Release of noradrenaline from isolated rat tail artery induced by bafilomycin A1

Abstract

The macrolide antibiotic bafilomycin A1, a selective inhibitor of the vesicular H(+)-transporting ATPase, increased irreversibly the overflow of 3,4-dihydroxyphenylethylene glycol from isolated segments of the rat tail artery. Maximum increase in the overflow was produced by exposing the tissues to 0.5 mumol/l bafilomycin A1. Unless the Na(+)-dependent neuronal amine carrier (uptake1) was inhibited, overflow of noradrenaline was below the detection limit. The bafilomycin A1-induced increase in overflow of noradrenaline from tissues with inhibited uptake1 was accompanied by a significant decrease in the (noradrenaline overflow:glycol overflow) ratio. Unlike reserpine and tetrabenazine, the antibiotic did not alter the (noradrenaline overflow:glycol overflow) ratio in arteries incubated in Ca(2+)-free, 120 mmol/l K+ medium. Bafilomycin A1 increased overflow of noradrenaline and normetanephrine from tissues with inhibited monoamine oxidase. Inhibitors of extraneuronal catecholamine transport (uptake2), corticosterone, 3-O-methylisoprenaline and 1,1'-diethyl-2,2'-cyanine, suppressed overflow of normetanephrine while increasing that of noradrenaline. Further increase in overflow of noradrenaline was produced by concomitant inhibition of uptake1. A similar effect was observed in tissues previously exposed to phenoxybenzamine. After exposure to bafilomycin A1, tyramine and (+) amphetamine (10 mumol/l) were equally effective in increasing overflow of noradrenaline from tissues with inhibited monoamine oxidase into corticosterone-containing medium. Bafilomycin A1 promotes leakage of noradrenaline from storage vesicles without affecting its conversion to 3,4-dihydroxyphenylethylene glycol. When uptake1 is inhibited, axoplasmic noradrenaline can be translocated effectively across the axonal membrane by the 'diffusional efflux'. When uptake1 is inhibited, spontaneous quantal release contributes significantly to overflow of noradrenaline into normal media. The 'diffusional efflux' of noradrenaline is unaffected by inhibitors of uptake2. Even at highly elevated concentrations of axoplasmic noradrenaline, the uptake1-mediated influx of noradrenaline exceeds the uptake1-mediated efflux. Enhancement of noradrenaline overflow from tissues with inhibited monoamine oxidase by indirectly acting sympathomimetic amines depends primarily on their ability to induce leakage of the transmitter from storage vesicles rather than its translocation across the axonal membrane.