Potassium-selective ion channels in a transformed insulin-secreting cell line.

K+ channels in inside-out patches from hamster insulin tumor (HIT) cells were studied using the patch-clamp technique. HIT cells provide a convenient system for the study of ion channels and insulin secretion. They are easy to culture, form gigaohm seals readily and secrete insulin in response to glucose. The properties of the cells changed with the passage number. For cell passage numbers 48 to 56, five different K+-selective channels ranging from 15 to 211 pS in symmetrical 140 mM KCl solutions were distinguished. The channels were characterized by the following features: a channel with a conductance (in symmetrical 140 mM KCl solutions) of 210 pS that was activated by noncyclic purine nucleotides and closed by H+ ions (pH = 6.8); a 211 pS channel that was Ca2+-activated and voltage dependent; a 185 pS channel that was blocked by TEA but was insensitive to quinine or nucleotides; a 130 pS channel that was activated by membrane hyperpolarization; and a small conductance (15 pS) channel that was not obviously affected by any manipulation. As determined by radioimmunoassay, cells from passage number 56 secreted 917 +/- 128 ng/mg cell protein/48 hr of insulin. In contrast, cells from passage number 77 revealed either no channel activity or an occasional nonselective channel, and secreted only 29.4 +/- 8.5 ng/mg cell protein/48 hr of insulin. The nonselective channel found in the passage 77 cells had a conductance of 25 pS in symmetrical 140 mM KCl solutions. Thus, there appears to be a correlation between the presence of functional K+ channels and insulin secretion.