β-Adrenoceptor stimulation enhances transmitter output from the rat phrenic nerve

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Abstract
1 Neurally-evoked output of newly synthesized [3H]-acetylcholine from the rat phrenic nerve was measured in the absence of cholinesterase inhibitors. 2 Noradrenaline and isoprenaline enhanced neurally-evoked transmitter output markedly. Moreover, immediately after the application of noradrenaline the basal tritium efflux increased significantly. 3 Pretreatment with propranolol (0.1 μmol l−1) or atenolol (0.3 μmol l−1) completely prevented the stimulatory effect of noradrenaline and isoprenaline on evoked transmitter output. 4 The facilitatory effect of isoprenaline declined, when the exposure time was increased. This observation supports the assumption that β-adrenoceptors can be desensitized or inactivated during continued exposure to agonists. 5 It was shown for the first time that stimulation of β-adrenoceptors enhances transmitter output from the motor nerve. It is proposed that these β-adrenoceptors are of the β1-subtype and are localized on the endings of motor nerves. Circulating catecholamines may facilitate neuromuscular transmission by stimulation of presynaptic β-adrenoceptors.