ATP-sensitive inward rectifier and voltage- and calcium-activated K+ channels in cultured pancreatic islet cells

Abstract

K+ channels in cultured rat pancreatic islet cells have been studied using patch-clamp single-channel recording techniques in cell-attached and excised inside-out and outside-out membrane patches. Three different K+-selective channels have been found. Two inward rectifier K+ channels with slope conductances of about 4 and 17 pS recorded under quasi-physiological cation gradients (Na+ outside, K+ inside) and maximal conductances recorded in symmetrical K+-rich solutions of about 30 and 75 pS, respectively. A voltage- and calcium-activated K+ channel was recorded with a slope conductance of about 90 pS under the same conditions and a maximal conductance recorded in symmetrical K+-rich solutions of about 250 pS. Single-channel current recording in the cell-attached conformation revealed a continuous low level of activity in an apparently small number of both the inward rectifier K+ channels. But when membrane patches were excised from the intact cell a much larger number of inward rectifier K+ channels became transiently activated before showing an irreversible decline. In excised patches opening and closing of both the inward rectifier K+ channels were unaffected by voltage, internal Ca2+ or externally applied tetraethylammonium (TEA) but the probability of opening of both inward rectifier K+ channels was reduced by internally applied 1-5 mM adenosine-5'-triphosphate (ATP). The large K+ channel was not operational in cell-attached membrane patches, but in excised patches it could be activated at negative membrane potentials by 10(-7) to 10(-6) M internal Ca2+ and blocked by 5-10 mM external TEA.

Effects of sulphonylureas and diazoxide on insulin secretion and nucleotide‐sensitive channels in an insulin‐secreting cell line

“…We found in single channel studies that the sulphonylurea drugs, tolbutamide and glibenclamide, both reduced the open state probability of the ATP-sensitive K+ channel and that their relative potency was of the same order as their pharmacological efficacy in man (Sturgess et al, 1985;Ashford et al, 1986). These findings have been confirmed and extended by Trube et al (1986), who also showed that the drugs did not affect Ca2+-dependent K+ channels present in B-cells Findlay et al, 1985) and the CRI-GL cell line (Sturgess et al, 1986a), and that the drug diazoxide could reverse inactivation of the ATP-sensitive K+ channel. Subsequently we have discovered in the insulin-secreting cell line, CRI-G1, a second cation channel which can be inhibited by adenine nucleotides (Sturgess et al, 1986b); therefore, this represents another possible target for the drugs.…”

“…Another major class of K + selective channels present in B-cells Findlay et al, 1985) and this cell line (Sturgess et al, 1986a) (Findlay et al, 1985) and in cardiac (Trube & Hescheler, 1984) and skeletal (Spruce et al, 1987) An alternative method of assessing the concentration-dependence of these compounds is to use the whole-cell configuration (Trube et al, 1986). By allowing dialysis of the cell interior with the pipette solution (and washout of ATP), the overall (cellular) potassium conductance observed, under voltage clamp, is due to the increased activation of the ATP-K+ channel (Rorsman & Trube, 1985).…”

Mechanism for reactivation of the ATP‐sensitive K+ channel by MgATP complexes in guinea‐pig ventricular myocytes.

“…This phenomenon, usually called 'run-down', distributed widely in various tissues and has known has been reported for KATP channels from f-cells (Findlay, physiological importance, increasing efforts have been Dunne & Petersen, 1985a; Ohno-Shosaku, Ziinkler & made to clarify the functions and regulatory mechanisms Trube, 1987), insulin-secreting cell lines (Findlay, Dunne, of this channel (see Ashcroft, 1988). In these studies, Ullrich, Wollheim & Petersen, 1985b;Sturgess, Kozlowski, however, characterization of this channel has been Carrington, Hales & Ashford, 1988), mammalian cardiac complicated by a major problem, namely, loss of channel cells (Trube & Hescheler, 1984;Kakei, Noma & Shibasaki, activity with time after patch excision in the single 1985), and amphibian skeletal muscle (Spruce, Standen & channel recordings or cell dialysis during whole-cell Stanfield, 1987).…”

The effects of cromakalim on ATP‐sensitive potassium channels in insulin‐secreting cells

“…Potassium selective ion channels whose gating is regulated by changes in the internal concentration of adenosine-5'-triphosphate (ATP) (KATP channels) have been described in the membranes of cardiac muscle (Noma, 1983;Kakei & Noma, 1984), skeletal muscle (Spruce et al, 1985;, smooth muscle (Davies et al, 1989), hypothalamic neurones (Ashford et al, 1989) and pancreatic #-cells (Cook & Hales, 1984;Findlay et al, 1985;Rorsman & Trube, 1985). In intact f-cells, where openings can be recorded under resting conditions, KATP channels govern the regulation of insulin secretion.…”