The mechanism of neuronal noradrenaline and dopamine β-hydroxylase release by a sodium-deficient solution substituted with urea

Abstract

Isolated rabbit hearts were perfused with a ‘low Na-urea’ solution (the substitution of 132 mM NaCl of Tyrode's solution by 242 mM urea) for 1–30 min. The effect on noradrenaline overflow was compared with that evoked by other NaCl substitutes. ‘Low Na’ solutions released noradrenaline at greatly differing rates and time courses. ‘Low Na-urea’ medium caused a large noradrenaline overflow which in time course and magnitude was similar to that evoked by ‘high K-low Na’. However, the effect of ‘high K-low Na’ was largely dependent on calcium and little affected by lowering the temperature from 36 to 16°C (Q 10 = 1.1) while the noradrenaline output evoked by ‘low Na-urea’ medium was only transiently calcium-dependent and greatly reduced by a drop in temperature (Q 10 = 6.7). Both ‘low Na-urea’ and calcium-free ‘high K-low Na’ solutions evoked the release of α-methyladrenaline which had been incorporated into adrenergic nerves of the hearts of rabbits previously treated with reserpine. Since reserpine blocks the vesicular storage mechanism the amine must have been released from the axoplasm through the neuronal membrane. The re-introduction of Tyrode's solution did not reverse the α-methyladrenaline output evoked by ‘low Na-urea’ although it abolished that after ‘low Na-sucrose’ or calcium-free ‘high K-low Na’ solutions. Similarly, the noradrenaline loss from the normal heart evoked by ‘low Na-urea’ medium was also unaltered by returning to Tyrode's solution. The irreversibility of the action of ‘low Na-urea’ solution on amine release, an initial loss of potassium into the perfusate and previous morphological evidence of myocardial cell damage suggested that the medium produced a hyposmotic shock. As a result, there was a brief calcium-dependent release of noradrenaline and of dopamine β-hydroxylase that was superimposed upon a much larger and persisting non-exocytotic amine overflow which was unaccompanied by release of dopamine β-hydroxylase. The present findings corroborate our previous observation that electron-dense cores in the small vesicles of adrenergic neurons of the rabbit heart are preserved while most of the cardiac noradrenaline is depleted by perfusion with ‘low Na-urea’ solution, and the suggestion by other authors that proteins, such as dopamine β-hydroxylase, rather than amine constitute the electron-dense material.