Facilitatory action of adrenergic drugs on muscle twitch evoked by nerve stimulation in the curarized rat phrenic hemidiaphragm.

The facilitatory effects of alpha-adrenergic drugs on neuromuscular transmission has been studied in the curarized rat phrenic-hemidiaphragm. The reversal effect of blocking action of (+)-tubocurarine on indirectly-induced twitch by alpha-adrenergic agents was higher with noradrenaline (EC50 = 6.51 +/- 0.18 microM) compared with adrenaline (EC50 = 11.80 +/- 1.06 microM) or phenylephrine (EC50 = 17.36 +/- 1.02 microM). Methoxamine, clonidine and azepexol showed no effect. The action of noradrenaline was blocked by prazosin (IC50 = 3.96 +/- 0.76 nM) and tolazoline (IC50 = 10.30 +/- 0.5 nM) but not by yohimbine at the concentration range 0.01-0.1 microM. From these results it could be concluded that an alpha 1-adrenergic mechanism might be involved in the facilitatory action of alpha-adrenergic agents at the level of motor nerve terminals.