Beta adrenergically mediated cardiac chronotropic and vascular smooth muscle responses during propranolol therapy and withdrawal in young and elderly persons.

Aging is associated with diminished beta-adrenergic responsiveness in a variety of tissues. Desensitization of tissues secondary to the age-associated increase in sympathetic nervous system activity has been proposed as a potential explanation for diminished beta-adrenergic responsiveness. If this hypothesis is correct, then beta-blockade in older people might be expected to reverse the blunted beta-adrenergic responses of tissues having diminished responsiveness. Cardiac chronotropic responses to bolus isoproterenol (ISO) doses and ISO-induced venous smooth muscle dilatation in superficial hand veins were examined in 8 young (26.2 +/- 2.6 yrs) and 9 elderly (68.0 +/- 2.2 yrs) healthy subjects before, during, and 3 and 7 days following 2 weeks of treatment with propranolol. Baseline cardiac chronotropic responsiveness, CD25, was 1.55 +/- 0.77 mcg in the young and 5.97 +/- 2.77 mcg in the elderly subjects (p less than .01), increasing to 84 +/- 56 mcg and 194 +/- 172 mcg during treatment with propranolol. At 3 and 7 days following withdrawal of propranolol, CD25s were respectively 1.24 +/- 0.79 (p = .14) and 1.10 +/- 0.42 (p = .04) in the young and 5.63 +/- 2.34 (p = .31) and 4.85 +/- 2.37 (p = .05) in the elderly subjects. In contrast, there was no decrease in the ED50 or increase in Emax for ISO-induced venodilation of hand veins following propranolol withdrawal. These results demonstrate that both young and elderly subjects have similar increases in cardiac chronotropic responsiveness following discontinuation of beta-blockade and do not support the concept that desensitization is responsible for the diminished beta-adrenergic responsiveness seen with aging.