Forskolin and ethanol both perturb the structure of liver plasma membranes and activate adenylate cyclase activity

Response of rat small intestinal active aldohexose transport to elevation of mucosal cyclic AMP by forskolin and 3-isobutyl-1-methylxanthine in vitro

The phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine (IBMX) was able to elevate rat small intestinal cyclic AMP levels to 300% of basal values. Active jejunal D-glucose transport was enhanced parallel to the rise of intracellular cyclic AMP levels to 140% of control values at 100 mumol/l IBMX. Transport parameters, as determined in a three compartment model in vitro using a dual label method, indicate increased 'uphill' glucose transport at the site of the brush border membrane, higher intracellular accumulation of the sugar, with unchanged passive permeabilities. Phlorizin-inhibited D-glucose transport and L-glucose transfer in the rat were not affected by the persisting cyclic AMP elevation produced by IBMX. Stimulating effects could also be demonstrated with D-galactose as a substrate. IBMX 100 mumol/l also increased active D-glucose as well as 3-O-methylglucose transport in mouse jejunum. Stimulatory effects on intestinal hexose transport and mucosal cyclic AMP levels were also found with the adenylate-cyclase activator forskolin. In the present study, forskolin effects on jejunal mucosal cyclic AMP levels were enhanced in the presence of 100 mumol/l IBMX, resulting in a 20-fold increase compared to controls at 20 mumol/l forskolin. The concentration response for the effect of forskolin in the presence of 100 mumol/l IBMX on D-glucose transport did not produce a significant increase compared to transport stimulation with IBMX alone. At higher concentrations of forskolin however, glucose transport decreased to levels well below the IBMX controls. The elevation of cellular cyclic AMP levels had no effects on passive permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by ethanol of forskolin-stimulated adenylate cyclase in a murine neuroblastoma clone (N1E-115)

Forskolin, a diterpene activator of adenylate cyclase, stimulated the formation of cyclic AMP in intact murine neuroblastoma clone N1E-115 cells and stimulated adenylate cyclase activity in a membranal preparation from these cells. Ethanol caused a concentration-dependent inhibition of the forskolin-stimulated responses in both preparations. In intact cells, the inhibition appeared to be noncompetitive. However, in the membranal preparation the inhibition was more of a competitive nature. In addition, there was also a large difference in the amount of inhibition in the two systems. Thus, the inhibition by ethanol was nearly twice as much with intact cells as with membranes. Sucrose appeared to mimic these effects of ethanol, suggesting that with intact cells the effect of this alcohol may be due, in part, to changes in cellular osmotic pressure.

Stimulation and inhibition of rat basophilic leukemia cell adenylate cyclase by forskolin

The influence of the diterpene, forskolin, was studied on adenylate cyclase activity in membranes of rat basophilic leukemia cells. Forskolin increased basal adenylate cyclase activity maximally 2-fold at 100 microM. However, adenylate cyclase activity stimulated via the stimulatory guanine nucleotide-binding protein, Ns, by fluoride and the stable GTP analog, guanosine 5'-O-(3-thiotriphosphate), was inhibited by forskolin. Half-maximal and maximal inhibition occurred at about 1 and 10 microM forskolin, respectively. The inhibition occurred without an apparent lag phase, whereas the enzyme stimulation by forskolin was preceded by a considerable lag period. The inhibition was not affected by treating intact cells or membranes with pertussis toxin and proteolytic enzymes, respectively, which have been shown in other cell types to prevent adenylate cyclase inhibition mediated by the guanine nucleotide-binding regulatory component, Ni. The forskolin inhibition of the stable GTP analog-activated adenylate cyclase was impaired by increasing the Mg2+ concentration and was reversed into a stimulation by Mn2+. Under optimal inhibitory conditions, forskolin even decreased basal adenylate cyclase activity. Finally, forskolin largely reduced the apparent affinity of the rat basophilic leukemia cell adenylate cyclase for its substrate, MgATP, which reduction resulted in an apparent inhibition at low MgATP concentrations and a loss of the inhibition at higher MgATP concentrations. The data indicate that forskolin can cause both stimulation and inhibition of adenylate cyclase and, furthermore, they suggest that the inhibition may not be mediated by the Ni protein, but may be caused by a direct action of forskolin at the adenylate cyclase catalytic moiety.

Adenylate cyclase and a fatty acid spin probe detect changes in plasma membrane lipid phase separations induced by dietary manipulation of the cholesterol:phospholipid ratio

Rats fed with a cholesterol supplement to their diet exhibited an increase in their plasma membrane cholesterol phospholipid (C/P)-lipid molar ratio from 0.72 to 0.98, whereas those fed the hypocholesterolaemic drug clofibrate in their diet exhibited a decrease in this ratio to 0.62. The properties of these membranes were analysed with regard to ligand-stimulated adenylate cyclase activity and the mobility of a fatty acid spin probe which allowed lipid phase separations to be identified. Membranes with elevated C/P ratios exhibited two distinct lipid phase separations, one at around 36 degrees C that was attributed to the external half of the bilayer and one at around 22 degrees C which was attributed to the inner half of the bilayer. Membranes with lowered C/P ratios exhibited a single lipid phase separation occurring at around 21 degrees C which was attributed to the lipids of the inner half of the bilayer. These results were compared with those obtained by manipulation of C/P ratios in vitro using liposome-cholesterol exchange techniques. Dietary manipulation of the C/P ratio of plasma membranes in vivo led to alterations in the fold stimulation of adenylate cyclase by various stimulatory ligands.

Ethanol's effects on cortical adenylate cyclase activity

The effects of ethanol on beta-adrenergic receptor-coupled adenylate cyclase (AC) of mouse cerebral cortex were examined. The addition of ethanol (20-500 mM) to incubation mixtures containing cortical membranes demonstrated that ethanol could increase AC activity and potentiate the stimulatory effects of guanylyl-imidodiphosphate [Gpp(NH)p] on AC activity. Ethanol increased the rate of activation of AC by guanine nucleotides and concomitantly decreased the EC50 for magnesium required to achieve maximal stimulation of cortical AC. The EC50 values for Gpp(NH)p and isoproterenol stimulation of AC activity were also altered by ethanol. Ethanol was capable of stimulating AC extracted by use of digitonin. The AC activity in the digitonin extract was no longer sensitive to the addition of Gpp(NH)p or NaF, but was still stimulated by ethanol. We propose multiple sites of action for ethanol in stimulating cortical AC activity. These sites include actions at the beta-adrenergic receptor, at the G/F coupling proteins, and at the catalytic unit of cortical AC. Comparison of ethanol's actions on cortical beta receptor coupled AC activity with prior reported actions of ethanol on striatal dopamine (DA)-sensitive AC indicated differential sensitivities of these two AC systems to ethanol. These differences may be determined by specific coupling characteristics of the striatal and cortical AC systems or by differences in the plasma membranes in which striatal and cortical AC systems are located.

Activation of cAMP dependent protein kinase during surfactant release from type II pneumocytes

Release of surfactant from pulmonary type II epithelial cells was stimulated by the beta-adrenergic agonist terbutaline and the diterpene forskolin. Cytosolic cyclic adenosine monophosphate (cAMP) concentrations increased significantly following exposure to terbutaline or forskolin and reached maximal levels within 5 min after treatment. Terbutaline and forskolin had a synergistic effect on cytosolic cAMP levels when added simultaneously. cAMP-dependent protein kinase activity was identified in cytosolic preparations of type II pneumocytes by phosphorylation of the peptide substrate Kemptide (Leu-Arg-Arg-Ala-Ser-Leu-Gly) and binding of 3H-cAMP to the regulatory components of cAMP-dependent protein kinase. Type I and type II regulatory subunits of the cANP-dependent kinase were present in approximately equal concentrations in type II cell cytosol. Activation ratio of cAMP-dependent protein kinase in cultured type II cells increased significantly in the presence of terbutaline, forskolin, or terbutaline plus forskolin. Activation ratios increased from 0.45 +/- 0.03 for control cells to 0.96 +/- 0.06 for cells exposed to terbutaline (10 microM) plus forskolin (5 microM) for 20 min. Release of 3H-phosphatidylcholine was also stimulated by terbutaline and forskolin. Effects of terbutaline and forskolin on surfactant release were approximately additive. Our results demonstrated increased cytosolic cAMP levels, increased cAMP-dependent protein kinase activation ratios, and subsequent augmented surfactant release from isolated type II pneumocytes in response to terbutaline and forskolin. These data support a role for activation of cAMP-dependent protein kinase as a mediator of surfactant release and document the utility of forskolin for study of cAMP-mediated effects in isolated type II cells.

Biochemical mechanisms responsible for alcohol-associated myocardiopathy

An overview of alcohol metabolism is presented followed by a discussion of the unique pathways for the metabolism of ethanol by the heart. The evidence for cardiac metabolic injury produced as a result of ethanol metabolism is then presented. Such injury involves the mitochondria, contractile proteins, and alterations in calcium fluxes and storage.

Islet-activating protein blocks glucagon desensitization in intact hepatocytes

Treatment of intact hepatocytes with islet-activating protein, from Bordatella pertussis, led to a pronounced increase in the ability of glucagon to raise intracellular cyclic AMP concentrations. Islet-activating protein, however, caused no apparent increase in the intracellular concentration of cyclic AMP under basal conditions. These effects were attributed to an enhanced ability of adenylate cyclase, in membranes from hepatocytes treated with islet-activating protein, to be stimulated by glucagon. When forskolin was used to amplify the basal adenylate cyclase activity, elevated GTP concentrations were shown to inhibit adenylate cyclase activity in membranes from control hepatocytes. This inhibitory effect of GTP was abolished if the hepatocytes had been pre-treated with islet activating protein. In isolated liver plasma membranes, islet-activating protein caused the NAD-dependent ribosylation of a Mr-40000 protein, the putative inhibitory guanine nucleotide regulatory protein, Ni. This effect was inhibited if guanosine 5'-[beta-thio]diphosphate rather than GTP was present in the ribosylation incubations. The ability of glucagon to uncouple or desensitize the activity of adenylate cyclase in intact hepatocytes was also blocked by pre-treating hepatocytes with islet-activating protein. Islet-activating protein thus heightens the response of hepatocytes to the stimulatory hormone glucagon. It achieves this by both inhibiting the expression of desensitization and also removing a residual inhibitory input expressed in the presence of glucagon.

N6-(Phenylisopropyl)adenosine prevents glucagon both blocking insulin's activation of the plasma-membrane cyclic AMP phosphodiesterase and uncoupling hormonal stimulation of adenylate cyclase activity in hepatocytes

Glucagon (10nM) prevented insulin (10nM) from activating the plasma-membrane cyclic AMP phosphodiesterase. This effect of glucagon was abolished by either PIA [N6-(phenylisopropyl)adenosine] (100nM) or adenosine (10 microM). Neither PIA nor adenosine exerted any effect on the plasma-membrane cyclic AMP phosphodiesterase activity either alone or in combination with glucagon. Furthermore, PIA and adenosine did not potentiate the action of insulin in activating this enzyme. 2-Deoxy-adenosine (10 microM) was ineffective in mimicking the action of adenosine. The effect of PIA in preventing the blockade by glucagon of insulin's action was inhibited by low concentrations of theophylline. Half-maximal effects of PIA were elicited at around 6nM-PIA. It is suggested that adenosine is exerting its effects on this system through an R-type receptor. This receptor does not appear to be directly coupled to adenylate cyclase, however, as PIA did not affect either the activity of adenylate cyclase or intracellular cyclic AMP concentrations. Insulin's activation of the plasma-membrane cyclic AMP phosphodiesterase, in the presence of both glucagon and PIA, was augmented by increasing intracellular cyclic AMP concentrations with either dibutyryl cyclic AMP or the cyclic AMP phosphodiesterase inhibitor Ro-20-1724. PIA also inhibited the ability of glucagon to uncouple (desensitize) adenylate cyclase activity in intact hepatocytes. This occurred at a half-maximal concentration of around 3 microM-PIA. However, if insulin (10 nM) was also present in the incubation medium, PIA exerted its action at a much lower concentration, with a half-maximal effect occurring at around 4 nM.

The phorbol ester, TPA inhibits glucagon-stimulated adenylate cyclase activity

The ability of glucagon (10 nM) to increase hepatocyte intracellular cyclic AMP concentrations was reduced markedly by the tumour-promoting phorbol ester TPA (12-O-tetradecanoyl phorbol-13-acetate). The half-maximal inhibitory effect occurred at 0.14 ng/ml TPA. This action occurred in the presence of the cyclic AMP phosphodiesterase inhibitor isobutylmethylxanthine (1 mM) indicating that TPA inhibited glucagon-stimulated adenylate cyclase activity. TPA did not affect either the binding of glucagon to its receptor or ATP concentrations within the cell. TPA did inhibit the increase in intracellular cyclic AMP initiated by the action of cholera toxin (1 microgram/ml) under conditions where phosphodiesterase activity was blocked. TPA did not inhibit glucagon-stimulated adenylate cyclase activity in a broken plasma membrane preparation unless Ca2+, phosphatidylserine and ATP were also present. It is suggested that TPA exerts its inhibitory effect on adenylate cyclase through the action of protein kinase C. This action is presumed to be exerted at the point of regulation of adenylate cyclase by guanine nucleotides.