STIMULATION OF BETA-1-ADRENOCEPTORS ENHANCES ELECTRICALLY EVOKED [H-3] ACETYLCHOLINE-RELEASE FROM RAT PHRENIC-NERVE

1. The effects of isoprenaline, noradrenaline and fenoterol on the electrically evoked release of [3H]‐acetylcholine from the rat phrenic nerve were investigated. 2. Isoprenaline (0.1 μmol/L) and noradrenaline (1 μmol/L) enhanced evoked [3H]‐acetylcholine release by about 90%, an effect which was abolished by CGP 20712A (0.1 μmol/L), a specific antagonist at β1‐adrenoceptors. Noradrenaline still enhanced [3H]‐acetylcholine release in the presence of phentolamine (1 μmol/L). 3. The enhancing effect of both isoprenaline and noradrenaline decreased at prolonged exposure times (24–32 min). A pre‐exposure of the tissue to a low concentration (0.01 μmol/L) of isoprenaline prevented the enhancing effect of 0.1 μmol/L isoprenaline. 4. Fenoterol, a specific agonist at β2‐adrenoceptors, did not modify evoked [3H]‐acetylcholine release. 5. The present results indicate the existence of facilitatory β1‐adrenoceptors on the motor nerve. These receptors appear to be desensitized either by a high concentration of, or by a long exposure to, agonists. Under the present conditions noradrenaline enhances the release of newly synthesized transmitter mainly by stimulation of β‐adrenoceptors. Copyright © 1990, Wiley Blackwell. All rights reserved