Subtype classification of the presynaptic α‐adrenoceptors which regulate [3H]‐noradrenaline secretion in guinea‐pig isolated urethra

Abstract

The following experiments were carried out to investigate the presence and type of functional presynaptic receptors in adrenergic nerves of the guinea‐pig urethra. The urethra from male guinea‐pigs was incubated with [3H]‐noradrenaline and superfused with Tyrode solution in vitro. The fractional secretion of [3H]‐noradrenaline evoked by 300 electrical pulses was measured. The [3H]‐noradrenaline secretion was positively frequency‐dependent, yielding a half‐maximal secretion at 8 ± 5 Hz. Stimulation was usually applied at 5 Hz. The [3H]‐noradrenaline secretion was not altered by noradrenaline (1 or 100 μm), norephedrine (1 μm), isoprenaline (0.1 μm), 5‐hydroxytryptamine (10 μm), oxotremorine (10 μm), adenosine (0.2 mm), propranolol (1 μm), atropine (1 μm) or 8‐phenyltheophylline (10 μm). The [3H]‐noradrenaline secretion was enhanced by clonidine (3 μm), chlorpromazine (10 μm), metitepine (1 μm), 4‐aminopyridine (0.5 mm), tetraethylammonium (2 mm), 3‐isobutyl‐1‐methylxanthine (4 mm), 8‐bromo cyclic AMP (1 mm) and forskolin (25 μm). The α‐adrenoceptor antagonists rauwolscine, yohimbine, phentolamine, prazosin and AR‐C 239 maximally enhanced the [3H]‐noradrenaline secretion to about 300% of control. The partial α‐adrenoceptor agonist oxymetazoline maximally enhanced the secretion to about 200% of control. The order of apparent EC50 values was rauwolscine < yohimine < phentolamine < oxymetazoline < prazosin < AR‐C 239. The enhancing effects of yohimbine (1 μ m) with tetraethylammonium (2 mm), 8‐bromo cyclic AMP (1 mm), or forskolin (25 μm) were additive, but not those of yohimbine (1 μm) with prazosin (10 μm), 4‐aminopyridine (0.5 mm), or 3‐isobutyl‐1‐methylxanthine (4 mm). These results suggest that the [3H]‐noradrenaline secretion in the guinea‐pig urethra is regulated by presynaptic α2A‐adrenoceptors which may, in a cyclic AMP‐independent manner, be coupled to a 4‐aminopyridine‐sensitive potassium channel. The secretion is not influenced by compounds acting at β‐adrenoceptors, muscarinic cholinoceptors or adenosine receptors.