Noradrenaline output induced by calcium, strontium and barium during exposure of guinea-pig vas deferens to ouabain.

The effects of tetrodotoxin (TTX, 5 x 10(-7) g/ml) and excess Mg2+ (20 mM) on the noradrenaline output induced by the addition of 2.5 mM of Ca2+, Sr2+ and Ba2+ during exposure to ouabain were observed using isolated guinea-pig vas deferens. Excess Mg2+ reduced the noradrenaline output induced by Ba2+ or Sr2+, but not that induced by Ca2+, regardless of whether it was added before or after the start of exposure to ouabain. In contrast, when added before the exposure to ouabain, TTX was most effective in retarding and in decreasing the noradrenaline output induced by Ca2+, La3+ (0.25 mM) inhibited, and transmural electrical stimulation potentiated the Ca2+-induced response during exposure to ouabain. These results confirmed that Ca2+-induced response during exposure to ouabain in mainly mediated by a Na+-dependent Ca2+ influx system. In contrast, Sr2+ and Ba2+ may enter into adrenergic nerve terminals mainly through depolarization and induce the output of noradrenaline when the Na+-K+ pump is inhibited.