CARDIAC NORADRENALINE STORES

The effects of single doses of reserpine or of guanethidine in increasing the heart rate of the dog heart-lung preparation have been studied. Compared with reserpine, guanethidine increased heart rate more and more rapidly, and the increase lasted longer. When the effects of reserpine had subsided, guanethidine now caused a similar rate of change and maximum increase in heart rate, but recovery was quicker. Guanethidine given alone caused less release of noradrenaline in the right atrium than did reserpine alone, but both drugs together caused greater release than did either alone. Thus liberation of noradrenaline in the right atrium was unrelated to the effectiveness of either drug to increase heart rate. Guanethidine and reserpine did not increase the heart rate of heart-lung preparations from dogs which had received reserpine for 2 days and which contained no noradrenaline. Guanethidine had similar actions on the heart rate of preparations either given a slow intravenous infusion of noradrenaline or a single dose of reserpine. Treatment of a normal heart-lung preparation with noradrenaline increased the atrial noradrenaline content, but subsequent addition of reserpine or of guanethidine changed heart rate in the same manner as for preparations not given noradrenaline. Treatment with reserpine increased the heart's sensitivity to noradrenaline without changing initial heart rate. These results are discussed in relation to the idea that there may be more than one noradrenaline compartment in the heart.