Mechanisms of altered β-adrenergic modulation of the cardiovascular system with aging

Perspectives on mammalian cardiovascular aging: humans to molecules

Age-related changes in cardiovascular function and structure in healthy adult volunteer community dwelling subjects (from 20 to 85 years) is remarkable for changes in pump function [impaired left ventricular (LV) ejection reserve capacity manifest by a reduced ejection fraction and accompanied by diminished cardioacceleration, LV dilation at end diastole and an altered diastolic filling pattern] and increased vascular afterloading. There is also evidence for a reduction in the number of cardiac myocytes with advancing age. Subcellular changes with aging (best understood in rodents) include certain regulatory factors of excitation-contraction-relaxation coupling (i.e. calcium handling), modulation by adrenergic receptor (AR) stimulation, and changes in the generation and sensitivity to the damaging effects of ROS. Coordinated changes in gene expression and/or protein function with aging result in a prolonged action potential (AP), Ca(i) transient, and contraction. L-type Ca(2+) current (I(Ca)) inactivates more slowly, and outwardly-directed K(+) currents are reduced, and likely contribute to AP-prolongation. The rate of Ca(2+) sequestration by the sarcoplasmic reticulum (SR) decreases in the senescent myocardium, in part underlying the prolonged Ca(i) transient. An age-associated reduction in transcription of the SERCA2 gene, coding for the SR Ca(2+) pump, accounts in part for a decrease in the SR pump site density. The contractile response to both beta(1)-AR and beta(2)-AR stimulation diminishes with aging due to decreased adrenergic augmentation of I(Ca), and thus the Ca(i) transient, in senescent vs. young hearts. The age-associated reduction in the postsynaptic response of myocardial cells to beta(1)-AR stimulation appears to be due to multiple changes in molecular and biochemical receptor coupling and post-receptor mechanisms. An increased basal production of ROS is paralleled by increased ROS-sensitivity, markers of chronic ROS damage and mitochondrial functional decline. Overall, these changes lead to a diminished (but not necessarily exhausted) capacity of the heart to adapt to physiological or pathological stress with advancing age.

Age-related changes in adrenergic neuroeffector transmission

In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission in the cardiovascular system, in terms of alpha2-adrenoceptors, beta2-adrenoceptors and the noradrenaline re-uptake process. These studies show diminished prejunctional alpha2- and prejunctional beta-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional alpha2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced beta-adrenoceptor facilitation, at least in normal conditions. Hence, assuming that such changes also occur in man, we might expect to find evidence of increased release of noradrenaline from noradrenergic nerves, and this could be reflected in plasma levels of noradrenaline.