Alterations in the properties of beta-adrenergic receptors of myocardial membranes in aging: impairments in agonist-receptor interactions and guanine nucleotide regulation accompany diminished catecholamine-responsiveness of adenylate cyclase

Effect of increasing age on adrenergic control of heart rate in the rat

To determine if decreased cardiac rate with increasing age in Fischer-344 rats was due to changes in the heart itself, in adrenergic nerves innervating the heart or in both, we studied heart rate in vivo and in vitro, and atrial and ventricular pacemaker activity in vitro following atrioventricular block, in control and in chemically sympathectomized rats [pretreated with 6-hydroxydopamine (6-OHDA), 20 mg/kg, s.c., 24 h prior to testing] at ages 1 to 28 months. With increasing age, heart rate (bpm) in vivo decreased from 440 +/- 12 to 385 +/- 10 in the control and from 403 +/- 20 to 318 +/- 11 in 6-OHDA pretreated rats; heart rate in vitro decreased from 353 +/- 9 to 243 +/- 8 in the control, and from 346 +/- 15 to 214 +/- 18 in 6-OHDA pretreated rats; the atrial rate (AR) decreased from 304 +/- 9 to 210 +/- 8 in the control and from 288 +/- 13 to 161 +/- 32 in 6-OHDA pretreated rats while the ventricular pacemaker rate (VR) decreased from 121 +/- 8 to 92 +/- 5 in the control, and from 100 +/- 14 to 70 +/- 7 in 6-OHDA pretreated rats. With age, AR decreased to a greater extent than VR and 6-OHDA had a greater effect in decreasing AR than VR. Using cardiac rate as a measure, it appears that with age changes in the pacemakers of the heart themselves (postjunctional) as well as in the adrenergic nerve endings innervating the heart (prejunctional) contribute to decreased cardiac rate and pacemaker activity in older rats.

Adenylate cyclase in senescence: catecholamine and parathyroid hormone pathways

There are significant changes in the adenylate cyclase complex of many tissues with age. These changes are not due to a single defect in the system. Rather, the defects may vary from tissue to tissue and may be multiple in nature. There are age-related changes described in the receptor, N-protein, and catalytic components of the adenylate cyclase receptor. Alteration of the level of the receptor may reflect increases in the circulating levels of the agonist with age. The mechanisms responsible for alterations in the N-protein and catalytic unit activity remain to be elucidated. Further work is also needed to determine if the age-related changes in cAMP production result in age-related changes in cAMP-dependent protein kinase activity and the phosphorylation of specific proteins. However, the age-related changes in adenylate cyclase activity that have already been described may at least partially explain the decreased responsiveness of some target tissues to hormones and drugs.

Effects of chronic administration of doxorubicin on myocardial beta-adrenergic receptors

The effects of multiple doses of doxorubicin (DXR) on myocardial beta-adrenergic receptor density and dissociation constant were investigated in male Sprague Dawley rats. The rats received DXR (2 mg/kg) or vehicle weekly by the SC route for 13 weeks. One group of DXR-treated rats plus corresponding controls were sacrificed at 14 weeks, one week after the last dose. Another group of DXR-treated rats plus corresponding controls were sacrificed at 19 weeks, six weeks after the last dose. The myocardial beta-adrenergic receptor was characterized by radio-ligand binding studies using [125I]iodocyanopindolol. Beta--receptor densities in DXR-treated rats of 7.0 and 7.4 fm/mg protein were unchanged from control levels of 7.2 fm/mg protein at both 14 and 19 weeks, respectively. Receptor dissociation constants in DXR-treated rats of 36.7 and 36.9 pM were increased over control levels of 24.6 and 30.0 pM at 14 and 19 weeks, respectively. However, the change in dissociation constant is only significant at 14 weeks. The increased dissociation constants suggest diminished agonist binding affinity of the myocardial beta-receptor. This impaired response of the receptor to catecholamines would tend to diminish the ability of myocardium to adequately respond to adrenergic stimuli.

Beta-adrenergic agonist-mediated desensitization in senescent rats

The capacity of senescent rats to develop the catecholamine refractory state was investigated in CDF (F-344) rats of 3 and 24 months of age. Beta-adrenergic receptor number, receptor-agonist affinity, and adenylate cyclase activity in heart membranes were assessed, following the chronic in vivo administration of the beta-adrenergic agonist, metaproterenol. Drug treatment leads to marked myocardial hypertrophy, receptor down-regulation, and reduced isoproterenol-stimulated adenylate cyclase activity. The extent of catecholamine-refractoriness was not different in the older rats, indicating the catecholamine desensitization of myocardial beta-adrenergic responsiveness is not impaired in senescence. Receptor agonist affinity and the percent of receptors in the high-affinity state decrease with age. These parameters are further reduced by agonist treatment but to a lesser extent in the older animals. Thus, the effects of age and agonist desensitization are not additive and suggest that aged animals may already be partially desensitized.

Autonomic interactions in the aging heart: age-associated decrease in muscarinic cholinergic receptor mediated inhibition of beta-adrenergic activation of adenylate cyclase

The ability of the muscarinic receptor agonist, carbachol, to inhibit beta-adrenergic activation of adenylate cyclase was examined in cardiac membranes from 6-month (young adult) and 24-month (aged) old rats in an effort to assess the effect of aging on adrenergic-cholinergic interactions in the heart. At varying concentrations (0.1-100 microM) of carbachol, GTP plus isoproterenol-stimulated adenylate cyclase activity was inhibited 5-39% in cardiac membranes from 6-month-old rats; this inhibition was statistically significant at all but the lowest concentration (0.1 microM) of carbachol used. In contrast, in cardiac membranes from 24-month-old rats, the inhibition of GTP plus isoproterenol-stimulated adenylate cyclase activity by carbachol was very weak (3-20% with 0.1-100 microM carbachol), and statistically insignificant. The muscarinic receptor antagonist, atropine, blocked the inhibition of GTP plus isoproterenol-stimulated enzyme activity by carbachol showing that the observed inhibitory effect of carbachol was muscarinic receptor dependent. The basal adenylate cyclase activity (which showed no significant age-related difference) was unaffected by carbachol. No significant age-related differences were evident in: (a) the concentration of carbachol required for half-maximal inhibition of GTP plus isoproterenol-stimulated adenylate cyclase activity; (b) the density of muscarinic receptor sites; and (c) their agonist and antagonist binding affinities. The GTP plus isoproterenol-stimulated cyclase activity measured in the absence of carbachol was approximately 70% lower in cardiac membranes from 24-month-old, compared to 6-month-old rats, confirming an age-associated decline in beta-adrenergic activation of the cyclase observed in our previous study [Mech. Ageing Dev., 19: (1982) 127-139]. The above findings suggest an apparent age-related decline in the postsynaptic antiadrenergic action of cholinergic stimulus in the heart; thus, exaggerated cholinergic antagonism of beta-adrenergic stimulus does not seem to contribute to the impaired adrenergic control of the heart in aging. On the other hand, autonomic imbalance, due to excessive weakening of the antiadrenergic influence of cholinergic stimulus, may compromise the ability of the cholinergic system to counteract the tendency of unrestrained adrenergic drive to increase ventricular vulnerability to fibrillation; this, in turn, may favor the high incidence of cardiac arrhythmias in aging.

Effect of chronic administration of doxorubicin on cardiac adenylate cyclase activity in mice

Cardiac adenylate cyclase activity was examined in mice treated chronically with doxorubicin. Mice received a subcutaneous dose of either 2 or 4 mg/kg doxorubicin twice weekly for 5 weeks. Mice were sacrificed five weeks after the last injection. Basal cardiac adenylate cyclase activity was significantly elevated in both the 2 and 4 mg/kg DXR-treated groups over the control level. GTP, isoproterenol (plus GTP), NaF, and forskolin stimulated activities in both the 2 and 4 mg/kg DXR-treated groups were also significantly elevated over control levels.

Aging and the cardiovascular system

What has preceded has been largely a listing of contradictory data: what follows is an attempt to gleam a pattern from all of this. In general, responsiveness is either reduced or unchanged by aging; reduced responsiveness has been demonstrated most frequently for cardiac beta-adrenoreceptors, cardiac muscarinic receptors, vascular beta-adrenoreceptors and vascular alpha 2-adrenoreceptors. A reduced ability of the amine uptake system has been demonstrated in some but not all studies: any reduction in re-uptake would tend to potentiate the effects of NA and counteract the reduced receptor responsiveness (or vice versa). This may explain why the most consistent cardiovascular alteration reported in the elderly is an increased plasma NA. While there are clinical reports that beta-blockers and converting enzyme inhibitors are less effective in elderly hypertensives at lowering blood pressure, this may reflect more the pathological development of hypertension with time rather than a true aging phenomenon. Overall, it appears that resting function of the cardiovascular system is near normal in the aged, but since the mode of control is somewhat altered, in particular with a blunted baroreflex, perturbations in the system produced by drugs may cause a higher incidence of adverse effects.

Factors controlling beta 1-adrenoceptor affinity and selectivity

A membrane preparation of the calf heart left ventricle has been used after identification and characterization, as a source of myocardial beta 1-adrenoceptor for radioligand binding studies. The displacement of specifically bound (-)-[3H]dihydroalprenolol by some beta-adrenoceptor ligands appeared to be pH-dependent, which could be related to the ionization characteristics of the compounds. Among the usually four ionic species of the ligand, present at physiological pH, the cation was shown to govern beta 1-adrenoceptor affinity. Furthermore, quantitative structure affinity relationships for the interaction with beta 1- and beta 2-adrenoceptors were established for the phenoxypropanolamines, a class of beta-adrenoceptor ligands. The N-isopropyl-oxypropanolamine side chain itself does not discriminate between beta 1- and beta 2-adrenoceptors, whereas aromatic substitution ortho to the side chain induces some beta 2-selectivity. Selectivity for myocardial beta 1-adrenoceptors is mainly obtained by aromatic substitution para to the side chain. This substitution pattern yields a decrease in beta 2-adrenoceptor affinity, far more pronounced than the decrease in beta 1-adrenoceptor affinity.

Effects of age on the adrenergic cardiac neuroeffector junction

Although it is clear that adrenergic nervous system control of cardiac function decreases with age and that the effector organ fails to adjust to this decreased control, it is not completely evident which of the many mechanisms operant at the adrenergic-cardiac neuroeffector junction contribute to this state. Prejunctionally, it appears that norepinephrine content decreases with age and that adrenergic axonal degeneration occurs. Also, evidence is available to suggest that modulation by prejunctional alpha adrenergic receptors of norepinephrine release is altered with increasing age, as is neuronal uptake of norepinephrine. Postjunctionally, it appears that beta-adrenergic receptor sensitivity to agonists undergoes age-related alterations, and possibly post receptor mechanisms involved in receptor-response coupling. Other mechanisms, such as those involved in transmitter uptake into extraneuronal sites, adrenergic neuronal responsiveness to stimulation, transmitter release and turnover, calcium and prejunctional receptor modulation of transmitter release, postjunctional receptor development of supersensitivity or subsensitivity, need further elucidation in order to have an understanding of the factors that contribute to the breakdown of homeostatic mechanisms that regulate the heart.

The effect of age and cholesterol on the rat lung beta-adrenergic system

To assess the influence of membrane lipid composition on beta-adrenergic receptor number and adenylate cyclase activity in aging, we investigated the effect of cholesteryl hemisuccinate on these parameters in lung membranes of 3-, 12-, and 24-month-old CDF (F-344) rats. When cholesteryl hemisuccinate (0.5 mg/ml) was incubated with lung membranes, beta-adrenergic receptor density was increased by 70%. This effect was the same for each age group studied and indicated that the density of both basal and CHS-sensitive receptors is unaltered in rat lung with age. Forskolin, NaF, p[NH]ppG, and isoproteronol-stimulated adenylate cyclase activity is 30% lower in lung membranes from aged rats. Since enzyme activity is affected by the lipid environment and membrane composition often changes with age, we assessed adenylate cyclase activity following cholesteryl hemisuccinate incorporation. There was up to a 75% decrease in adenylate cyclase activity following cholesteryl hemisuccinate incorporation in lung membranes in each of the three age groups. In untreated membranes, there was no significant difference in cholesterol or lipid phosphate content with age. These data suggest that cholesterol content does not account for alterations in senescent rat lung adenylate-cyclase activity.

Alterations in adenylate cyclase activity due to streptozotocin-induced diabetic cardiomyopathy

The effect of chronic experimental diabetes on the adenylate cyclase system (AC) in the rat heart was investigated. Rats were made diabetic by an intravenous injection of streptozotocin (65 mg/kg), hearts were removed 8 weeks later and washed cell particles were isolated. AC activity was measured in the absence and presence of different concentrations of forskolin, NaF, GTP analogue [Gpp(NH)p] or epinephrine. A significant depression in the epinephrine stimulated AC activity was observed in diabetic hearts. Basal AC activity and stimulation of AC with forskolin, NaF and Gpp(NH)p were not significantly different between control and diabetic preparations. These results indicate no apparent alterations in the regulatory or catalytic properties of AC in hearts from chronic diabetic rats. The observed depression in epinephrine stimulated AC activity may account for the depressed inotropic action of catecholamines in the diabetic cardiomyopathy.

Decreased response with age of the cardiac catecholamine sensitive adenylate cyclase system

The cardiac beta-adrenergic coupled adenylate cyclase system was examined in young and old male Wistar rats. The concentration of binding sites for (-)3H-DHA in membranes prepared from cardiac ventricles was 21.1 +/- 2.78 (SD) fmoles/mg protein in 3-4 month old rats (young rats) and 31.2 +/- 2.20 fmoles/mg protein in 24 month old rats (old rats). The dissociation constant, KD was 4.3 +/- 1.8 nM and 6.7 +/- 1.7 nM for young and old rats, respectively. Various compounds were used to study the characteristics of activation of adenylate cyclase in homogenates from cardiac ventricles. Basal adenylate cyclase was reduced 30% in old animals compared to young (6.1 pmoles/min/mg protein in 24 month vs. 8.6 pmoles/min/mg protein in 3-4 month). (-)Isoproterenol (10(-5) M) alone stimulated adenylate cyclase greater than two-fold in young rats (10.6 pmoles/min/mg protein above basal) and this stimulation was 34% lower in old animals. GppNHp (100 microM), fluoride (10 mM), and forskolin (100 microM) activation of adenylate cyclase above basal was reduced 38, 37, and 34%, respectively, in the old animals. No significant changes between the two groups were noted in the apparent affinity of GppNHp either alone or in the presence of (-)isoproterenol nor in the affinities of catecholamine agonists for activation of cyclase. These results suggest a reduction in the amount of functional regulatory protein or possibly cyclase in 24 month old rat ventricular tissue compared to 3-4 month old tissue. However, this data does not rule out the possibility of altered molecular interactions of a full complement of regulatory protein(s) with beta-adrenergic receptor and/or catalytic adenylate cyclase.