Role of the cAMP sensor Epac as a determinant of K ATP channel ATP sensitivity in human pancreatic β‐cells and rat INS‐1 cells

Protein kinase A (PKA)‐independent actions of adenosine 3′,5′‐cyclic monophosphate (cAMP) are mediated by Epac, a cAMP sensor expressed in pancreatic β‐cells. Evidence that Epac might mediate the cAMP‐dependent inhibition of β‐cell ATP‐sensitive K + channels (K ATP ) was provided by one prior study of human β‐cells and a rat insulin‐secreting cell line (INS‐1 cells) in which it was demonstrated that an Epac‐selective cAMP analogue (ESCA) inhibited a sulphonylurea‐sensitive K + current measured under conditions of whole‐cell recording. Using excised patches of plasma membrane derived from human β‐cells and rat INS‐1 cells, we now report that 2′‐ O ‐Me‐cAMP, an ESCA that activates Epac but not PKA, sensitizes single K ATP channels to the inhibitory effect of ATP, thereby reducing channel activity. In the presence of 2′‐ O ‐Me‐cAMP (50 μ m ), the dose–response relationship describing ATP‐dependent inhibition of K ATP channel activity ( NP o ) is left‐shifted such that the concentration of ATP producing 50% inhibition (IC 50 ) is reduced from 22 μ m to 1 μ m for human β‐cells, and from 14 μ m to 4 μ m for rat INS‐1 cells. Conversely, when patches are exposed to a fixed concentration of ATP (10 μ m ), the administration of 2′‐ O ‐Me‐cAMP inhibits channel activity in a dose‐dependent and reversible manner (IC 50 12 μ m for both cell types). A cyclic nucleotide phosphodiesterase‐resistant ESCA (Sp‐8‐pCPT‐2′‐ O ‐Me‐cAMPS) also inhibits K ATP channel activity, thereby demonstrating that the inhibitory actions of ESCAs reported here are unlikely to arise as a consequence of their hydrolysis to bioactive derivatives of adenosine. On the basis of such findings it is concluded that there exists in human β‐cells and rat INS‐1 cells a novel form of ion channel modulation in which the ATP sensitivity of K ATP channels is regulated by Epac.