Electrical field-stimulated release of norepinephrine-H3 from rat atrium: effects of ions and drugs.

Electrical field stimulation-induced release of norepinephrine-H 3 accumulated in the rat atria is dependent upon calcium only at lower current strengths. Such evoked release is blocked by bretylium, tetrodotoxin or ouabain. Inhibition of release by bretylium, but not by ouabain, is reversed by elevating the levels of calcium. Stimulation-induced release of norepinephrine-H 3 at higher current strengths appears to be independent of calcium and is not diminished by bretylium, tetrodotoxin or ouabain. The similarity of the effects of drugs and of changes in frequency and duration of stimuli to responses observed with peripheral nerve stimulation suggests that, in the isolated rat atria, field currents of low intensity depolarize axons, and the conducted impulses are responsible for the evoked release. At higher current strengths, direct effects on the nerve ending result in norepi-nephrine-H 3 release by a calcium-independent mechanism.