Block of ATP‐regulated potassium channels by phentolamine and other α‐adrenoceptor antagonists

1 The patch clamp technique has been used to characterize the effects of phentolamine, an unselective blocker of α 1 and α 2 ‐adrenoceptors, on the electrical activity of isolated RINm5F insulin‐secreting cells and the gating of ATP‐regulated potassium (K + ATP ) channels. 2 Current‐clamp experiments carried out by use of both conventional whole‐cell recordings and nystatin‐perforated cells, have demonstrated that phentolamine (5–20 μ m ) in the complete absence of α‐adrenoceptor agonists, caused a sharp depolarization of the cell membrane from approximately −66 mV to −42 mV. This depolarization was associated with the generation of calcium action potential‐like spikes. In the continued presence of phentolamine, diazoxide (100 μ m ) reversed these effects by causing a hyperpolarization of the cell, thereby preventing Ca 2+ spikes. 3 Unitary current events from K + ATP channels were recorded from both outside‐out membrane patches and saponin permeabilized or open‐cells. When added to either the inside or the outside of the plasma membrane, phentolamine (0.1–100 μ m ) blocked openings from these channels. The effects of phentolamine were rapid, sustained and fully reversible. Phentolamine was apparently a more effective blocker of channels from the inside than the outside of the membrane. 4 The K 1 value, corresponding to 50% inhibition of channels was estimated to be approximately 0.7 μ m when phentolamine was added to the inside of the membrane and the Hill coefficient approximately 1. 5 Yohimbine (1–10 μ m ) and the chemically 2‐substituted imidazoline α‐adrenoceptor antagonists, antazoline (25 μ m ) and tolazoline (25 μ m ) were also found to block K + ATP channels in isolated patches of membrane. 6 In conclusion the present study demonstrates that phentolamine and other imidazoline adrenoceptor antagonists have effects upon ATP‐sensitive K + channels that are not associated with stimulation of the adrenoceptor.