Effect of age on presynaptic beta2 receptor mediated responses in the rat heart

Orchidectomy increases beta-adrenoceptor activation-mediated neuronal nitric oxide and noradrenaline release in rat mesenteric artery

BACKGROUND/AIMS

A previous study has demonstrated that endogenous male sex hormones do not alter neuronal nitric oxide (NO) release in rat mesenteric artery. However, the regulatory role of endogenous male sex hormones on noradrenaline (NA) release in rat mesenteric artery is not known. The present study was designed to analyze whether endogenous male sex hormones influence the NA release induced by electrical field stimulation (EFS), as well as the possible modification in NA and neuronal NO release by presynaptic beta-adrenoceptor activation.

METHODS

For this purpose, mesenteric arteries from control and orchidectomized male Sprague-Dawley rats were used. Basal and EFS-induced neuronal NO and NA release, as well as the contractile effect induced by EFS, was measured.

RESULTS

Basal and EFS-induced neuronal NO and NA release were similar in arteries from control and orchidectomized rats. The beta-adrenoceptor agonist clenbuterol did not modify EFS-induced neuronal NO and NA release in arteries from control rats. In contrast, in arteries from orchidectomized animals, clenbuterol increased both neuronal NO and NA release; this increase was prevented by incubation with the beta-adrenoceptor antagonist propranolol. However, the contractile response elicited by EFS was not modified by clenbuterol in either group of rats.

CONCLUSIONS

These results show that orchidectomy does not alter the EFS-induced NA release. What is more, activation of presynaptic beta-adrenoceptors does not modify EFS-induced NA and neuronal NO release in arteries from control rats although it increases the release of both neurotransmitters in arteries from orchidectomized rats. Despite these modifications, the EFS-induced contractile response is preserved in arteries from orchidectomized rats.

Modulation of neuroendocrine--immune signaling by L-deprenyl and L-desmethyldeprenyl in aging and mammary cancer

The aging process is characterized by a decline in cellular functions of diverse systems of the body, including the neuroendocrine-immune network. One neuroendocrinological theory of aging is based on findings that the loss of hypothalamic neurotransmitter functions and an imbalance in hormonal secretion contribute to the cessation of reproductive cycles and the development of mammary and pituitary tumors. One potential cause of immunosenescence is an age-related decline in the regulatory functions of sympathetic noradrenergic nerve fibers whose neurotransmitters signal lymphoid cells in the bone marrow, thymus, spleen, and lymph nodes. In addition to impairment caused by the generation of free radicals during numerous biochemical processes, there is a shift in the pro-oxidant/anti-oxidant balance resulting in cellular oxidative stress and hastening the aging process. Altered interactions between the neuroendocrine system and the immune system are associated with increased incidence, development, and growth of breast cancer and other neoplastic diseases. We have demonstrated that the disruption in the neuroendocrine-immune interactions in old rats, and in female rats with mammary tumors, can be reversed by deprenyl, a monoamine oxidase inhibitor. Deprenyl treatment leads to enhanced central and peripheral catecholaminergic activity and a readjustment of immunological responses. In this brief review, the nature and changes in the bi-directional communication between the neuroendocrine system and immune system and the possible mechanism(s) of actions of deprenyl in restoring these interactions during aging and mammary cancer are discussed.

Angina-induced protection against myocardial infarction in adult and elderly patients: a loss of preconditioning mechanism in the aging heart?

OBJECTIVES

The present study examined whether angina 48 h before myocardial infarction provides protection in adult and elderly patients.

BACKGROUND

The mortality rate for coronary artery disease is greater in elderly than in young patients. In experimental studies, ischemic preconditioning affords an endogenous form of protection against ischemia-reperfusion injury in adult but not in senescent hearts. Angina before myocardial infarction, a clinical equivalent of experimental ischemic preconditioning, has a protective effect in adult patients. It is not known whether angina before myocardial infarction is also protective in aged patients.

METHODS

We retrospectively verified whether antecedent angina within 48 h of myocardial infarction exerts a beneficial effect on in-hospital outcomes in adult (< 65 years old, n = 293) and elderly (> or = 65 years old, n = 210) patients.

RESULTS

In-hospital death was more frequent in adult patients without than in those with previous angina (10% vs. 2.6%, p < 0.01), as were congestive heart failure or shock (10.7% vs. 3.3%, p < 0.02) and the combined end points (in-hospital death and congestive heart failure or shock) (20.7% vs. 5.9%, p < 0.0003). In contrast, the presence or absence of previous angina before acute myocardial infarction in elderly patients seems not to influence the incidence of in-hospital death (14.4% vs. 15.2%, p = 0.97), congestive heart failure or shock (11.0% vs. 11.9%, p = 0.99) and the combined end points (25.4% vs. 27.1%, p = 0.89). Logistic regression analysis models for in-hospital end points show that previous angina is a positive predictor in adult but not in elderly patients.

CONCLUSIONS

The presence of angina before acute myocardial infarction seems to confer protection against in-hospital outcomes in adults; this effect seemed to be less obvious in elderly patients. This study suggests that the protection afforded by angina in adult patients may involve the occurrence of ischemic preconditioning, which seems to be lost in senescent patients.

Differential changes in presynaptic modulation of transmitter release during aging

The purpose of this study was to assess the functional role of presynaptic alpha 2-autoreceptors in noradrenergic transmission in the hippocampus and dopamine-2 heteroreceptors in cholinergic transmission in the striatum in young, adult, and senescent rats. Male and female Wistar rats (4, 12, and 24 months old) were used and the release of radioactivity from striatal and hippocampal slices that had been loaded either with [3H]choline or with [3H]norepinephrine was measured at rest and in response to field stimulation (2 Hz, 360 shocks). The release was challenged by sulpiride, a selective dopamine-2 receptor antagonist, and CH-38083, a selective alpha 2-adrenoceptor antagonist. The dissociation constant and the number of alpha 2-adrenoceptors was also determined by binding studies using [3H]yohimbine as ligand in crude membrane preparations of frontal cortex. There were an age-related changes in alpha 2-adrenoceptor-mediated negative feedback modulation of norepinephrine release and in the density and dissociation constant of alpha 2-adrenoceptors. They were reduced in senescent rats. In contrast the presynaptic modulation of striatal cholinergic transmission by dopamine-2 receptors was not altered during aging, but the storage capacity of and the release of acetylcholine from cholinergic interneurons was significantly lower.

Influence of exercise and age on myocardial beta-adrenergic receptor properties

The bradycardia following physical training may be mediated by an alteration in beta-adrenergic receptor number or agonist affinity. We characterized the interaction between age and exercise on myocardial beta-adrenergic receptor number and agonist affinity in 4- and 24-month-old F344 rats to test the hypothesis that the effects of training should be blunted in older rats. beta-adrenergic receptor density was unchanged with age or training. The total number of receptors per heart increased with age due to increased ventricle weight. With training, in the senescent rats the total number of receptors decreased due to a reduced amount of homogenate protein recovered from the ventricle, the significance of which is unknown. The receptor agonist dissociation constant for isoproterenol was determined in both the absence and presence of beta,gamma-imidoguanosine 5'-triphosphate [Gpp(NH)p] and did not change with age or training. Neither training nor age influenced beta-adrenergic receptor characteristics, suggesting that training bradycardia is not mediated by an alteration in beta-adrenergic receptors.

Gender differences in the effect of age on adrenergic neurotransmission in the heart

In male rats, there is an age-associated decline in the capacity of cardiac sympathetic nerves to release norepinephrine. To investigate whether this phenomenon also occurs in female rats, we examined adrenergic neurochemical transmission in the hearts of female and male Fischer 344 rats. Rats aged 6, 12, and 24 months were employed. Hearts with the right cardiac sympathetic nerve intact were isolated and perfused with Krebs-Ringer solution. Stimulations were performed following the administration of cocaine or metanephrine separately and in combination or following the combination of cocaine, metanephrine, and yohimbine. Cocaine (1 microM) was used to block uptake 1, metanephrine (1 x 10(-5) M) to block uptake 2, and yohimbine (3 microM) to block alpha 2 receptor presynaptically. The nerve was stimulated with frequencies of 2, 6, and 12 Hz, and norepinephrine amount in the effluent was determined by high performance liquid chromatography/electrochemical detection methods. Neither norepinephrine release nor the effects of yohimbine, cocaine, or metanephrine were found to be affected in older female rats (24 month). This suggests that age-related changes in adrenergic neurochemical transmission in the heart of female rats differ from those seen in older male rats.