Aging and aortic impedance in beagle dogs

Previous studies of the effect of aging on aortic impedance have proved inconclusive. Studies in man could be affected by clinically inapparent atherosclerotic effects which cannot be separated from aging effects. Therefore, to more directly examine the role of aging, hemodynamic studies were carried out in awake, resting young and senescent non-atherosclerotic beagles before and after the administration of various vasoactive agents. On separate days graded infusions of a rapid acting beta-adrenergic agonist and antagonist (isoproterenol and propranolol), and alpha-adrenergic agonist and antagonist (phenylephrine and phenoxybenzamine), combined alpha- and beta-adrenergic antagonists (propranolol plus phenoxybenzamine), and a non-specific vasodilator (nitroprusside) were administered. Aortic impedance was calculated from high-fidelity pressure and flow signals using standard Fourier analysis methods. Under baseline conditions no age-associated differences in any of the hemodynamic parameters were determined to be likely. The only significant age-associated difference in response was to graded doses of isoproterenol wherein there was a larger increase in flow and a greater decrease in resistance in the young compared to the senescent dogs. There were no age-associated differences in response to any of the other drugs. Thus, these data are further evidence in an intact circulation for an age-associated decrease in another beta-adrenergically mediated response.