DMPP and the adrenergic nerve terminal: Mechanisms of noradrenaline release from vesicular and extravesicular compartments

Summary

DMPP (1,1-dimethyl-4-phenylpiperazine) in various concentrations between 1.6×10⁻⁶ M and 6.2×10⁻⁵ M was infused into isolated rabbit hearts to study the neuronal release and uptake of noradrenaline.

• 1. In the untreated heart, 2.2×10⁻⁵ M or 6.2×10⁻⁵ M DMPP caused an overflow of noradrenaline (22 or 300 ng/g, respectively) that was Ca²⁺-dependent. During infusion of 1.6×10⁻⁵ M DMPP, however, a Ca²⁺-independent overflow of noradrenaline was detected which was about 3.5-fold higher than the resting overflow (0.22 ng g⁻¹ min⁻¹) of noradrenaline and was maintained over the 21-min period of infusion of DMPP; the rate of contraction (155 beats/min) was also markedly elevated by maximally 60 beats/min.

• 2. The isolated heart removed about 50% of infused noradrenaline (3×10⁻⁸ M) from the perfusion fluid. The removal, which reflects neuronal netuptake, was reversibly reduced to about 5% during exposure to 1.6×10⁻⁵ M DMPP; half-maximal inhibition was caused by 3×10⁻⁶ M DMPP.

• 3. After pretreatment with reserpine and pargyline, the heart was perfused for 30 min with 1.2×10⁻⁶ M (−)noradrenaline (³H-labelled or unlabelled) and then washed with an amine-free solution. DMPP was infused beginning at 20 or 30 min after the start of the wash-out period, i.e. at a time when the efflux of noradrenaline originated from the adrenergic neurone. DMPP in concentrations of 1.6×10⁻⁵ M and 6.2×10⁻⁵ M caused an about 10-fold increase of the rate of efflux which was not altered by Ca²⁺-deprivation or by 10⁻⁵ M hexamethonium. Half-maximal facilitation was observed using 3.5×10⁻⁶ M DMPP. The facilitated efflux reached a maximum after 3 min and then declined gradually. The decline consists of 3 components with half-times of 3, 7 and 65 min.

• 4. It is concluded that various non-nicotinic effects of DMPP on the adrenergic neurone were separated from the well-known nicotinic release of noradrenaline by using low concentrations (<2×10⁻⁵ M). The non-nicotinic increase in heart rate and in transmitter overflow evoked by DMPP presumably was due to (1) inhibition of re-uptake of spontaneously released noradrenaline and (2) to displacement of vesicular noradrenaline. Furthermore, the drug increased the release of noradrenaline from extravesicular stores (present after inhibition of monoaminoxidase), presumably by acceleration of the transmembranal efflux and by displacement of noradrenaline from extravesicular binding sites.