Metabolic control of the potassium permeability in pancreatic islet cells

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Abstract
The K+ permeability of pancreatic islet cells was studied by monitoring the efflux of 86Rb+ (used as tracer for K+) from perifused rat islets and measuring the uptake of 42K+. Glucose markedly and reversibly decreased 86Rb+ efflux from islet cells and this effect was antagonized by inhibitors of the metabolic degradation of the sugar, i.e. mannoheptulose, iodoacetate, glucosamine and 2-deoxyglucose. Among glucose metabolites, glyceraldehyde reduced the K+ permeability even more potently than did glucose itself; pyruvate and lactate alone exhibited only a small effect, but potentiated that of glucose. Other metabolized sugars, like mannose, glucosamine and N-acetylglucosamine, also decreased 86Rb+ efflux from islet cells. Fructose was effective only in the presence of glucose. Non-metabolized sugars like galactose, 2-deoxyglucose and 3-O-methylglucose had no effect. The changes in K+ permeability by agents known to modify the concentrations of nicotinamide nucleotides, glutathione or ATP in islet cells were also studied. Increasing NAD(P)H concentrations in islet cells by pentobarbital rapidly and reversibly reduced 86Rb+ efflux; exogenous reduced glutathione produced a similar though weaker effect. By contrast, oxidizing nicotinamide nucleotides with phenazine methosulphate or Methylene Blue, or oxidizing glutathione by t-butyl hydroperoxide increased the K+ permeability of islet cells. Uncoupling the oxidative phosphorylations with dicumarol also augmented 86Rb+ efflux markedly. In the absence of glucose, but not in its presence, methylxanthines reduced 86Rb+ efflux from the islets; such was not the case for cholera toxin or dibutyryl cyclic AMP. Glucose and glyceraldehyde had no effect on 42K+ uptake after a short incubation (10min), but augmented it after 60min; the effect of glucose was suppressed by mannoheptulose and not mimicked by 3-O-methylglucose. The results clearly establish the importance of the metabolic degradation of glucose and other substrates for the control of the K+ permeability in pancreatic islet cells and support the concept that a decrease in the K+ permeability represents a major step of the B-cell response to physiological stimulation.