Biochemical evidence for re‐use of noradrenergic storage vesicles in the guinea‐pig heart

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Abstract
The present investigation was specifically aimed to study the question of reuse of sympathetic storage vesicles of the isolated guinea pig heart. Tetraethylammonium (TEA, 20 mM) caused a massive enhancement of noradrenaline (NA) [norepinephrine] overflow upon transmural stimulation of the heart. The enhancement was inversely related to the frequency of stimulation, and maximum amounts (expressed per pulse) overflowed upon stimulation with 1 pulse. After 1 pulse, the overflow reached a maximum level in about 20 s and gradually declined to a basal level in about 80 s. Enhanced overflow of NA by TEA and intermittent stimulation was accompanied by reduction in ventricular NA content. Reduction was inversely related to the frequency of stimulation. Almost 50% reduction occurred 15 min after TEA and stimulation (1 Hz), and it reached a maximum value (80%) in 60-70 min. Substantially higher quantities of NA were recovered in the perfusion fluid than were lost from the heart after TEA plus stimulation. Reduction in NA content effected by TEA plus stimulation (for 70 min) was not accompanied by any decrease in dopamine-.beta.-hydroxylase. After incubation with 6 .mu.M NA, the normal ventricular portions showed a net accumulation of NA (0.87 .mu.g/g). Tissue NA content was not changed upon incubation with 6 .mu.M dopamine or 25 .mu.M L-dopa. Partially depleted stores of ventricular NA (by TEA and stimulation) were restored to a significant extent by exogenous NA, dopamine or L-dopa. The process of partial depletion of and repletion with NA was repeated 2 times in the same tissue. Restoring effects of NA, dopamine and L-dopa on the NA stores of the partially depleted ventricle were almost completely blocked by desipramine. Newly retained NA or that synthesized from L-dopa in a partially depleted ventricle was released upon electrical stimulation. The release was totally dependent on Ca. The capacity of a partially depleted tissue to take up and retain exogenous NA remained identical to that of a normal ventricle, provided the tissue was stimulated in the presence of TEA for only 10 min rather than for 70 min. The functional integrity of the noradrenergic storage vesicles of cardiac sympathetic nerves remains normal after exocytotic release of their transmitter substance. One possibility is that these vesicles can be re-utilized by the terminal region of the neuron to synthesize, store and release their transmitter substance.