Abstract-ATP-sensitive K (K ATP ) channels are inhibited by cytosolic ATP, a defining property that implicitly links these channels to cellular metabolism. Here we report a direct link between fatty acid metabolism and K ATP channels in cardiac muscle cells. Long-chain (LC) acyl-coenzyme A (CoA) esters are synthesized from fatty acids and serve as the principal metabolic substrates of the heart. We have studied the effects of LC acyl-CoA esters and LC fatty acids on K ATP channels of isolated guinea pig ventricular myocytes and compared them with the effects of phosphatidylinositol 4,5-bisphosphate (PIP 2 ). Application of oleoyl-CoA (0.2 or 1 mol/L), a naturally occurring acyl-CoA ester, to the cytosolic side of excised patches completely prevented rundown of K ATP channels, but not of Kir2 channels. The open probability of K ATP channels measured in the presence of oleoyl-CoA or PIP 2 was voltage dependent, increasing with depolarization. Oleoyl-CoA greatly reduced the ATP sensitivity of K ATP channels. At a concentration of 2 mol/L, oleoyl-CoA increased the half-maximal inhibitory concentration of ATP Ͼ200-fold. The time course of the decrease in ATP sensitivity was much faster during application of oleoyl-CoA than during application of PIP 2 . The effects of PIP 2 , but not of oleoyl-CoA, were inhibited by increasing Ca 2ϩ to 1 mmol/L. Oleate (C18:1; 10 mol/L), the precursor of oleoyl-CoA, inhibited K ATP channels activated by oleoyl-CoA. Palmitoleoyl-CoA and palmitoleate (C16:1) exerted similar reciprocal effects. These findings indicate that LC fatty acids and their CoA-linked derivatives may be key physiological modulators of K ATP channel activity in the heart. (Circ Res. 2001;88:918-924.) Key Words: free fatty acids Ⅲ acyl-CoA esters Ⅲ PIP 2 Ⅲ K ATP channels A TP-sensitive potassium (K ATP ) channels link energy metabolism with the electrical activity of the heart. [1][2][3] This link is functionally important during hypoxia or ischemia. Any imbalance in the ratio between energy supply through coronary arteries and energy expenditure by cardiac myocytes will indirectly modulate electrical activity, because a change in the ATP/ADP ratio, together with acidosis, promotes activation of K ATP channels. 4 -6 By increasing K ϩ efflux and shortening the duration of the action potential, K ATP channel activation reduces transsarcolemmal Ca 2ϩ influx and thereby the energy costs of Ca 2ϩ -ATPases and actomyosin-ATPase. Hence, in the face of reduced energy supply, K ATP channels may provide a means to decrease cytosolic energy demand.We report here a direct effect of long-chain (LC) acylcoenzyme A (CoA) esters and fatty acids on the activity of K ATP channels in isolated cardiac myocytes. We have shown that LC acyl-CoA esters facilitate the opening of K ATP channels by reducing their ATP sensitivity. The precursors of LC acyl-CoA esters, LC fatty acids, were found to inhibit K ATP channels. The reciprocal effects of free fatty acids and acyl-CoA esters on K ATP channels represent a novel link between energy metab...