An ultrastructural study of the effects of age on sympathetic innervation and atrial tissue in the rat

Previous work on the rat heart has demonstrated an age-related reduction in catecholamines and a decline in myocardial cell sensitivity to catecholamines in vitro. We used ultrastructural cytochemical techniques to label noradrenergic vesicles of the sympathetic nerve terminals of the rat heart atrium, and addressed the question of whether these deficits are accompanied by a decrease in the number of synaptic vesicles or by progressive axonal degeneration. Our results demonstrate a significant sympathetic axonal degeneration between 3 and 24 months of age. No decrease in noradrenergic vesicle population in the intact nerve terminals could be discerned over this age span. Atrial cell structural alterations observed with age include: (1) increased quantities of residual bodies; (2) infrequent but definite myofibrillar disorganization at cell peripheries; (3) infrequent regional discontinuity of cell attachments and (4) increased extracellular collagen. We suggest that the apparent integrity of noradrenergic vesicle populations is consistent with reports by other investigators that levels of the catecholamine synthesizing enzyme, tyrosine hydroxylase, in sympathetic ganglia increase with age. The previously observed decline in cardiac catecholamines with age may be due to axonal degeneration rather than to reduced noradrenergic vesicles in intact terminals.