated control of ion channels in the beta-cell membrane. We have isolated and characterized cDNA and genomic clones encoding a voltage-dependent K+ channel isoform expressed in human islets and in a human insulinoma. This K+ channel isoform, designated hPCN1, with a deduced amino acid sequence of 613 residues (M(r) = 67,097), is related to the Shaker family of Drosophila K+ channels. hPCN1 is homologous to two other human K+ channel isoforms we have isolated, hPCN2 and hPCN3, with 55% and 65% amino acid sequence identity, respectively. The electrophysiological characteristics of hPCN1 were determined after microinjection of synthetic RNA into Xenopus oocytes. Two-microelectrode voltage-clamp recordings of oocytes injected with hPCN1 RNA revealed a voltage-dependent outward K+ current that inactivated slowly with time. Outward currents were inhibited by 4-aminopyridine with a K(i) less than 0.10 mM and were relatively insensitive to tetraethylammonium ion or Ba2+. A delayed rectifier K+ channel such as hPNCN1 could restore the resulting membrane potential of beta-cells after depolarization and thereby contribute to the regulation of insulin secretion.