Role of cyclic AMP in the release of noradrenaline from isolated rat atria

Abstract
The possible role of cyclic AMP (cAMP) on tritium overflow evoked by stimulation of the cardioaccelerant nerves was studied in rat atria preincubated with [3H]-noradrenaline. Addition of the activator of adenylate cyclase forskolin (1 µmol/l), or of the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine (IBMx, 100 µmol/l), did not affect both basal and evoked overflow. However, in the presence of the α2-adrenoceptor antagonist yohimbine (0.03 µmol/l) both forskolin and IBMx increased the stimulation-induced transmitter overflow by 49% and 141%, respectively (compared to yohimbine 0.03 µmol/l). Thus, in rat atria the cAMP-dependent facilitation of noradrenaline release is only present when the autoinhibition exerted by activation of prejunctional α2-adrenoceptors is blocked. Propranolol (0.1 µmol/l) that did not produce any effect on noradrenaline release markedly reduced the facilitatory response induced by forskolin in the presence of yohimbine. When rats were pretreated with the β2-adrenoceptor agonist clenbuterol (0.3 mg · kg−1, s. c., twice daily, 14 days), a treatment which desensitizes β-adrenoceptor-me-diated facilitation of noradrenaline release (Kazanietz and Enero 1989), the facilitatory effect of forskolin and IBMx in the presence of yohimbine was abolished. The results indicate that in rat atria the effect of forskolin and IBMx on noradrenaline release are only to be observed after blockade of presynaptic α2-adrenoceptor autoinhibition. β-adrenoceptor blockade or clenbuterol pre-treatment decreases the facilitatory response to forskolin and hence prejunctional β-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase.

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