Modes of hexamethonium action on acetylcholine receptor channels in frog skeletal muscle
1 The antagonism between hexamethonium and cholinoceptor agonists was investigated in frog skeletal muscle fibres with voltage-clamp techniques. Hexamethonium caused a voltage-dependent reduction in the amplitude of endplate currents. For neurally evoked endplate currents, the reduction increased e-fold with a 38 mV membrane hyperpolarization. 2 The effect of hexamethonium on the time course of endplate currents was small, and was most apparent as a slight prolongation of the decay phase at hyperpolarized potentials (more negative than -100 mV). A similar small prolongation of single channel lifetime was detected with fluctuation analysis techniques. Hexamethonium produced a voltage-dependent reduction in apparent single channel conductance as the membrane was hyperpolarized. 3 Log (concentration-response) curves for acetylcholine (ACh)-induced currents, determined either from currents accompanying ramp changes in membrane potential or from steady state currents in voltage-jump experiments, were less steep for responses in the presence of hexamethonium. This reduction in slope became more pronounced at more negative membrane potentials. Observations at +50 mV suggested that the equilibrium constant for competitive antagonism was approximately 200 μM. 4 In voltage-jump experiments with a two-microelectrode voltage clamp, the current evoked by ACh in the presence of hexamethonium differed from that recorded with ACh alone. In the presence of hexamethonium, the expected 'instantaneous' ohmic increase in membrane current in response to a hyperpolarizing step was not detected; instead a decrease in current was observed. This problem was further investigated with a vaseline-gap voltage-clamp technique which provides improved temporal resolution. With this method a rapid decrease in the ACh-induced inward current was observed with step hyperpolarizations in the presence of hexamethonium. 5 When the membrane potential was stepped back to its resting level from a more hyperpolarized potential in the presence of hexamethonium, there was a surge of ACh-induced inward current that decayed with a time constant of less than 100 μs. 6 The slow relaxation in the ACh-induced current that followed a voltage step recorded in the presence of hexamethonium was slower than that recorded with ACh alone. In the presence of hexamethonium the time constant of this relaxation increased e-fold for a 67 mV hyperpolarization. 7 The results are consistent with a rapid voltage-dependent block of ACh-activated channels by hexamethonium with hyperpolarization, and voltage-dependent unblock with depolarization. The voltage-dependent block is combined with competitive antagonism at the ACh receptors. However, not all observations appear to be compatible with a simple sequential block of open ion channels, but rather suggest that occupation of the channel by hexamethonium may not prevent channel closure.