Membrane potential responses to ionophoretically applied alpha-adrenoceptor agonists in the mouse anococcygeus muscle

1 Noradrenaline phenylephrine, naphazoline and oxymetazoline were applied by ionophoresis to the mouse anococcygeus muscle and the membrane potential was recorded with an intracellular microelectrode. 2 The ionophoretic application of noradrenaline and phenylephrine produced dose-related depolarizations in 96% of the cells tested; in contrast, naphazoline and oxymetazoline depolarized only 62% of the cells although contraction was always seen. 3 The depolarizations produced by all four drugs had similar characteristics in shape and time course except that the latency of responses induced by the imidazoline-related drugs was significantly longer than the value obtained with the phenylethanolamines. This discrepancy was not due to the difference in susceptibility to neuronal uptake of the two groups of drugs. 4 The time to peak depolarization for naphazoline and oxymetazoline was longer than that for noradrenaline and phenylephrine but was not sufficient to account for the considerably slower contraction produced by the former drugs. 5 At room temperature the sensitivity of the mouse anococcygeus to ionophoretically applied naphazoline and oxymetazoline was significantly lower than that to noradrenaline and phenylephrine but at 35 degrees C the sensitivity was similar for all drugs. 6 These results suggest that there might be two subclasses of alpha 1-adrenoceptor in the mouse anococcygeus; stimulation of one type leads to depolarization and contraction and activation of the other class produces contraction with no change in membrane potential.