Role of ATP-sensitive potassium channels in action potential shortening during ischemia: Differential responses of endocardial and epicardial cells

Modulation of N- and L-type Ca2+ channels by oxotremorine-M (oxo-M) acting on muscarinic receptors and norepinephrine (NE) acting on α-adrenergic receptors was studied in superior cervical ganglion neurons. Oxo-M depresses dihydropyridine-augmented tail currents in whole-cell recordings, whereas NE does not. This modulation of L-type Ca2+ channels by oxo-M is abolished by adding 20 mM BAPTA to the pipette solution. Oxo-M, acting via a diffusible messenger, reduces the probability of opening of single N- and L-type channels recorded in cell-attached patches. We conclude that a diffusible messenger signaling pathway activated by oxo-M inhibits both N- and L-type Ca2+ channels, whereas a membrane-delimited pathway activated by oxo-M and NE inhibits only N-type Ca2+ channels.