Effect of ageing and propranolol administration on myocardial beta-adrenoceptor receptor function in mature rats

Assessment of the Effects of Age, Gender, and Exercise Training on the Cardiac Sympathetic Nervous System Using Positron Emission Tomography Imaging

BACKGROUND

Using positron emission tomography (PET) imaging, we sought to determine whether normal age or exercise training cause changes in the cardiac sympathetic nervous system function in male or female healthy volunteers.

METHODS

Healthy sedentary participants underwent PET studies before and after 6 months of supervised exercise training. Presynaptic uptake by the norepinephrine transporter-1 function was measured using PET imaging of [(11)C]-meta-hydroxyephedrine, a norepinephrine analog, and expressed as a permeability-surface area product (PSnt in mL/min/mL). Postsynaptic function was measured as β-adrenergic receptor density (β'max in pmol/mL tissue) by imaging the β-receptor antagonist [(11)C]-CGP12177. Myocardial blood flow (MBF in mL/min/mL tissue) was measured by imaging [(15)O]-water.

RESULTS

At baseline, there was no age difference in β'max or MBF but PSnt declined with age (1.12±0.11 young vs 0.87±0.06 old, p = .036). Before training, women had significantly greater MBF (0.87±0.03 vs 0.69±0.03, p < .0001) and PSnt (1.14±0.08 vs 0.75±0.07, p < .001) than men. Training increased VO2 max by 13% (p < .0001), but there were no training effects on β'max, PSnt, or MBF. Greater MBF in females and a trend to increased PSnt post-training persisted.

CONCLUSION

With age, presynaptic uptake as measured by PSnt declines, but there were no differences in β'max. Endurance training significantly increased VO2 max but did not cause any changes in the measures of cardiac sympathetic nervous system function. These findings suggest that significant changes do not occur or that current PET imaging methods may be inadequate to measure small serial differences in a highly reproducible manner.

Effects of beta-adrenoceptor blockade on beta-adrenergic signal transduction in cardiomyopathic hamster (BIO 8262) hearts

In myopathic BIO 8262-hamsters beta1-adrenergic stimulation of cardiac adenylyl cyclase has been found to be markedly reduced compared to that of healthy controls. In order to test the hypothesis that the functional uncoupling of beta1-adrenoceptors in diseased hamster hearts is due to agonist-dependent desensitization, we investigated the effects of prolonged treatment with beta-adrenoceptor antagonists on cardiac beta-adrenergic signaling. Groups of hamsters aged 240 days received either drinking water, or drinking water containing metoprolol (10 or 100 mg/kg/day) or propranolol (4 or 40 mg/kg/day). After 4 weeks' treatment animals were killed and heart ventricles were prepared for determination of beta1- and beta2-adrenoceptor densities and their functional contribution to stimulation of adenylyl cyclase. Markers of myocardial hypertrophy, i.e. absolute and relative ventricular weight and 5-nucleotidase activity, were not affected by the different treatment regimens. Neither absolute densities nor relative proportions of beta-adrenoceptor subtypes differed between untreated and treated hamster groups. Metoprolol had no effects on the functional efficacy of beta1- and beta2-adrenoceptors. Hamsters treated with high dose propranolol showed unchanged beta1-adrenoceptor function but reduced beta2-adrenergic stimulation of adenylyl cyclase. The findings of the present study demonstrate that the disturbed coupling of cardiac beta1-adrenoceptors to adenylyl cyclase cannot be reversed by in vivo treatment with beta-adrenoceptor antagonists and, therefore, is unlikely to be due to agonist-dependent desensitization.

Propranolol treatment affects ventral prostate blood vessels and serum testosterone concentrations in adult rats

The influence of prolonged beta-adrenergic receptor blockade on stereologic parameters of the ventral prostate and serum testosterone concentrations was examined in adult rats injected with propranolol (0.1 or 0.4 mg kg-1/day) for 15 or 30 consecutive days. Both doses of propranolol reduced the relative and absolute volume of the ventral prostate blood vessels. This effect was prevented by simultaneous administration of urapidil, an alpha 1-adrenergic receptor antagonist, indicating that a compensatory vasoconstriction took place as a consequence of propranolol treatment. Serum testosterone concentration was significantly increased following the 30-day application of the lower dose of the drug. These results show that prolonged administration of propranolol, although not affecting the epithelial component of the gland, may indirectly influence prostatic function by reducing the blood flow to the gland.