Facilitation by ‘low sodium-urea’ medium of the washout of dopamine β-hydroxylase released by potassium ions from the perfused rabbit heart

Abstract

Rabbit hearts were perfused for 2 or 30 min at a constant flow with high K-low Na solution (equimolar substitution of 132 mM NaCl of Tyrode's solution by KCl). The perfusates were collected in 5 min periods and assayed for noradrenaline and dopamine β-hydroxylase activity. High K-low Na solution perfused for 30 min caused a pronounced noradrenaline output with the peak at 0–5 min which subsequently declined exponentially with time. In contrast, the dopamine β-hy-droxylase output rose slowly to its maximum at 15–20 min. If high K-low Na solution was administered for 2 min it failed to produce a peak in the output of dopamine β-hydroxylase. The lack of coincidence of noradrenaline and dopamine β-hydroxylase outputs might have been due to either a slow washout of the enzyme or to a larger ratio of dopamine β-hydroxylase to noradrenaline at 15 than at 5 min. Methacholine (320 μM) or calcium deprivation which preferentially blocked the initial noradrenaline overflow greatly reduced the total output of dopamine β-hydroxylase and abolished the dopamine β-hydroxylase peak at 15–20 min. In the following experiments low Na-urea solution (substitution of 132 mM NaCl of Tyrode's solution by 242 mM urea) was employed to accelerate the washout of dopamine β-hydroxylase. Perfusion of the hearts with high K-low Na for 2 min, followed by low Na-urea solution, evoked a dopamine β-hydroxylase peak already at 5–10 min. The total output of dopamine β-hydroxylase was larger than the combined outputs to be expected of high K-low Na and low Na-urea solutions. The total noradrenaline output was approximately the sum of that during high K-low Na and low Na-urea perfusion. It is concluded that the maximum rate of dopamine β-hydroxylase release evoked by high K-low Na medium occurs at the same time as the initial large noradrenaline overflow that is calcium-dependent and subject to presynaptic muscarinic inhibition. However, there is a delay in the washout of the large compared with the small molecule. Thus, low Na-urea solution can be used as a tool to accelerate the washout of dopamine β-hydroxylase from the perfused rabbit heart.