The effect of aging on alpha-adrenoceptors and their responses in rabbits

Development of fetal vascular responses to endothelin-1 and acetylcholine in the sheep

Responses to K(+), endothelin-1 (ET-1), and acetylcholine (ACh) of isolated adrenal, femoral, middle cerebral, and renal arteries from fetal [110--145 days gestational age (dGA, term approximately 148 dGA)] and 0- to 24-h newborn (NB) lambs were evaluated using the technique of wire myography. Responses at distinct developmental ages for each vascular bed were compared. In all arteries sensitivity to K(+)-induced vasoconstriction was similar at all fetal age points examined. In contrast, sensitivity to ET-1 increased with increasing fetal age in arteries from all vascular beds. The magnitude of the maximal vasoconstriction was positively correlated with GA for K(+) in adrenal, femoral, and cerebral arteries and for ET-1 in femoral, cerebral, and renal arteries. Cerebral arteries showed a greater sensitivity when compared with the other systemic arteries to K(+) and ET-1 at all fetal ages and to K(+) in NB. ACh evoked relaxatory responses in fetal and NB femoral and adrenal arteries. However, renal arteries relaxed comparatively less in response to ACh, and no vasodilation was noted in middle cerebral arteries at any age points examined. For femoral arteries ACh-induced vasorelaxation decreased with increasing GA but was restored in arteries from NB lambs. In summary, the responsiveness of isolated resistance arteries varies with developmental age in the fetal and perinatal sheep and these effects are both agonist and vascular bed specific. The augmented sensitivity in response to ET-1 of middle cerebral compared with other systemic arteries may reflect the importance of cerebral blood flow control during this critical developmental period.

Age-related changes in vascular responses: a review

Normal aging is associated with different changes in the cardiovascular system that lead to an increase in pathological processes, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. The vascular alterations consist of changes in the function and structure of the arteries, and increasing vascular stiffness, mainly when atherosclerosis is present, whose incidence is increased with age. The arteries accumulate lipids, collagen, and minerals. Cerebral perfusion may be reduced in the elderly, mainly regional cerebral blood flow, which leads to a deterioration of mental and physical functions. The degree of deterioration is increased when aging is associated with hypertension. Aging alters endothelial cells, which play an important role in vascular tone regulation. Such a process tends to reduce endothelium-dependent relaxations, and clearly reduces the vasodilation elicited by beta-adrenoceptor agonists. The contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium and angiotensin are barely affected with aging, whereas those elicited by noradrenaline or endothelin are usually reduced. However, plasma noradrenaline levels are increased with age, mainly due to a reduction in the sensitivity of presynaptic alpha 2-adrenoceptors and also of noradrenaline uptake. Sodium pump activity, that controls cellular ionic homeostasis, may be altered depending on animal species. Finally, vascular Ca2+ regulation appears to be altered and the extracellular Ca2+ dependence of contractile responses elicited by agonists is increased, which justifies the enhanced sensitivity to Ca2+ antagonists in senescence.

Phosphoinositide metabolism and aging in aorta from SHR and WKY rats: effects of endothelin-1 and noradrenaline

Inositol phosphate formation in aortic rings from SHR and WKY rats of different ages (6, 14, and 40 weeks) was examined. Both basal and agonist (noradrenaline and endothelin-1) stimulated levels of inositol phosphates were studied. No differences in basal levels of inositol phosphate formation were observed between SHR and WKY nor were differences in agonist stimulated levels found in 6 week old animals. However, at 14 and 40 weeks both noradrenaline and endothelin-1 stimulated levels were reduced in SHR compared to WKY. Both endothelin and noradrenaline stimulated inositol phosphate formation decreased with age in SHR. In WKY there was a decrease in endothelin but not noradrenaline stimulated levels with age. Age related decreases were consistently greater in SHR than WKY. Thus, both age and rat strain modulated agonist stimulated inositol phosphate formation.

Modelling and modification of the binding site of endothelin and other receptors

From three-dimensional models of its receptors, residues which bind the carboxy-terminus of endothelin were predicted. This site is in a pocket consisting of five putative transmembrane helices and includes a histidine in the sixth helix. This residue is either phenylalanine or asparagine in cationic neurotransmitter receptors. The histidine alkylating agent diethylpyrocarbonate potently inhibited binding of [125I]endothelin-1 to its receptors in bovine cerebellum, where a single population of endothelin ETB receptors was shown to exist. From the absence of pH sensitivity of inhibition above pH 5 and the reversal by hydroxylamine of inhibition, diethylpyrocarbonate is concluded to inhibit by histidine modification. Diethylpyrocarbonate inhibited ligand binding to several receptors with the potency order endothelin ETB > or = bombesin > or = dopamine D2 > or = m2 muscarinic > alpha 1-adrenoceptor > or = m 1 muscarinic > 5-HT2. This is consistent with histidine in the binding site of endothelin (and some other peptidergic) receptors and the proposed model.

Reduced high-affinity alpha 1-adrenoceptors in liver of senescent rats: implications of assessment at various temperatures

1. We investigated the changes occurring as a result of aging in alpha 1-adrenoceptors in the livers of Fisher 344 females rats. For comparison, we also measured beta-adrenoceptors in this tissue. Three age groups were studied, including young adults (aged 6 months), mature adults (aged 16 months) and senescent animals (aged 25 months). 2. The density of alpha 1-receptors was measured by use of [3H]-prazosin and was found to be reduced 39% (P less than 0.01) at 25 months compared with 6 months. The percentage of alpha 1-receptors displaying high affinity for adrenaline was also reduced from 85.6% at 6 months to 51.6% at 25 months (P less than 0.02). 3. In contrast, the density of beta-receptors, which was measured with [125I]-iodocyanopindolol, was increased 104% between 6 months and 25 months. The affinity of both alpha 1- and beta-adrenoceptors for antagonists was unchanged with age. 4. We found that receptor affinity for agonists may be measured accurately in binding studies conducted at 4 degrees C or 25 degrees C, but that the apparent affinity for agonist was artifactually reduced in studies conducted at 37 degrees C. This effect is poorly reversible, in that reduced agonist-affinity is also observed in tissue which has been incubated at 37 degrees C and then cooled to 4 degrees C before performing the binding studies. 5. It is concluded that liver alpha 1-adrenoceptor function is reduced and beta-adrenoceptor function increased in senescence.