Changes in cardiac norepinephrine in spontaneously hypertensive and stroke-prone rats

The norepinephrine (NE) concentration of cardiac ventricles was determined by radioenzymatic assay in normotensive Wistar Kyoto rats (WKY), spontaneously hypertensive rats (SHR), and stroke-prone rats (SPR) at 3-6, 14-19, and over 31 weeks of age. There was no difference between strains prior to hypertension, but a progressive decrease in cardiac NE concentration occurred in SHR and particularly in SPR relative to WKY after hypertension was established. This decrease was not due to cardiac hypertrophy. The cardiac neuronal NE storage capacity in rats over 31 weeks of age was analyzed by determining the maximum concentration of NE obtained in a cardiac microsomal fraction, after saturation in vivo with exogenous NE. The results indicated that, after a long period of hypertension, there was a reduction in cardiac NE storage capacity resulting from a loss either of sympathetic nerve endings or of storage vesicles. Moreover, in addition to this reduction in the total size of the cardiac NE store, there was an independent reduction in the degree of filling of this store in both SHR and SPR. This could reflect an increased turnover of cardiac NE in chronically hypertensive SHR and SPR.