Facilitatory and inhibitory muscarine receptors on the rat phrenic nerve: effects of pirenzepine and dicyclomine

Neuronal transmitter stores of the rat phrenic nerve were labelled by an incubation with [3H]choline. Release of [3H]acetylcholine was elicited either by a short (100 pulses, 5 Hz) or by a long (1500 pulses, 5 or 25 Hz) period of electrical nerve stimulation. Pirenzepine and dicyclomine enhanced transmitter release evoked by the short stimulation period. Both antagonists reduced transmitter release evoked by the long stimulation period. Pirenzepine reduced transmitter release at low concentrations (1 nmol/l) whereas a higher concentration was necessary for the enhancing effect; the opposite pattern was found for dicyclomine. A low concentration of oxotremorine (10 nmol/l) enhanced and a hig…