Age-related deficit in beta receptor stimulation of cAMP binding in blood vessels

Both vascular relaxation and cAMP accumulation mediated by beta adrenergic agonists declines with increasing age. We have developed a method to measure the biologically relevant cAMP bound to the regulatory subunits of cAMP-dependent protein kinase in rat aorta. In homogenates of rat aorta, binding of [3H]cAMP was saturable with a Kd of 8.0 +/- 1.5 nM; the dissociation of [3H]cAMP from the binding sites was comprised of fast and slow components at 4 degrees C. Endogenous cAMP binding in aortas stimulated with isoproterenol (10(-5) M) or forskolin (10(-5) M) was quantitated by radioimmunoassay. Compared to basal values, isoproterenol increased cAMP binding by 37% (P less than 0.05) in aortas from young animals (5-6 weeks) but had essentially no effect on binding in older animals (9-11 months). In contrast, forskolin, which causes full relaxation in aortas from older rats, equally elevated bound cAMP values in aortas in the two age groups. In addition, the ratio of total cAMP binding sites to cAMP-dependent protein kinase catalytic activity was 45% higher in aortas from the older rats, suggesting that there were proportionately more regulatory subunits in those vessels. The deficit in isoproterenol-stimulated cAMP binding in vascular smooth muscle in older animals may in part explain the loss in relaxation mediated by beta adrenergic agonists.