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

Sarcolemmal cholesterol and caveolin-3 dependence of cardiac function, ischemic tolerance, and opioidergic cardioprotection

Cholesterol-rich caveolar microdomains and associated caveolins influence sarcolemmal ion channel and receptor function and protective stress signaling. However, the importance of membrane cholesterol content to cardiovascular function and myocardial responses to ischemia-reperfusion (I/R) and cardioprotective stimuli are unclear. We assessed the effects of graded cholesterol depletion with methyl-β-cyclodextrin (MβCD) and lifelong knockout (KO) or overexpression (OE) of caveolin-3 (Cav-3) on cardiac function, I/R tolerance, and opioid receptor (OR)-mediated protection. Langendorff-perfused hearts from young male C57Bl/6 mice were untreated or treated with 0.02-1.0 mM MβCD for 25 min to deplete membrane cholesterol and disrupt caveolae. Hearts were subjected to 25-min ischemia/45-min reperfusion, and the cardioprotective effects of morphine applied either acutely or chronically [sustained ligand-activated preconditioning (SLP)] were assessed. MβCD concentration dependently reduced normoxic contractile function and postischemic outcomes in association with graded (10-30%) reductions in sarcolemmal cholesterol. Cardioprotection with acute morphine was abolished with ≥20 μM MβCD, whereas SLP was more robust and only inhibited with ≥200 μM MβCD. Deletion of Cav-3 also reduced, whereas Cav-3 OE improved, myocardial I/R tolerance. Protection via SLP remained equally effective in Cav-3 KO mice and was additive with innate protection arising with Cav-3 OE. These data reveal the membrane cholesterol dependence of normoxic myocardial and coronary function, I/R tolerance, and OR-mediated cardioprotection in murine hearts (all declining with cholesterol depletion). In contrast, baseline function appears insensitive to Cav-3, whereas cardiac I/R tolerance parallels Cav-3 expression. Novel SLP appears unique, being less sensitive to cholesterol depletion than acute OR protection and arising independently of Cav-3 expression.

Cholesterol-dependent separation of the beta2-adrenergic receptor from its partners determines signaling efficacy: insight into nanoscale organization of signal transduction

Determining the role of lipid raft nanodomains in G protein-coupled receptor signaling remains fraught by the lack of assays directly monitoring rafts in native membranes. We thus combined extensive biochemical and pharmacological approaches to a nanoscale strategy based on bioluminescence resonance energy transfer (BRET) to assess the spatial and functional influence of cholesterol-rich liquid-ordered lipid nanodomains on beta2 adrenergic receptor (beta2AR) signaling. The data revealed that whereas beta2AR did not partition within liquid-ordered lipid phase, a pool of G protein and adenylyl cyclase (AC) were sequestered in these domains. Destabilization of the liquid-ordered phase by cholesterol depletion led to a lateral redistribution of Galphas and AC that favored interactions between the receptor and its signaling partners as assessed by BRET. This resulted in an increased basal and agonist-promoted beta2AR-stimulated cAMP production that was partially dampened as a result of constitutive protein kinase A-dependent phosphorylation and desensitization of the receptor. This restraining influence of nanodomains on beta2AR signaling was further substantiated by showing that liquid-ordered lipid phase stabilization using caveolin overexpression or increasing membrane cholesterol amount led to an inhibition of beta2AR-associated signaling. Given the emerging concept that clustering of receptors and effectors into signaling platforms contributes to the efficacy and selectivity of signal transduction, our results support a model whereby cholesterol-promoted liquid-ordered lipid phase-embedding Gs and AC allows their lateral separation from the receptor, thus restraining the basal activity and controlling responsiveness of beta2AR signaling machinery within larger signaling platforms.

The effect of temperature on adrenergic receptors of alveolar type II cells of a heterothermic marsupial

Fat-tailed dunnarts, Sminthopsis crassicaudata, survive dramatic changes in body temperature during torpor without experiencing surfactant dysfunction. Adrenergic factors regulate surfactant secretion through beta(2)-adrenergic receptors on alveolar type II cells. Temperature has no effect on the secretory response of dunnart type II cells to adrenergic stimulation. We hypothesise that during torpor, dunnart type II cells up-regulate the number of adrenergic receptors present on type II cells to enable stimulation at lower concentrations of agonist. Here, we isolated type II cells from warm-active (35 degrees C) and torpid (15 degrees C) dunnarts and examined the effects of an in vitro temperature change on the number and activity of adrenergic receptors. Torpor did not affect the beta-adrenergic receptor number. However, we observed a significant decrease in adrenergic receptor number when cells from warm-active animals were incubated at 15 degrees C and when cells from torpid animals were incubated at 37 degrees C. cAMP production was significantly higher in type II cells from torpid dunnarts than warm-active dunnarts and this may contribute, in part, to the temperature insensitivity we have previously observed in the adrenergic regulation of surfactant secretion.

Hypercholesterolemia is associated with a reduced response of smooth muscle guanylyl cyclase to nitrovasodilators

A diminished relaxant response of atherosclerotic arteries to nitrovasodilators has been frequently observed in advanced stages of hypercholesterolemia. In the present study, we investigated whether this effect might be a result of reduced activity of smooth muscle guanylyl cyclase. Experimental atherosclerosis was induced by feeding rabbits a cholesterol-rich diet (1%) over a period of 4 months. Aortas were removed and homogenized, and guanylyl cyclase activity was measured in the 100,000 g supernatants. Sodium nitroprusside, which stimulated cyclic GMP (cGMP) formation in control tissues almost 200-fold (from 3 to 585 pmol cGMP.mg-1 x min-1), increased enzyme activities in atherosclerotic aortas only approximately 90-fold (from 3 to 257 pmol cGMP.mg-1 x min-1). Similarly, the maximal stimulatory effects of S-nitroso-glutathione were reduced from 200-fold (controls) to 114-fold in atherosclerotic tissues. Basal guanylyl cyclase activities were identical in both atherosclerotic and control vessels. Hypercholesterolemia also reduced the activity of smooth muscle adenylyl cyclase. In control aortas, basal and NaF-stimulated enzyme activities were 24 and 349 pmol cAMP.mg-1 x min-1, respectively, whereas cAMP formation was reduced in atherosclerotic aortas to 7 (basal) and 96 (NaF) pmol cAMP.mg-1.min-1. The stimulatory effect of NaF (approximately 14-fold) remained unchanged. Since adenylyl and guanylyl cyclase have important functions in regulating vascular tone, reduced activities of both enzymes may contribute to the diminished relaxant and/or enhanced vasoconstricting effects of vasoactive compounds in atherosclerotic blood vessels.

Circadian and seasonal rhythms of 5-HT receptor subtypes, membrane anisotropy and 5-HT release in hippocampus and cortex of the rat

Specific serotonin binding (5-HT1, 5-HT1A, and 5-HT2 subtypes) and membrane anisotropy were measured at 2 h intervals over a 24 h period in the hippocampus and cortex of Wistar WU rats, housed under a 12 h light-dark cycle, with lights on at 07.00. All experiments were performed both in March and December. In the hippocampus significant circadian rhythms could be ascertained for 5-HT1 binding sites in March and December while for 5-HT1A (subtype of 5-HT1) binding sites the circadian rhythm was only significant in March. The membrane anisotropy also showed significant variations only in March. Circadian rhythms were also found in the cortex for 5-HT1 (December) and 5-HT2 (March and December) binding sites as well as for the membrane anisotropy (December). A correlation was found between membrane anisotropy and 5-HT1 and 5-HT2 binding sites in hippocampus and cortex, respectively. A circadian rhythmicity was also observed for serotonin release as measured by in vivo voltammetry in both brain areas. The results obtained on the diurnal variations of serotonin receptor subtypes and serotonin release and the probable inverse relationship of these two parameters may be relevant in understanding the coupling of pre- and postsynaptic activity.

Cholesterol modulation of membrane fluidity and VIP receptor/effector system in rat prostatic epithelial cells

Treatment of rat prostatic epithelial cells with cholesteryl hemisuccinate (ChH) resulted in a time- and dose-dependent inhibition of the stimulatory effect of the neuropeptide vasoactive intestinal peptide (VIP) on cyclic AMP accumulation, with a 40% decrease in the response to a maximally effective VIP concentration. Cell treatment with ChH led also to a similar blocking of isoproterenol (a beta-adrenergic agonist) action but did not modify forskolin (which is assumed to act directly on the catalytic unit of adenylate cyclase) activity upon cyclic AMP levels. The levels of the transduction protein Gs were similar in membranes from both control and ChH-treated cells as suggested by experiments on cholera toxin-catalyzed ADP-ribosylation. The inhibitory effect of ChH was accompanied by an increase of membrane microviscosity as estimated by measurements of fluorescence polarization. Experiments on VIP binding indicated that increasing cholesterol concentration in the plasma membrane led to a higher VIP binding capacity without changes in the affinity of VIP receptors. These data suggest that membrane cholesterol incorporation diminishes the coupling efficiency between adenylate cyclase and the VIP-receptor complex or other receptor systems (i.e., desensitization) due to an increase of plasma membrane rigidity.

Beta-adrenergic function in aging. Basic mechanisms and clinical implications

Catecholamines have an important endocrine and neuroendocrine role in mediating a variety of autonomic functions. One consequence of normal aging, in particular in the cardiovascular system, is a decline in beta-adrenergic function associated with an alteration in responsiveness to beta-adrenergic therapy. The intrinsic ability for muscle contractility or relaxation is maintained with age and there appears to be an alteration in the process linking the receptor with the contractile or relaxation mechanisms. In rats, beta-adrenergic receptor density decreases with age in adipose tissues and most brain areas, is unchanged in lymphocytes, heart and lung, and increases in the liver. In humans, there are no receptor changes with age in either lymphocytes or brain. In contrast, the number of high-affinity receptors (or coupled receptors) decreases with age in most tissues. In addition, there is a decrease in membrane adenylate cyclase activity or cellular production of cyclic adenosine monophosphate (adenosine 3',5'-cyclic phosphate; cAMP). Plasma noradrenaline (norepinephrine) concentration increases with age. The reduced receptor number in some tissues (down-regulation), the reduced high-affinity receptors and the reduced hormone-stimulated adenylate cyclase activity with age suggests receptor desensitisation to increased plasma noradrenaline concentration. The inability of older animals to desensitise to beta-adrenergic agonists further supports this hypothesis. However, there is an additional post-receptor reduction in catalytic unit activity with age independent of desensitisation. Medications directed at the beta-adrenergic system are commonly used in the elderly. Many of the data on the impact of age on clinical responses are conflicting or unavailable. Concomitant disease, functional status, nutritional state and polypharmacy may play an even greater role than age. However, the available data can be used to guide the selection of therapy, anticipate side effects, and predict potential interactions with other medications and diseases.

Modulation of the beta-adrenergic stimulation of cyclic AMP accumulation in rat prostatic epithelial cells by membrane fluidity

1. The influence of membrane lipid composition on the beta-adrenergic stimulation of cyclic AMP accumulation in rat prostatic epithelial cells was assessed after treating the cells with cholesteryl hemisuccinate (ChH). 2. ChH treatment resulted in a 40% inhibition of the beta-adrenergic response after 30 min of lipid preincubation at 37 degrees C. 3. The inhibitory effect of ChH was dose-dependent and was accompanied by an increase of microviscosity as measured by a fluorescence polarization technique with the probe diphenylhexatriene. 4. Experiments with increasing concentrations of isoproterenol indicated that the efficiency, but not the potency, of the beta-adrenergic response was affected by the increasing of the cholesterol content in the cell membrane.

Age-related changes in vasoactive intestinal polypeptide levels and distribution in the rat lung

Vasoactive intestinal polypeptide (VIP) levels and distribution were studied in the lung of young-adult (3-month-old) and aged (28-month-old) male Wistar rats by radioimmunoassay and immunofluorescence. VIP concentrations were reduced approximately by 60% as the animal ages. The density of VIP-immunoreactive nerve fibres was remarkably reduced within bronchial smooth muscle and bronchial glands. Moreover, the number of VIP-immunoreactive nerve cell bodies located in intraparenchymal ganglia was decreased in old rats. The density of VIP-containing perivascular plexuses was slightly reduced in senescence. The present data are indicative that VIP neuronal system is impaired in the lung of old rats. In view of the significant age-dependent loss of VIP-immunoreactive nerve fibres that supply the bronchial tree and bronchial glands it cannot be excluded that the relaxant action exerted by peptide on airway smooth muscle and the control of bronchial secretion exerted by VIP are impaired in old age.

Neurotransmitter receptor interactions with G-proteins: a critical link in receptor response mechanisms

The interaction of membrane bound receptors with guanine nucleotide binding regulatory proteins is necessary for the transduction of many extracellular signals across the cell membrane. Age-related impairments of this interaction illustrate that the efficiency of receptor effector coupling is complex, and is an additional factor for consideration in the future study of receptor mediated functions.

Cyclic nucleotides and atherosclerosis: studies in primary culture of human aortic cells

A primary culture of cells derived from uninvolved and atherosclerotic intima of human aorta was used to elucidate the role of cyclic nucleotides in atherogenesis. The cells cultured from fatty streaks and atherosclerotic plaques had a 2- to 8-fold lower cyclic AMP level and a 1.5- to 2-fold higher level of cyclic GMP compared with those of a grossly normal intima. Medial cells cultured from nonlesioned and atherosclerotic aortic segments showed no differences in the cyclic nucleotide concentrations. Reduction of the intracellular cyclic AMP with 2'-deoxyadenosine or a cyclic GMP elevation with its dibutyryl derivative, or liposomes containing cyclic GMP stimulated the uptake of [3H]thymidine and protein synthesis in the cells cultured from unaffected intima. On the contrary, a rise of the intracellular cyclic AMP caused by adenylate cyclase activators, a phosphodiesterase inhibitor, dibutyryl cyclic AMP, and liposomes containing cyclic AMP inhibited cell proliferation and protein synthesis. Elevation of the intracellular cyclic AMP stimulated the hydrolysis of lipids which led to reduction of lipid levels in the cells cultured from atherosclerotic lesions. The results of this study corroborate the existence of a relationship between the alterations of intracellular cyclic nucleotide levels and the metabolic disorders occurring in atherosclerosis.

Cholesterol modulation of beta-adrenergic receptor characteristics

Cholesterol, a major structural component of plasma membranes, has a profound influence on cell surface receptor characteristics and on adenylate cyclase activity. beta-Adrenergic receptor number, adenylate cyclase activity, and receptor-cyclase coupling were assessed in rat lung membranes following preincubation with cholesteryl hemisuccinate. beta-Adrenergic receptor number increased by 50% without a change in antagonist affinity. However, beta-adrenergic receptor affinity for isoproterenol increased 2-fold as a result of an increase in the affinity of the isoproterenol high-affinity binding site. The increase in agonist affinity did not potentiate hormone-stimulated adenylate cyclase activity, which decreased 3-fold following cholesterol incorporation. However, the ratio of isoproterenol to GTP-stimulated activity was unchanged with cholesterol. Stimulation distal to the receptor by GTP, NaF, GppNHp, Mn2+ and forskolin also demonstrated 50-80% reduced enzyme activity following cholesterol incorporation. These data suggest that membrane cholesterol incorporation decreases catalytic unit activity without affecting transduction of the hormone signal.