The effect of fatty acids on calcium-activated and inwardly-rectifying potassium channel activity in bovine chromaffin and endothelial cells
The effect of a variety of fatty acids was examined on two types of potassium channels in bovine chromaffin (BCC) and bovine aortic endothelial cells (BAEC). Large-conductance calcium-activated potassium channels were recorded from inside-out patches excised from cultured BCC. Fatty acids were applied to the intracellular surface of the patch at a concentration of 10μM. All the fatty acids examined (arachidonic acid (C20:4 n-6), eicosapentaenoic acid (C20:5 n-3), docosahexaenoic acid (C22:6 n-3) and myristic acid (C14:0)) increased the patch open probability at a potential of +40mV. The effect was immediate and reversible. The n-3 fatty acids were more effective than either the n-6 or saturated fatty acids. Fatty acids shifted the voltage required to half-activate the channel in a hyperpolarising direction. The current-voltage relationship was unaffected. The effect was independent of the intracellular calcium concentration as the same response was observed in a "calcium-free" solution (no added CaCl2, 5mM EGTA). The fatty acid-mediated increase in BK channel activity is proposed to provide a negative feedback on exocytosis from bovine chromaffin cells. The effect of fatty acids on the whole-cell current of cultured BAEC was examined. Acute exposure of BAEC to 10μM fatty acids in the presence of 10-8M intracellular calcium did not affect the predominant whole-cell current, characterised as an inwardly rectifying potassium current. Arachidonic acid did not affect the whole-cell current whether it was applied to the extra- or intracellular surface of the membrane. Increasing the intracellular calcium concentration to 10-6M revealed the presence of an outward calcium-sensitive potassium current. Application of arachidonic acid (10μM) significantly increased the magnitude of this current. The effect was immediate and reversible. The activation of this calcium-sensitive potassium current hyperpolarises the cell, sustaining the influx of calcium and nitric oxide/prostacyclin production.