Prejunctionally mediated inhibition of neurotransmission by isoprenaline in rat vas deferens

Abstract

Effects of isoprenaline on monophasic contractions evoked by electric field stimulation were studied in rat isolated prostatic vas deferens. Isoprenaline reduced electrically evoked contractions (EC 50: 0.27 ± 0.05 μM) and propranolol concentration-dependently antagonized the effect of isoprenaline. In contrast, isoprenaline (0.3–3 μM) did not affect the contractile response induced by exogenous noradrenaline or ATP, while forskolin (100 nM) attenuated agonist-induced contraction. In some tissues, adrenergic and purinergic components of the electrically evoked contraction were isolated by exposure to α,β-methylene ATP (3 μM) and prazosin (3 μM), respectively. Isoprenaline induced a greater inhibition of purinergic than adrenergic component of the electrically evoked contraction. Iberiotoxin (50 nM), glibenclamide (3 μM), 4-aminopyridine (0.3 mM) and tetraethylammonium ions (1 mM) attenuated the effect of isoprenaline. These results indicate that isoprenaline-induced inhibition of the electrically evoked (both purinergic and adrenergic) contraction was mediated primarily through activation of prejunctional β-adrenoceptors, which probably inhibited release of contractile transmitters from sympathetic nerves supplying vas deferens. Lack of effect of isoprenaline on agonistinduced contraction does not favour a functional role of β-adrenoceptors in vas smooth muscle.