Effect of aging on vasopressin, catecholamines, and alpha 2-adrenergic receptors

To characterize normal changes with aging, we measured plasma levels of epinephrine, norepinephrine, and vasopressin, as well as alpha 2-adrenergic receptor numbers (Bmax) and the antagonist dissociation constant (Kd) from platelet-derived membranes of white, younger (aged 28 +/- 6 years, n = 30) and older (aged 70 +/- 4 years, n = 41) normotensive, healthy volunteers. There were no differences in resting vasopressin or epinephrine levels at 0.83 +/- 0.83 and 360 +/- 120 pmol/L in the younger versus 1.0 +/- 0.2 and 450 +/- 420 in the older subjects, respectively. However, plasma norepinephrine was significantly higher in the older (2.87 +/- 1.34 nmol/L) versus the younger subjects (1.50 +/- 0.53 nmol/L, P less than .01). Platelet alpha 2-receptor numbers were significantly lower in the older subjects at 289 +/- 79 fmol/mg protein versus 388 +/- 81 fmol/mg protein in the younger subjects (P less than .01), compatible with but not proof of down-regulation by norepinephrine. However, Kd, representing receptor affinity, was similar in both groups. Therefore, studies of hormone and receptor status in various pathologic conditions should always take into account the normal changes attributable to the age of the subject population.