Mechanism of butyrate-induced hyperpolarization of cultured rat myenteric neurones

Short-chain fatty acids produced by the bacterial fermentation of carbohydrates are present in high concentrations within the colonic lumen and have been shown to alter the excitability of enteric neurones. The present study was designed to investigate the mechanisms of butyrate-induced changes in membrane potential of myenteric neurones. Myenteric neurones from 4-10-day-old rats were isolated from the small and large intestine by an enzymatic digestion with collagenase and kept in culture. Membrane potential was measured with the whole-cell patch-clamp technique and the intracellular Ca2+, concentration was measured with the fura-2 method. The short-chain fatty acid butyrate (10-100 mmol L-1) induced a reversible and concentration-dependent hyperpolarization of the membrane with a half-maximal effect at 30 mmol L-1. The hyperpolarization evoked by butyrate (50 mmol L-1) was strongly inhibited by charybdotoxin (10(-7) mol L-1), a specific blocker of Ca2+ -dependent K+ channels. The butyrate-induced hyperpolarization was resistant against blockade of phospholipase C by U-73122 (10(-5) mol L-1), and resistant against inclusion of heparin (6 x 10(-7) mol L-1), an inositol-1,4,5-trisphosphate receptor antagonist, in the patch-pipette. In contrast, ruthenium red (3 x 10(-5) mol L-1), an inhibitor of ryanodine receptors, significantly reduced both the hyperpolarization of the membrane as well as the increase in the intracellular Ca2+ concentration evoked by butyrate. Even in neurones permeabilized with saponin (10 mg L-1), butyrate was able to stimulate a release of stored intracellular Ca2+, suggesting a direct action of the short-chain fatty acid at the stores without mediation of a soluble intracellular second messenger.