Activation Of Catalytic Unit Research Articles

Functional effects of chronic electroconvulsive shock on serotonergic 5-HT 1A and 5-HT 1B receptor activity in rat hippocampus and hypothalamus

The aims of this work were to determine the influence of chronic electroconvulsive shock (ECS) on presynaptic 5-HT 1A receptor function, postsynaptic 5-HT 1A receptor function in hippocampus and hypothalamus, and presynaptic 5-HT 1B receptor function in hippocampus and hypothalamus. This represents part of an on-going study of the effects of ECS on serotonergic receptor activity in selected brain areas which may be relevant to the effects of electroconvulsive therapy (ECT) in humans. Chronic ECS reduced the ability of the 5-HT 1A receptor agonist 8-hydroxy-2(di- n-propylamino)tetraline (8-OH-DPAT) (0.2 mg/kg s.c.) to decrease 5-HT levels in hypothalamus as shown by in vivo microdialysis, indicative of a reduction in sensitivity of presynaptic 5-HT 1A autoreceptors. The ability of the 5-HT 1B receptor antagonist GR 127935 (5 mg/kg s.c.) to increase 5-HT levels in both hippocampus and hypothalamus was unaffected by chronic ECS. 8-OH-DPAT (0.2 mg/kg s.c.) increased cyclic AMP levels in hippocampus measured by in vivo microdialysis approximately 2-fold. The degree of stimulation of cyclic AMP formation was not altered by chronic ECS. However the cyclic AMP response to forskolin (50 μM) administered via the microdialysis probe, which was approximately 4-fold of basal in sham-treated rats, was almost completely abolished in ECS-treated rats. Since this indicates that either adenylate cyclase catalytic unit activity or Gs protein activity is reduced in the hippocampus after chronic ECS, the lack of change in 8-OH-DPAT-induced cyclic AMP formation may be taken as possible evidence of an increase in sensitivity of postsynaptic 5-HT 1A receptors in the hippocampus by chronic ECS. Chronic ECS increased basal plasma levels of corticosterone, ACTH and oxytocin. The ACTH response to s.c. injections of 0.2 mg/kg or 0.5 mg/kg 8-OH-DPAT was reduced by chronic ECS. Postsynaptic 5-HT 1A receptor activity in the hypothalamus, in contrast to the hippocampus, thus appears to be desensitized after chronic ECS. We conclude that chronic ECS has regionally specific effects on both pre- and post-synaptic 5-HT 1A receptors, but, in contrast to some antidepressant drugs, does not affect presynaptic 5-HT 1B receptor activity.

Discriminant Responses of the Catalytic Unit and Glucose 6-phosphate Transporter Components of the Hepatic Glucose-6-Phosphatase System in Ehrlich Ascites-Tumor-Bearing Mice

The effect of Ehrlich ascites tumor cells, in vivo, on the hepatic glucose-6-phosphatase (G6Pase) system was examined. The Vmax for glucose 6-phosphate hydrolysis by G6Pase was reduced by 40% and a greater than 15-fold decrease in mRNA encoding the catalytic unit of the G6Pase system was observed 8 days after injection with tumor cells. Blood glucose concentration was decreased from 169 ± 17 to 105 ± 9 mg/dl in tumor-bearing mice. There was no change in the G6P transporter (G6PT) mRNA level. However, there was a significant decrease in G6P accumulation into hepatic microsomal vesicles derived from tumor-bearing mice. Decreased G6P accumulation was also associated with a decrease in G6Pase hydrolytic activity in the presence of vanadate, a potent catalytic-unit inhibitor. In addition, G6P accumulation was nearly abolished in microsomes treated with N-bromoacetylethanolamine phosphate, an irreversible inhibitor of the G6PT. These results demonstrate that the catalytic unit and G6PT components of the G6Pase system can be discriminantly regulated, and that microsomal glucose 6-phosphate uptake is dependent on catalytic unit activity as well as G6PT action.

Beta-adrenergic signal transduction in aging parotid and submandibular salivary glands.

This study assessed agonist- and post-receptor-stimulated adenylate cyclase (AC) activity in parotid and submandibular salivary glands from female F-344 rats of 3, 12, and 24 months of age. Isoproterenol-stimulated dose-response activation of adenylate cyclase was unchanged between 3 and 12 months but decreased at 24 months (p less than .05). Forskolin-stimulated AC activity, representing catalytic unit activity, was decreased at 24 months in the parotid (p less than .05) and at 12 months (p less than .05) and 24 months (p less than .01) in the submandibular gland. Beta-adrenergic signal transduction in salivary glands stimulates the secretion of salivary proteins that have important functions in the maintenance of oral health.

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.

Altered Glucagon- and Catecholamine Hormone- Sensitive Adenylyl Cyclase Responsiveness in Rat Liver Membranes Induced by Manipulation of Dietary Fatty Acid Intake

Glucagon-stimulated adenylyl cyclase activity has been shown to change in liver membranes manipulated to alter either their fatty acid composition or fluidity. We examined whether membrane alterations induced by dietary manipulation affected receptor function. Glucagon- and beta-adrenergic-stimulated receptor-adenylyl cyclase systems were examined in liver membranes of rats fed diets containing 10% corn oil, 10% coconut oil (essential FFA deficient), or 8.5% coconut oil with 1.5% corn oil (essential FFA repleat). Basal and maximal nonreceptor-mediated adenylyl cyclase activity (stimulated by NaF, guanylylimidodiphosphate, and forskolin) was the same in membranes of each of the dietary groups, suggesting that Gs-protein and the catalytic unit activity per se were unaltered by the manipulations. Glucagon-stimulated adenylyl cyclase activity increased with increasing unsaturation of dietary fatty acids; activity in coconut oil-fed rats was 527 +/- 30 (mean +/- SEM) pmol/mg.10 min, that in coconut/corn oil-fed rats was 752 +/- 74 pmol/mg.10 min, and that in corn oil-fed rats was 981 +/- 94 pmol cAMP/mg.10 min. [125I]Monoiodoglucagon binding did not increase in parallel to the adenylyl cyclase alterations; coconut oil-fed animals (614 fmol/mg) differed from the other groups (450 and 430 fmol/mg). Isoproterenol (beta-adrenergic)-stimulated adenylyl cyclase activity was also highest in the corn oil-fed animals, but was similar in the other dietary groups, with no difference in other characteristics of [125I]iodopindolol binding between the groups. The results demonstrate that alterations in the glucagon-stimulated adenylyl cyclase response are different from those in the beta-adrenergic adenylyl cyclase response. Further, they suggest that although direct activations of the catalytic unit or its interaction with the guanine nucleotide-sensitive protein are apparently not affected, hormone receptor-mediated adenylyl cyclase activity may be altered by these dietary manipulations.

Forskolin activation of adenylate cyclase in rat myocardium with age: Effects of guanine nucleotide analogs

Beta-adrenergic and post-receptor activation of adenylate cyclase decreases with age in the rat myocardium. Forskolin-stimulated adenylate cyclase activity decreases with age to the same extent as NaF or isoproterenol stimulation, suggesting that a loss of catalytic unit activity accounts for the loss of activity with age. However, recent evidence suggests that there are both a guanine nucleotide-dependent and independent component of forskolin activation. We assessed the possible role of each of these components with age by measuring forskolin dose-response stimulation of adenylate cyclase activity in the presence and absence of guanosine 5′- O-(2-thiodiphosphate (GDP-βS) and β,γ-imidoguanine 5′-triphosphate (Gpp(NH)p) in myocardial membranes from F-344 rats of 3, 12, and 24 months of age. Maximal forskolin stimulation of adenylate cyclase was least in the 24- (121 ± 21 pmol cAMP/min/mg) as compared with either the 12- (212 ± 29) or 3- (190 ± 19) month-old rats. The presence of GDP-βS had no effect on either the EC 50 or maximum activity, and the age-related decline persisted. Gpp(NH)p plus isoproterenol enhanced forskolin activation but the effect was additive and not synergistic. There was no effect on the EC 50, and the maximum activity was least in the 24-month-old rats. These data reconfirm the loss of catalytic unit activity with age and indicate that the reduced activity with age is independent of the presence or absence of activated G-protein.

P-182 - Inhibition of the catalytic unit activity by the βγ-subunits of the GTP-binding regulatory proteins in the receptor-adenylate cyclase system: Evidence for the molecular interaction between the βγ-subunits and the catalytic unit

P-182 - Inhibition of the catalytic unit activity by the βγ-subunits of the GTP-binding regulatory proteins in the receptor-adenylate cyclase system: Evidence for the molecular interaction between the βγ-subunits and the catalytic unit

Adenylate cyclase in senescence: catecholamine and parathyroid hormone pathways.

There are significant changes in the adenylate cyclase complex of many tissues with age. These changes are not due to a single defect in the system. Rather, the defects may vary from tissue to tissue and may be multiple in nature. There are age-related changes described in the receptor, N-protein, and catalytic components of the adenylate cyclase receptor. Alteration of the level of the receptor may reflect increases in the circulating levels of the agonist with age. The mechanisms responsible for alterations in the N-protein and catalytic unit activity remain to be elucidated. Further work is also needed to determine if the age-related changes in cAMP production result in age-related changes in cAMP-dependent protein kinase activity and the phosphorylation of specific proteins. However, the age-related changes in adenylate cyclase activity that have already been described may at least partially explain the decreased responsiveness of some target tissues to hormones and drugs.

Cholesterol modulation of β-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. β-Adrenergic receptor number, adenylate cyclase activity, and receptor-cyclase coupling were assessed in rat lung membranes following preincubation with cholesteryl hemisuccinate. β-Adrenergic receptor number increased by 50% without a change in antagonist affinity. However, β-adrenergic receptor affinity for isoproterenol increased 2-fold as a result of an increase in the affinity of the isoproterenol high-affinity binding site. This 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, Mn 2+ 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.

Age-associated decrease in the catalytic unit activitiy of rat myocardial adenylate cyclase

Two techniques were employed to define the site of diminished adenylate cyclase activity observed in aged-rat myocardium. We previously reported no change in beta-adrenergic receptor number or affinity but decreased Naf- and 5′-guanylylimidodiphosphate (GppNHp)-stimulated adenylate cyclase activity with age in the rat myocardium. These data suggest a defect in either N-protein or the catalytic unit component of adenylate cyclase. N-protein activity was assessed by the ability of myocardial membrane extractsf from 3-, 12, and 24-month-old Fischer 344 rats to complement the N-protein deficient cyc − S49 mouse lymphoma cell line. Catalytic unit activity was assessed by the ability of forskolin to stimulate adenylate cyclase in myocardial membranes from young and old rats. The results demonstrate that both N-protein activity and catalytic unit activitiy are diminished with age in the rat myocardium. However, since N-protein is present in excess relative to catalytic unit, the data are consistent with the hypothesis that the loss of catalytic unit activity accounts for the loss of overall enzyme activity. The effect is tissue-specific, since erythrocyte membranes do not show the same age-dependent loss of adenylate cyclase activitiy.