Atrial sympathetic nerves after perfusion of the rabbit heart with low sodium solutions containing potassium chloride or urea—a biochemical, histochemical and fine structural study

Abstract

Rabbit hearts were perfused in vitro with Tyrode's solution or low sodium media (17 mm Na +) substituted with either KCl (low Na-high K) or urea (low Na-urea). Samples of right atrial tissues were removed for fluorescence and electron microscopy after 5, 15 and 120 min. In separate experiments, the noradrenaline concentrations in the right and left atria and ventricles were estimated at these times, and the amine overflow into the perfusion fluid determined. The cardiac noradrenaline concentrations and ultrastructural appearance of adrenergic axon terminals were not significantly affected by perfusion with Tyrode's solution for 120 min. Low Na-urea caused a progressive loss of noradrenaline from all parts of the heart which resulted in a 86–88% decrease of amine concentrations. Similar changes were observed after low Na-high K and also if the noradrenaline concentrations were expressed in terms of protein content of tissue. The amounts lost from the hearts were almost quantitatively recovered from the perfusates. There was greater tissue edema in hearts perfused with low Na-urea than after perfusion with the other media. After 5 min perfusion with either of the low Na solutions, little change was apparent in the ultrastructure of nerve terminals; there was no obvious alteration in histofluorescence, but after 15min it was reduced and at 120 min nearly abolished. Low Na-high K reduced the size of dense cores within the vesicles at 15 min. At 120 min the vesicles were agranular and their number per unit axon profile had dropped by 50%. In contrast, low Na-urea did not significantly alter the number of small dense-cored vesicles per profile despite causing myocardial cell damage from 5 min onwards which intensified with prolonged perfusion. The concomitant loss of histofluorescence, dense-cored vesicles and noradrenaline after low Na-high K is compatible with the view that the amine was released by exocytosis, as reported by others. However, despite myocardial tissue damage by low Na-urea, the noradrenaline overflow in this case is not due to disruption of vesicles and axon membranes. The preservation of electron-dense material in the vesicles is consistent both with previous reports that macromolecules rather than amines are responsible for the electron opaque cores and that amine release by low Na-urea is probably not by exocytosis.