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Biomedical subjects

J N Fain

Publications and source records attributed to J N Fain.

At least 127 records · Page 7Linked to original sources
PubMed1980

Effect of insulin, catecholamines and calcium ions on phospholipid metabolism in isolated white fat-cells.

The incorporation of [(32)P]P(i) into phosphatidylinositol by rat fat-cells was markedly increased in the presence of adrenaline. Phosphatidic acid labelling was also increased, but to a lesser extent. These effects are due to alpha(1)-adrenergic stimulation since they were unaffected by propranolol, blocked by alpha-blockers in the potency order prazosin<<phentolamine<yohimbine and mimicked by methoxamine. The alpha-adrenergic stimulation of phosphatidylinositol labelling did not require extracellular Ca(2+), which supports the hypothesis that an increased turnover of phosphatidylinositol is involved in alpha-adrenergic activation of Ca(2+) entry. Insulin and the ionophore A23187 gave a small increase in (32)P labelling of phosphatidylinositol in Ca(2+)-free medium containing 1mm-EGTA. The increases due to insulin or ionophore A23187 were abolished if 2.5mm-Ca(2+) was added to medium containing EGTA. However, the increases in labelling of phosphatidylinositol due to alpha-adrenergic amines were still evident in medium containing EGTA and Ca(2+). Lipolytic agents such as corticotropin, dibutyryl cyclic AMP, adrenaline in the presence of phentolamine and isoproterenol decreased [(32)P]P(i) incorporation into phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. This inhibitory effect may be secondary to accumulation of intracellular unesterified fatty acids, since it was decreased by incubating fewer cells in medium with 6 rather than 3% albumin and was restored by the addition of oleate to the medium. The incorporation of [(32)P]P(i) into phosphatidylcholine was unaffected by lipolytic agents. The data suggest that there is an inhibition of the synthesis of certain phospholipids in the presence of lipolytic agents, which may be secondary to intracellular accumulation of unesterified fatty acids.

Adenosine↗
PubMed1980

Agonist versus antagonist binding to alpha-adrenergic receptors.

The binding properties of two alpha-adrenergic radioligands, [3H]epinephrine (an agonist) and [3H]dihydroergocryptine (an antagonist), were compared in two model systems--membranes derived from human platelets and membranes from rat liver. The platelet contains exclusively alpha 2 and the liver mostly (approximately 80%) alpha 1 receptors. Agonists induce the formation of a guanine nucleotide-sensitive high-affinity state of alpha 2 but not alpha 1 receptors. [3H]Dihydroergocryptine labels all the alpha receptors, whereas [3H]epinephrine at low concentrations labels predominantly the high-affinity form of the alpha 2 receptor in both platelet and liver. However, in the liver, alpha-adrenergic effects such as glycogen phosphorylase activation are shown to be mediated via alpha 1 receptors. Thus, in liver membranes the endogenous "physiological" agonist may not label the physiologically relevant alpha 1 receptors in typical radioligand binding assays using low concentrations of [3H]epinephrine.

Adrenergic alpha-Agonists↗
PubMed1979

Effect of lipolytic agents on adenosine and AMP formation by fat cells.

The release and metabolism of adenosine was examined using rat fat cells in which the nucleotide pool has been labeled by incubation with radioactive adenine. The accumulation of adenosine in the medium was near maximal at the start of the incubation and increased only slightly thereafter. Adenosine was rapidly deaminated to inosine and subsequently oxidized to uric acid. In the presence of allopurinol, and inhibitor of xanthine dehydrogenase, hypoxanthine accumulated in the medium as the end-product of adenosine catabolism. Adenosine accumulated in the medium only if fat cells were incubated in the presence of erythro-9-(2-hydroxy-3-nonyl)adenine, an inhibitor of adenosine deaminase. Even in the presence of this inhibitor there was no acceleration of adenosine release by norepinephrine in the presence of theophylline. However, there was an increase in labeled intracellular AMP accumulation by norepinephrine plus theophylline. The increase in labeled AMP correlated with the final free fatty acid to albumin ratio suggesting that the rise in AMP was related to an accumulation of intracellular free fatty acids. The addition of sodium oleate to the medium mimicked the effect of norepinephrine plus theophylline on the accumulation of labeled AMP. These results indicate that AMP rather than adenosine accumulates in isolated fat cells during incubation with lipolytic agents.

Adenine Nucleotides↗
PubMed1979

Relationship between phosphatidylinositol synthesis and recovery of 5-hydroxytryptamine-responsive Ca2+ flux in blowfly salivary glands.

Each salivary gland contains about 135 pmol of phosphatidylinositol. In glands prelabelled by incubation for 1 h with [32P]Pi or [3H]inositol there was a subsequent breakdown of 80% of the labelled phosphatidylinositol over a 2 h incubation period with 10 micrometer-5-hydroxytryptamine. However, there was no detectable decrease either in total phosphatidylinositol based on phosphorus analysis by chemical estimation or in the radioactivity of [32P]phosphatidylinositol in salivary glands of flies raised from the larval stage on diets containing[32P]Pi and whose phospholipids were uniformly labelled. These results suggest that the pool of phosphatidylinositol involved with Ca2+ gating is a small fraction of the total phosphatidylinositol content. Furthermore it is this small compartment that is preferentially radioactively labelled during short-term incubations with radioactively labelled precursors. In salivary glands incubated for 2 h with 10 micrometer-5-hydroxytryptamine there was a marked decrease in the flux of 45Ca2+ across the gland. After removal of the hormone, incubation of salivary glands for 1 h in the presence of 2mM-inositol, but not choline or ethanolamine, resulted in a recovery of hormone-responsive 45Ca2+ flux. Quantitative studies revealed that less than 9 pmol of phosphatidylinositol must be formed to fully restoret he 5-hydroxytryptamine-responsive 45Ca2+ flux.

Biological Transport↗
PubMed1979

Relationship between hormonal activation of phosphatidylinositol hydrolysis, fluid secretion and calcium flux in the blowfly salivary gland.

The addition of 5-hydroxytryptamine to the isolated blowfly salivary gland stimulates fluid secretion, transepithelial calcium transport and the breakdown of 32P- or 3H-labelled phosphatidylinositol The breakdown of [32P]phosphatidylcholine and [32P]-phosphatidylethanolamine was not stimulated by 5-hydroxytryptamine. In salivary glands incubated with myo-[2-3H]inositol for 1--3 h, more than 95% of the label retained by the tissue was in the form of phosphatidylinositol. The addition of 5-hydroxytryptamine resulted in an increase in the accumulation of label in intracellular inositol 1:2-cyclic phosphate, inositol 1-phosphate and free inositol along with an increase in the release of [3H]inositol to the medium and saliva. The release of [3H]inositol to the medium served as a sensitive indicator of phosphatidylinositol breakdown. The release of [3H]inositol was not increased by cyclic AMP or the bivalent-cation ionophore A23187 under conditions in which salivary secretion was accelerated. The stimulation of fluid secretion by low concentrations of 5-hydroxytryptamine was potentiated by 3-isobutyl-1-methylxanthine, which had no effect on inositol release. The stimulation of fluid secretion by 5-hydroxytryptamine was greatly reduced in calcium-free buffer, but the breakdown of phosphatidylinositol continued at the same rate in the absence of calcium. These results support the hypothesis that breakdown of phosphatidylinositol by 5-hydroxytryptamine is involved in the gating of calcium.

Animals↗
PubMed1979

Inhibition of phosphatidylinositol synthesis and the inactivation of calcium entry after prolonged exposure of the blowfly salivary gland to 5-hydroxytryptamine.

The incorporation of [32P]Pi into all salivary-gland phospholipids except phosphatidic acid was inhibited by 5-hydroxytryptamine. The accumulation of [32P]Pi into phosphatidic acid was actually enhanced by 5-hydroxytryptamine. There was an inhibition of labelled inositol incorporation into phosphatidylinositol by 5-hydroxytryptamine, which seems to be mediated by calcium because it was mimicked by the ionophore A23187, but was prevented if glands were stimulated with 5-hydroxytryptamine in the absence of external calcium. Inhibition of synthesis together with stimulation of breakdown will decrease the concentration of phosphatidylinositol, which could account for the inactivation of calcium transport observed at high 5-hydroxytryptamine concentrations. When salivary glands were stimulated with 1 micrometer-5-hydroxytryptamine, there was a rapid increase in the transfer of 45Ca2+ from the medium into the saliva, but with time this transport declined to a low value. If the glands were washed free of 5-hydroxytryptamine and incubated in the presence of 2mM-inositol for 1 h, the increase in calcium transport caused by 5-hydroxytryptamine was restored. There was little recovery in the absence of inositol. If glands were stimulated with 5-hydroxytryptamine in the absence of external calcium, a condition which prevents the inhibition of phosphatidylinositol synthesis, calcium transport in response to 5-hydroxytryptamine was greater than in glands preincubated with 5-hydroxytryptamine in the presence of calcium. The inactivation of calcium transport may result from a decrease in phosphatidylinositol concentration. These results support the hypothesis that the hydrolysis of phosphatidylinositol plays some role in either the opening or closing of calcium 'gates'.

Animals↗
PubMed1979

Inhibition of glucose transport in fat cells and activation of lipolysis by glucocorticoids.

The major effects of glucocorticoids on white fat are shown in Fig. 1. The glucocorticoid diffuses into the cytosol of fat cells where it binds to a soluble receptor. The steroid-receptor complex then enters the nucleus where RNA synthesis is increased. The next step may be a selective increase in synthesis of protein(s). In any event, there is an inhibition of the membrane-bound glucose transport system and an increase in the ability of lipolytic agents to activate triglyceride lipolysis. There is also a decrease in phosphoenolpyruvate carboxykinase, lipoprotein lipase, and fatty acid synthetase that occurs after a somewhat longer lag period than is required for inhibition of glucose transport or lipolysis activation. Whether these effects are independent or secondary to the glucose transport inhibition and lipolysis activation remains to be established.

Adipose Tissue↗
PubMed1979

Effects of bilirubin on fat cell metabolism and lipolysis.

Bilirubin (0.45 mM) inhibited lipolysis and stimulated [1-14C]glucose oxidation by rat fat cells in the presence of an equimolar concentration of bovine serum albumin. Bilirubin was an insulinlike agent with respect to inhibition of lipolysis and stimulation of glucose oxidation. There was a marked inhibition of adenylate cyclase activity of fat cell ghosts by 0.001--0.1 mM bilirubin in the absence of albumin, which was largely reversed by the addition of albumin. Although both bilirubin and free fatty acids bind to albumin, the primary binding sites appear to be separate. The effects of bilirubin at a molar ratio to albumin of 2 or less were not influenced by free fatty acid-to-albumin ratios up to 3. Triglyceride lipase activity of partially purified rat fat cell homogenates was inhibited by bilirubin in the presence of an equimolar concentration of bovine albumin. These data indicate that the antilipolytic action of bilirubin is probably due to direct inhibition of triglyceride lipase through a mechanism that does not involve competition with free fatty acids for binding to albumin.

Adenylyl Cyclase Inhibitors↗
PubMed1979

Regulation of cyclic AMP metabolism and lipolysis in isolated rat fat cells by insulin, N6-(phenylisopropyl) adenosine and 2',5'-dideoxyadenosine.

The increases in cyclic AMP accumulation and lipolysis by rat fat cells incubated in the presence of catecholamines were abolished by N6-(phenylisopropyl) adenosine. The same inhibition of cyclic AMP accumulation was seen in the presence of 2',5'-dideoxyadenosine but lipolysis was unaffected. In contrast, insulin inhibited lipolysis without affecting cyclic AMP accumulation by norepinephrine plus adenosine deaminase. These results suggest that there are either multiple pools of cyclic AMP or that ther exists some other mechanism which is involved in the regulation of lipolysis by hormones.

Adenosine↗