The effects of ageing on vascular alpha-adrenoceptors in pithed rat and rat aorta

Spirulina extract improves age-induced vascular dysfunction

CONTEXT

Vascular dysfunction is considered a hallmark of ageing that has been associated with altered vasomotor responses, in which nitric oxide (NO) and reactive oxygen species participate. The consumption of Spirulina extracts, with antioxidant properties, increased recently.

OBJECTIVE

This study investigates the effect of Spirulina aqueous extract (SAE) on the vascular function of the aorta from aged rats.

MATERIALS AND METHODS

Aortic segments from aged male Sprague-Dawley rats (20-22 months old) were exposed to SAE (0.1% w/v, for 3 h) to analyse: (i) the vasodilator response induced by acetylcholine (ACh), by the NO donor sodium nitroprusside (SNP), by the carbon monoxide releasing molecule (CORM) and by the K_ATP channel opener, cromakalim (CK); (ii) the vasoconstrictor response induced by KCl and noradrenaline (NA); (iii) the production of NO and superoxide anion, and (iv) the expression of the p-eNOS and HO-1 proteins.

RESULTS

Incubation with SAE increased the expression of p-eNOS (1.6-fold) and HO-1 (2.0-fold), enhanced NO release (1.4-fold in basal and 1.9-fold in ACh-stimulated conditions) while decreased the production of superoxide (0.7-fold). SAE also increased the sensitivity (measured as pEC₅₀) to ACh (control: -7.06 ± 0.11; SAE: -8.16 ± 0.21), SNP (control: -7.96 ± 0.16; SAE: -9.11 ± 0.14) and CK (control: -7.05 ± 0.39; SAE: -8.29 ± 0.53), and potentiated the response to KCl (1.3-fold) and to NA (1.7-fold).

CONCLUSION

The antioxidant properties of SAE improved the vasomotor responses of aorta from aged rats. These results may support the use of Spirulina as a protection against vascular dysfunction.

Effects of chronic treatment with a low dose of nicorandil on the function of the rat aorta during ageing

1. It is known that ATP-sensitive potassium (K(ATP)) channels regulate the membrane potential of smooth muscle cells and vascular tone. Because their activity is altered during ageing, many pharmacological treatments aimed at improving K(ATP) channel and cardiovascular functions have been evaluated. Nicorandil, a K(ATP) channel opener, nitric oxide (NO) donor and anti-oxidant, induces vasodilation, decreases blood pressure and exhibits cardioprotection in ageing, as well as after ischaemia-reperfusion. 2. In the present study, using tension myography and biochemical and histological techniques, we investigated the effects of chronic (2 months) low-dose nicorandil (0.1 mg/kg per day) treatment on the function of rat aorta during ageing (in 4-, 12- and 24-month old rats). 3. The results showed that chronic nicorandil treatment significantly improves mechanical relaxation and noradrenaline-induced vasoconstriction in aged rats. At all ages, the nicorandil-induced vasodilation was primarily mediated by its NO donor group. Nicorandil treatment resulted in an additional 0.5-1 elastic lamella in the aorta and decreased total protein, collagen and elastin content in the aortic wall at all ages. However, in 4-month-old rats, nicorandil significantly increased the elastin : total protein ratio by 19%. 4. In contrast with results of previous studies that used high doses of nicorandil (i.e. 60 mg/kg per day), low-dose nicorandil treatment in the present study did not lead to a progressive desensitization to nicorandil and may be beneficial in improving arterial function in ageing or cardiovascular diseases.

Elastin haploinsufficiency induces alternative aging processes in the aorta

Elastin, the main component of elastic fibers, is synthesized only in early life and provides the blood vessels with their elastic properties. With aging, elastin is progressively degraded, leading to arterial enlargement, stiffening, and dysfunction. Also, elastin is a key regulator of vascular smooth muscle cell proliferation and migration during development since heterozygous mutations in its gene (Eln) are responsible for a severe obstructive vascular disease, supravalvular aortic stenosis, isolated or associated to Williams syndrome. Here, we have studied whether early elastin synthesis could also influence the aging processes, by comparing the structure and function of ascending aorta from 6- and 24-month-old Eln+/- and Eln+/+ mice. Eln+/- animals have high blood pressure and arteries with smaller diameters and more rigid walls containing additional although thinner elastic lamellas. Nevertheless, longevity of these animals is unaffected. In young adult Eln+/- mice, some features resemble vascular aging of wild-type animals: cardiac hypertrophy, loss of elasticity of the arterial wall through enhanced fragmentation of the elastic fibers, and extracellular matrix accumulation in the aortic wall, in particular in the intima. In Eln+/- animals, we also observed an age-dependent alteration of endothelial vasorelaxant function. On the contrary, Eln+/- mice were protected from several classical consequences of aging visible in aged Eln+/+ mice, such as arterial wall thickening and alteration of alpha(1)-adrenoceptor-mediated vasoconstriction. Our results suggest that early elastin expression and organization modify arterial aging through their impact on both vascular cell physiology and structure and mechanics of blood vessels.

Effects of aging on alpha1-adrenoceptor mechanisms in the isolated mouse aortic preparation

The effects of aging on alpha(1)-adrenoceptor (alpha(1)-AR)-mediated contractile response in endothelium-removed aortic preparations isolated from 5- to 40-week-old (5-, 6-, 8-, 10-, 20-, 40-weeks) mice were studied in the presence of propranolol. The potency of noradrenaline, estimated as its pD(2) value, increased with age from 5- to 10-weeks, but decreased thereafter with age from 10- to 40-weeks. However, the affinity of prazosin (pA(2) value) did not change with aging. These results suggest that age-related change in noradrenaline potency is not attributable to the change of drug affinity to alpha(1)-ARs, but is possibly due to drug affinity-unrelated factors such as change of the reserve of alpha(1)-ARs.

Use of the sensory nerve stimulator to accelerate healing of a venous leg ulcer with sensory nerve dysfunction: a case study

A new therapy using sensory nerve stimulation [International Patent Application Number PCT/AU2004/001079: "nerve function and tissue healing" (Khalil, Z)] has been developed in our vascular physiology laboratory. This treatment has been found to improve the deficient sensory nerve function and associated deficient wound healing of older persons to levels seen in young people. An 82-year-old man with a small but persistent venous leg ulcer for 18 months, despite apparently appropriate wound dressings and compression therapy, was seen in a specialist wound management service. The patient's sensory and microvascular function was assessed in great detail using the vascular physiology laboratory techniques, and he was provided the sensory nerve stimulation therapy in addition to conventional therapy. His wound healed after 4 weeks. We report the case here. Prior to nerve stimulation therapy, cutaneous sensation, microvascular blood flow and oxygen tension were found to be reduced near the ulcer when compared with the opposite, non ulcerated leg. After therapy, oxygen tension and microvascular blood flow had improved. This case provides further evidence that sensory nerve stimulation therapy at the stipulated parameters improves wound healing. The observation that sensory nerve function improved provides support for the notion that improvement in healing is mediated by improved nerve function.

Age-related changes in microvascular blood flow and transcutaneous oxygen tension under Basal and stimulated conditions

BACKGROUND

Adequate cutaneous microvascular blood flow and tissue oxygen tension are important prerequisites for successful tissue repair. The efficacy of tissue repair decreases with age and is linked to the age-related functional decline of unmyelinated sensory neurons that are important for inflammation and tissue repair. However, available information on the effect of these neuronal changes on microvascular blood flow and tissue oxygen tension is limited, particularly under control and injury conditions. The authors had two aims in this study: (a) to assess age-related changes in the relationship between microvascular blood flow and tissue oxygen perfusion under basal and two different stimulated conditions (sensory dependent and sensory independent), and (b) to clarify the biological meaning of transcutaneous partial pressure of oxygen (tcPO2) measurements.

METHODS

The effects of a sensory-independent vasodilator (acetylcholine) and a sensory-dependent vasodilator (capsaicin) on microvascular blood flow and oxygen perfusion in persons of different ages were measured. Laser Doppler flowmetry and a commercially available transcutaneous oxygen monitor (with sensors set at 39 degrees C and 44 degrees C) were used. Healthy volunteers were recruited: 11 young, 14 middle aged, and 19 older.

RESULTS

Under basal conditions (skin temperature, 37 degrees C to 39 degrees C), both basal blood flow and tcPO2 increased with increasing age. However, with the sensor set at 44 degrees C, tcPO2 showed a significant decrease with age. Acetylcholine increased blood flow approximately equally in the three age groups. Capsaicin increased blood flow and tcPO2 in all age groups, with the young showing a greater increase compared with the older participants.

CONCLUSIONS

The age-associated changes in basal and stimulated microvascular blood flow and tcPO2 could be attributed in part to altered neuronal function. Measuring tcPO2 at 39 degrees C showed a trend toward an increase with age. In contrast, a decrease with age was observed when tcPO2 was measured at 44 degrees C, a temperature sufficient to activate sensory nerve endings. The results may reflect a decline in sensory nerve function with age rather than a decrease in oxygen delivery for vascular reasons. This is supported by the complementary data showing a significant age-related decrease in stimulated blood flow in response to capsaicin, with no change in the response to the sensory-independent vasodilator acetylcholine. Thus, for clinical purposes, data obtained using the tcPO2 monitor should be interpreted with full knowledge of the conditions under which the measurements were made. Furthermore, for scientific purposes, the tcPO2 monitor could be used to assess sensory nerve function when sensors are heated to 44 degrees C.

The effect of aging on phenylephrine response in normal subjects

INTRODUCTION

With aging, cardiac responses to β-adrenergic stimulation decline but the responses to α1-stimulation are less clear. Moreover, whether aging, in the absence of disease, influences the left ventricular response to an increase in afterload is unclear. This study examined the effect of aging on heart rate (HR), blood pressure (BP), cardiac index (CI) and several left ventricular contractility measurements during α 1-stimulation with a phenylephrine infusion.

METHODS

Subjects were rigorously screened to be normal by history, physical, blood tests, ECG, ETT and echocardiogram. Twelve young (mean 26 years, all male) and 15 aged (69 years, 11 males) subjects were studied during 10 minute infusions of phenylephrine at 0.5 and 1.0 mcg/ kg/min. HR, BP and radionuclide ventriculographic cardiac volumes were measured.

RESULTS

Systolic BP increased more in the aged than in the young (22 vs. 13%, p=0.003), while heart rate (16 vs. 21%, p=0.05) fell less. Contractile responses to phenylephrine, including EF, stroke volume index (SVI), stroke work index and left ventricular contractility index were not altered with aging. Systemic vascular resistance (SVR) was higher at baseline and at each infusion rate, but there was no age-associate change in the response to PE.

CONCLUSIONS

In a healthy normal aged population, a preserved SVI response in the setting of a higher baseline SVR results in an increased SBP response to α1-stimulation. Contractile responses to increased afterload are not altered with aging. Age-associated differences in the response to α1-stimulation are small and are explained by altered baroreflex sensitivity and a stiffer vasculature.

Paradoxically Enhanced Endothelin-B Receptor–Mediated Vasoconstriction in Conscious Old Monkeys

Background —We investigated the effects of aging on the responses to endothelin (ET) in conscious old (19.8±0.6 years) and young adult (6.8±0.3 years) monkeys and compared these results with those of other vasoconstrictors, eg, phenylephrine (PE) and angiotensin II (Ang II).

Methods and Results —The monkeys ( Macaca fascicularis ) were chronically instrumented. Baseline total peripheral resistance (TPR) was not different between the 2 groups. As expected, TPR rose less ( P <0.05) with PE (5 μg/kg) in old monkeys (34±3%) than in young monkeys (57±6%); TPR also rose less with Ang II. Surprisingly, TPR rose more ( P <0.05) with endothelin-1 (ET-1, 25 ng · kg −1 · min −1 ) in old monkeys (36±6%) than in young monkeys (10±2%). An ET B receptor agonist, sarafotoxin (S6c, 30 ng · kg −1 · min −1 ) was administered in the presence of an ET A receptor antagonist, BQ-123 (1 mg/kg). Under these conditions, TPR increased more ( P <0.05) in old monkeys (59±10%) than in young monkeys (31±4%). In the presence of nitric oxide synthase (NOS) inhibition with N W -nitro- l -arginine methyl ester (60 mg/kg), vasoconstriction induced by S6c no longer differed with age, because it was enhanced in young monkeys ( P <0.05) (68±9% versus 31±4%) but not in old monkeys (58±6% versus 59±10%). Thus, after NOS inhibition, vasoconstrictor responses to ET were no longer enhanced in old monkeys.

Conclusions —Peripheral vasoconstriction (PE and Ang II) is reduced in old monkeys, as expected. Paradoxically, vasoconstriction induced by ET-1 was actually enhanced in old monkeys, which appears to be a result of impaired endothelium-dependent vasodilation, which with ET-1 should involve the ET B receptor.

Maturation and aging-related differences in responsiveness of rat aorta and carotid arteries to alpha1-adrenoceptor stimulation

Functional correlates of alpha1-adrenoceptors were studied in thoracic aorta and carotid arteries from Wistar rats, at maturation (3- and 8-week-old) and aging (17- and 29-week-old) phases, using phenylephrine (PHE) and KCl. PHE and KCl effects on rat aorta did not differ with age. A reduction in maximal contractions to PHE or KCl occurred in carotid arteries during maturation. Concentration-inhibition curves to the alkylating agent chloroethylclonidine or WB4101 suggested at all ages studied a predominance of alpha1B-adrenoceptors in aorta. However, alpha1A-adrenoceptors may account for some activity in aortas of 29-week-old rats. In carotid arteries alpha1D-adrenoceptors account for functional correlates at all ages studied. The results indicate that maturation or aging affects vascular responsiveness or predominant receptor subtype differently in aorta and carotid arteries with intact endothelium.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Age and region-dependent contraction to alpha-adrenoceptor agonists in rat and guinea-pig isolated trachea

1. The influence of age and of region on alpha-adrenoceptor-mediated contraction to (-)-adrenaline and (-)-noradrenaline was examined in rat (4-136 weeks) and guinea-pig (2-156 weeks) isolated tracheal ring preparations with particular emphasis on the early (up to 12 weeks) maturation phase. 2. In rat tracheal rings, significant regional variation was observed with respect to maximal (-)-adrenaline-induced contraction, such that the greatest activity was seen in ring preparations from the laryngeal end of the trachea. Tracheal rings from the carinal end responded very poorly or were unresponsive to (-)-adrenaline, depending on animal age. These regional differences were seen across the age range. The potencies of (-)-adrenaline and (-)-noradrenaline remained unchanged with respect to animal age, but the maximum contractile tension that developed in response to these agonists increased with increasing animal age in all regions of the trachea. 3. In guinea-pig isolated tracheal tissue, maximum contractile responses (Emax) to (-)-adrenaline and (-)-noradrenaline remained unchanged with increasing animal age. In addition, there was no evidence for a region-dependence in the responsiveness of tracheal tissue to alpha-adrenoceptor-mediated contraction in this species. 4. In both guinea-pig and rat isolated tracheal tissue, alpha-adrenoceptor-mediated contraction appeared to involve the activation of alpha1-adrenoceptors.

Sympathetic modulation of sensory nerve activity with age: human and rodent skin models

1. Sensory nerves serve an afferent role and mediate neurogenic components of inflammation and tissue repair via an axon reflex release of sensory peptides at sites of injury. Dysfunction of these nerves with age could contribute to delayed tissue healing. 2. Complementary animal and human skin models were used in the present studies to investigate changes in the modulation of sensory nerve function by sympathetic efferents during ageing. Laser Doppler flowmetry was used to monitor neurogenic skin vascular responses. 3. The animal model used skin of the hind footpad of anaesthetized rats combined with electrical stimulation of the sciatic nerve, while the human model comprised capsaicin electrophoresis to the volar surface of the forearm. Sympathetic modulation was effected by systemic phentolamine pretreatment in animals and local application in the human model. 4. The results obtained from the human model confirmed the reported decline in sensory nerve function and showed no change in sympathetic modulation with age. The results from the animal model confirm and expand results obtained from the human model. 5. The use of low (5 Hz) and high (15 Hz) frequency electrical stimulation (20 V, 2 ms for 1 min) revealed a preferential response of aged sensory nerves to low-frequency electrical stimulation parameters with differential sympathetic modulation that is dependent on the frequency of stimulation.

Alpha 1-adrenoceptor responsiveness in the aging aorta

Previous studies from this laboratory have shown that aortic alpha 1-adrenoceptor-mediated responsiveness is altered during maturation and aging. This study examines the possibility that there is a change in the alpha 1-adrenoceptor subtypes in the aorta during maturation and aging. The apparent affinity of norepinephrine, as determined by partial receptor inactivation with the alpha 1-adrenoceptor antagonist phenoxybenzamine, was found to be higher in 1-month-old rats compared to 6- and 24-month-old rats. The alpha 1B-adrenoceptor subtype-selective antagonist chlorethylclonidine was used to examine possible heterogeneity in aortic alpha 1-adrenoceptors. The inhibitory effect of chlorethylclonidine on norepinephrine-stimulated contraction was greater in young animals compared to aged animals. Chlorethylclonidine blocked norepinephrine-stimulated inositol phosphate accumulation in 1-month-old aorta but it produced only partial inhibition in the 6- and 24-month-old aortas. The relatively non-selective alpha 1-adrenoceptor antagonists phenoxybenzamine (0.1 microM) and prazosin (0.1 microM) inhibited inositol phosphate accumulation and contractile responses in all ages. The complete block of alpha 1-adrenoceptor-mediated responses by chlorethylclonidine in younger animals shows that alpha 1-adrenoceptor-mediated responses are mediated by the chlorethylclonidine-sensitive alpha 1-adrenoceptor subtypes. The partial inhibition by chlorethylclonidine of alpha 1-adrenoceptor-mediated responses in 6- and 24-month-old animals indicates an increased role of an alpha 1-adrenoceptor subtype that is relatively insensitive to chlorethylclonidine.

Effects of portal hypertension on responsiveness of rat mesenteric artery and aorta

1. We have examined the effects of pre-hepatic portal hypertension on the responsiveness of rat small mesenteric arteries and aorta. Rats were made portal hypertensive by creating a calibrated portal vein stenosis, or sham-operated. 2. In rat mesenteric arteries, there was no significant difference between portal hypertensive and sham-operated animals in the contractile potency of noradrenaline (NA), but the maximum contractile responses to NA, U46619 and KCl were significantly increased in vessels from portal hypertensive animals. This altered maximum contractile response was not due to alterations in smooth muscle mass. 3. In rat mesenteric arteries, there were no significant differences between portal hypertensive and sham-operated animals in endothelium-dependent relaxations to acetylcholine (ACh). The difference between portal hypertensive and sham-operated rats in the maximum response to U46619 was maintained following a combination of methylene blue (1 microM) and NG-monomethyl-L-arginine (100 microM), suggesting that any differences in endothelial function do not explain differences in the response to vasoconstrictors. 4. In rat aorta, there were no significant differences between portal hypertensive and sham-operated animals in the contractile response to NA or KCl or in the endothelium-dependent relaxations to ACh. 5. In pithed rats, there was no difference between portal hypertensive and sham-operated animals in the pressor potency of NA. 6. It is concluded that portal hypertension produces an increase in the contractile response to the vasoconstrictors NA, U46619 and KCl in rat mesenteric arteries but not in the aorta. This suggests that the diminished responsiveness to vasoconstrictors reported in portal hypertensive rats in vivo is not due to a diminished responsiveness at the level of the vascular smooth muscle.

Aging and vasoconstrictor responses mediated by alpha 2-adrenoceptors and 5-HT1 and 5-HT2 receptors

The potencies of noradrenaline (NA) at producing alpha 2-adrenoceptor mediated contractions, and of 5-hydroxytryptamine (5-HT) at producing 5-HT1 and 5-HT2 receptor mediated contractions were investigated in rings of human saphenous vein obtained from varicose vein surgery. There was a significant negative correlation between agonist potency and age for NA at alpha 2-adrenoceptors (r = 0.52, n = 21, p < 0.05) and for 5-HT at both 5-HT1 (r = 0.47, n = 19, p < 0.05) and 5-HT2 (r = 0.54, n = 19, p < 0.05) receptors, so that both agonists were less potent with increasing age. This demonstrates an age-related decrease in alpha 2-adrenoceptor and 5-HT1 and 5HT2 mediated contractile responsiveness in the human saphenous vein.

Vascular responsiveness in young, diabetic, and aging hyperinsulinemic rats

The purpose of this study was to compare vascular responsiveness in young (12 week old), aging hyperinsulinemic-glucose intolerant (52 weeks old) and diabetic (streptozotocin; 14 weeks old) rats. Aortic rings with and without endothelium were maintained in organ chambers for isometric tension recording. The contractile response to KCl was significantly enhanced in aortae from diabetic animals when compared to the responses obtained in young and old ones. The contractile response to norepinephrine or U46619, was significantly shifted to the right in the aortae from aging animals, however the aortae from these hyperinsulinemic rats were hyperresponsive to serotonin. Acetylcholine and ADP provoked an endothelium-dependent relaxation which was markedly depressed in the aortae from diabetic animals. The relaxation to ADP was selectively inhibited in the aging animals. The effect of sodium-nitroprusside was not significantly different in the three groups. Isoproterenol and forskolin induced endothelium-independent relaxation. Isoproterenol responses were inhibited in aging and diabetic animals, however the forskolin-relaxation was inhibited only in the aortae from aging animals. These results suggest that in two models of diabetes (i.e. Type I insulin-dependent and type II non insulin-dependent) vascular responsiveness is differently affected. Aging hyperinsulinemic animals present a selective hyperresponsiveness to serotonin, a selective dysfunction of ADP-induced endothelium-dependent relaxation and smooth muscle adenylate cyclase deficit. In diabetic animals a beta adrenergic hyporesponsiveness, not linked to adenylate-cyclase dysfunction, and non-selective depression of endothelium-dependent responses can be observed.

Modulation by aging of the coronary vascular response to endothelin-1 in the rat isolated perfused heart

Changes with age in the coronary vascular response to endothelin-1 were investigated in perfused hearts isolated from 2-, 6- and 24-month-old (mo) male Fisher-344 rats. Endothelin-1 injected as a single bolus (0.3, 3 and 30 nmol) into the coronary artery supply caused dose-dependent vasoconstriction in all three age groups. While there was no age-related change in the vasoconstriction induced by the lower doses (0.3 and 3 nmol), the higher dose (30 nmol) elicited a more pronounced vasoconstriction in 6- and 24-mo rats than that in 2-mo rats. NG-nitro-L-arginine (L-NNA), an inhibitor of nitric oxide formation, markedly enhanced the vasoconstriction induced by 30 nmol endothelin-1 in 2- and 6-mo rats but only slightly and non-significantly enhanced that vasoconstriction in 24-mo rats. Haemoglobin, which inhibits activation of guanylate cyclase by nitric oxide, enhanced the endothelin-1-induced vasoconstriction in 2-mo rats, but not in 6- and 24-mo rats. The acetylcholine-induced coronary vasodilation was more pronounced in 2- and 6-mo rats than in 24-mo rats and was attenuated by L-NNA in 2- and 6-mo rats. The coronary vasodilation induced by nitroprusside (0.1 mmol), a pharmacological precursor of nitric oxide, did not change with age. Endothelin-1 (30 nmol) markedly increased the release of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) in all three age groups. The prostaglandin synthesis inhibitor indomethacin enhanced the endothelin-1-induced vasoconstriction in 2- and 6-mo rats to a similar extent.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenoceptor coupling mechanisms which regulate salivary secretion during aging

In parotid slices and membranes from Wistar rats 2, 12 and 24 months old, changes are noted in adrenoceptor-stimulated K+ fluxes, formation of [3H]inositol phosphates ([3H]IPs), cAMP production, and membrane environment. Norepinephrine-stimulated K+ efflux and formation of [3H]IPs in the slices proceed through an alpha 1-adrenergic mechanism and are reduced 20% and 40% during aging, respectively. In beta-adrenoceptor stimulation with isoproterenol, no age changes were observed in K+ influx and cAMP production. The cholesterol content in membranes was reduced with age; concomitantly, the membrane viscosity decreased with age. These results indicate that the alterations in the membrane environment may provide age-dependent modulation of alpha 1-adrenoceptor coupling mechanisms and their functions.

Effects of aging on postsynaptic alpha 1-adrenoceptor mechanisms in rat aorta

1. Effects of aging on alpha 1-adrenoceptor and S2-serotonin receptor mechanisms in rat aorta were studied. 2. In rat aorta, the potency (pD2 value) of norepinephrine or phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter with age from 10 to 80 weeks. The affinity (pKA value) of norepinephrine or phenylephrine and of prazosin (pA2 value) did not alter with aging. 3. In rat vas deferens, the efficacy of norepinephrine and the maximum binding sites of [3H]prazosin increased with age from 3 to 18 weeks, but decreased thereafter with age from 18 to 60 weeks. The affinity (pKA value) of norepinephrine and the dissociation constant (KD value) of prazosin did not alter with aging. 4. In rat aorta, the potency (pD2 value) and affinity (pKA value) of serotonin, and affinity (pA2 value) of ketanserin did not alter with aging. 5. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level [( Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats. 6. These results suggest that changes in alpha 1-adrenoceptor mechanisms with aging are due to changes in receptor density or receptor reserve, but not to changes in affinity of drugs to alpha 1-adrenoceptor or sensitivity of contractile system to Ca2+ mediated through alpha 1-adrenoceptor, and that S2-serotonin receptor mechanisms in rat aorta do not alter with aging.

Developmental changes in alpha 1-adrenoceptor coupling to G-protein in bovine aorta

Differences in epinephrine binding to alpha 1-adrenoceptors, epinephrine-induced contractile potency, susceptibility to phorbol ester (PDBu) modulation, and differences in membrane fluidity were studied in bovine aorta from young (3-8 weeks) and adult or mature (6-8 years) animals. Membranes prepared from aorta of adult animals exhibited a two-fold higher receptor density while [3H]prazosin affinity was unchanged. Epinephrine displacement studies revealed both high- and low-affinity binding in membranes from the aortas of young animals whereas, preparations from adult animals exhibited only a single class of low-affinity sites. In low-temperature binding studies, membranes prepared from aortas of adult animals exhibited both high- and low-affinity agonist binding, in proportions about equal to those of young animals. The ability of PDBu to uncouple alpha 1-adrenoceptor from G-protein interaction is demonstrable in young but not in adult animals which raises the possibility of prior phosphorylation of receptors in the latter tissues. Aortas from young animals showed increased contractile potency to epinephrine and, in addition, were significantly more fluid as compared to aortas from adult animals. Alterations in the membrane environment or phosphorylation state of the alpha 1-adrenoceptor may thus provide age-dependent modulation of its function.

Changes in expression of autonomic nerves in aging and disease

Mechanisms of autonomic neuroeffector transmission are summarised, including evidence for a multiplicity of transmitters, co-transmission, neuromodulation and 'chemical coding' of individual autonomic neurons, where the combination of transmitters they contain is known, as well as their projections and central connections. Changes in expression of autonomic nerves and co-transmitters that occur during development and aging, following trauma, surgery, after chronic exposure to drugs, and in a number of disease situations are described. It is suggested that in neuropathological analysis, compensatory increases in innervation should be considered as well as loss or damage to nerves. Studies of the molecular mechanisms involved in the control of co-transmitter and receptor expression are now needed.

Age-related differences in the depressor responses to sodium nitroprusside and nifedipine in anaesthetized rats

1. The blood pressure lowering actions of sodium nitroprusside (SNP) and nifedipine were compared in 5-month and 22-month pentobarbitone-anaesthetized Sprague-Dawley rats. 2. Five-month-old rats had significantly higher resting diastolic blood pressure (DBP) than 22-month rats and this may explain why SNP (1-30 micrograms kg-1) produced significantly greater falls of DBP in absolute terms in 5-month animals. 3. The depressor responses to nifedipine were similar in young and old rats, despite differences in resting DBP, suggesting that, in comparison to nifedipine, SNP is less effective at lowering DBP in aged rats.

Effect of aging on presynaptic alpha 2-adrenoceptor mechanisms in guinea pig ileum

1. Presynaptic alpha 2-adrenoceptor mechanisms in electrically stimulated longitudinal muscles of ilea isolated from 3, 10, 20 and 47 week-old guinea pigs were studied by analysis of the concentration-response curves of noradrenaline, a full agonist, and clonidine, a partial agonist, and the Scatchard plot of specific binding of [3H]-p-aminoclonidine to synaptosomal fractions from the longitudinal muscle of guinea pig ileum. 2. The pD2 value of noradrenaline and the maximum contraction induced by clonidine increased with age from 3 to 20 weeks and there after decreased to 47 weeks, while the pA2 value of yohimbine against noradrenaline did not alter with age. 3. The capacity of the maximum binding sites of [3H]-p-aminoclonidine increased with increasing age (3-20 weeks), while the dissociation constant (Kd) of [3H]-p-aminoclonidine did not change during the same period. 4. The changes in the presynaptic alpha 2-adrenoceptor mechanisms with age are considered to be due to the change in the total concentration of presynaptic of alpha 2-adrenoceptors.

Pertussis toxin and pre-junctional alpha 2-adrenoreceptors in rat heart and vas deferens

1. The effects of pretreatment with pertussis toxin (50-200 micrograms kg-1, i.p.) on the pre-junctional inhibitory actions of the alpha 2-adrenoreceptor agonist xylazine were examined by employing the pithed rat and rat isolated vas deferens. 2. In pithed rats, pertussis toxin attenuated the pressor response to xylazine, but did not alter the pre-junctional cardio-inhibitory actions of xylazine. In epididymal portions of rat vas deferens, pretreatment with pertussis toxin did not alter the pre-junctional inhibitory action of xylazine. 3. These data lend no support for the view that alpha 2-adrenoreceptor-mediated prejunctional inhibition involves inhibition of adenylate cyclase through the Ni regulatory protein, at least in the tissues examined.