Presynaptic effects of l-alanine on peripheral sympathetic neurotransmission

Concentrations ranging from 0.1 nM to 30 microM of the aminoacids l-alanine (l-ALA), taurine, hypotaurine and beta-ALA were assayed on the chronotropic responses of rat isolated atria stimulated through their cardioaccelerans nerves. The increases in atrial rate obtained in frequency-response curves to nerve stimulation (0.1-12.8 Hz) were not modified by taurine, hypotaurine and beta-ALA but were reduced by l-ALA (0.01 and 0.1 microM). L-ALA did not modify the chronotropic responses to exogenous norepinephrine (NE). In the atria labeled in vitro with [3H]NE, the release of radioactivity evoked by postganglionic nerve stimulation at 2 Hz (6 V, 0.5 msec duration for 2 min) was reduced to 70% of control values by 0.1 microM l-ALA. The release of [3H]NE evoked by 60 and 100 mM KCl was also reduced by 0.1 microM l-ALA to 40 and to 70% of control values, respectively. L-ALA (0.1 microM) did not affect the outflow of tritium provoked by a 1-min incubation with 1 microM tyramine. The chronotropic responses to nerve stimulation were not modified by the l-ALA metabolites pyruvate and glutamate but were reduced by an analog of l-ALA, alpha-(methylamino)-isobutyric acid, which also diminished the release of [3H]NE elicited by nerve stimulation at 2 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)