Abstract
β-Adrenergic responses have been shown to decline with aging, particularly in the cardiovascular system. We infused terbutaline, a selective β2-adrenoceptor agonist, into the brachial artery of 10 young (mean age 25 yr, range 2231 yr) and 9 elderly (mean age 73 yr, range 68-81 yr) healthy subjects to examine its effects on nutrient flux. Forearm K+, PO4, free fatty acid (FFA), and glycerol uptake were determined by measurement of forearm blood flow (using dye dilution) and brachial arterial and deep venous plasma substrate concentrations. Elderly subjects were less sensitive to terbutaline-mediated increases in forearm blood flow, net fluxes of K+, and glycerol but not net fluxes of FFA or PO4. The mean fitted slopes of each parameter vs. the log of the terbutaline concentration, a measure of forearm β-adrenergic sensitivity, for young and elderly groups were 4.9 ± 1.7 (SD) vs. 2.4 ± 2.3 for forearm blood flow (P < 0.05), 0.84 ± 0.46 vs. 0.43 ± 0.37 for K+ net flux (P < 0.05), -157 ± 113 vs. -26 ± 26 for glycerol net flux (P < 0.01), -336 ± 429 vs. -44 ± 457 for FFA net flux (P = 0.11), and 0.31 ± 0.24 vs. 0.18 ± 0.16 for PO4 net flux (P = 0.14). Terbutaline promoted net uptake of K+ into skeletal muscle less well in the elderly, although net PO4 flux was similar in the two groups. Terbutaline-stimulated vasodilation and net glycerol efflux but not FFA efflux were impaired with aging. These data demonstrate that heterogeneous changes in β-adrenergic responses occur with aging.