The effect of aging on neurohypophyseal function

Abstract

In healthy adults, secretion of the antidiuretic hormone, arginine vasopressin (AVP), is controlled largely by osmotic hemodynamic variables. Osmoregulation is mediated via neurons located in the anterior hypothalamus near but distinct from the neurosecretory cells. By some unknown mechanism, this osmostat is able to detect and respond to very small changes in the plasma concentration of sodium and certain other solutes. The sensitivity of this system varies considerably from person to person but is constant on repeat testing in a given subject. With advancing age, however, the sensitivity of the osmoregulatory system appears to increase progressively. Thus, by the sixth decade, the amount of AVP released per unit increment in plasma osmalality is about twice that secreted by subjects in the third decade. Whether this age related enhancement is due to increased sensitivity of the osmostat per se or to increased neurohypophyseal stores of AVP has not been determined. Aging also appears to affect the baroregulation of AVP but in the opposite direction. Whereas healthy young adults subjected to moderate hypovolemia and/or hypotension invariably exhibit pronounced increases in AVP secretion, the same or a greater hemodynamic stimulus evokes no AVP response in almost half of all subjects over 68 years of age. The lack of AVP response is not due to detects in the neurohypophysis or baroreceptor per se because osmotically mediated AVP release and orthostatically induced catecholamine release are normal to super normal in healthy elderly subjects. Thus, the age related defect in baroregulation probably is located in one or more pathways between the vasomotor center and neurohypophysis. Preliminary data suggests it is associated with abnormalities in the control of basal and orthostatic blood pressure and may also be linked to the age related change in osmoregulatory function. The predilection of elderly subjects to disorders of salt and water balance may be due in part to these age related alterations in neurohypophyseal regulation.