Isomerization of prostaglandin H2 into prostaglandin D2 in the presence of serum albumin

The prostaglandin endoperoxide, prostaglandin H2, decomposes in aqueous media mainly into prostaglandin E2. This paper shows that in the presence of serum albumin from a number of species prostaglandin H2 decomposes mainly into prostaglandin D2, an isomer of prostaglandin E2. The effect on endoperoxide decomposition exerted by serum albumin may well have physiological significance since intace endoperoxides can be released from tissues and since the biological properties of prostaglandins E2 and D2 are quite different.

Oxygen uptake and tissue oxygen tension during adrenergic stimulation in canine subcutaneous adipose tissue

The effect of sympathetic nerve stimulation (NS) and injected noradenaline (NA) or isoprenaline (Iso) on PVO2, VO2 and PtO2 was studied in isolated canine subcutaneous adipose tissue. These effects were compared to those produced by mechanical blood flow reduction (clamping). Resting VO2 measured 13.0+/- 2.3 mumol X min-1 X 100 g-1. When blood flow was reduced by 20% or less there was no significant change of VO2. Reducing blood flow to 50% of control or less by NS caused a parallel reduction in VO2, while clamping reduced VO2 significantly less. NA gave effects similar to those of NS. After NS or NA there was a period of hyperemia and increased oxygen extraction which more than compensated for the decrease in VO2 during vasoconstriction. Such a net increase in VO2 was not produced by clamping. Control PtO2 averaged 29+/-2 mmHg. NA reduced it by 70% and clamping to the same blood flow level only by 14% (p less than 0.01). Thus, a mere reduction in blood flow has little effect on PtO2, while blood flow reduction combined with redistribution of blood flow and an increased oxygen deman can lead to tissue hypoxia.

Metabolism and effect of prostaglandin H2 in adipose tissue

Prostaglandin H2 (PGH2) inhibited noradrenaline induced cyclic AMP accumulation in isolated rat fat cells in a dose-dependent manner. IC50 was 10-25 ng/ml both in the absence and in the presence of theophylline. The degree of inhibition produced by PGH2 increased with time of incubation. A stable PGH2 analog did not inhibit cyclic AMP accumulation. PGH2 was rapidly converted by isolated fat cells to PGD2, PGE2 and PGF2alpha' but no formation of thromboxane B2 was found either in vitro or in vivo. PGE2 was a more potent inhibitor than PGH2 of noradrenaline induced cyclic AMP accumulation. PGD2 enhanced cyclic AMP accumulation in a limited concentration interval, while PGF2alpha was essentially uneffective. Our results suggest that PGH2 is an inhibitor of cyclic AMP formation in isolated rat fat cells only after conversion to PGE2. A physiological role for PGH2 as a modulator of lipolysis is considered unlikely.

Release of adenosine-like material from isolated perfused dog adipose tissue following sympathetic nerve stimulation and its inhibition by adrenergic alpha-receptor blockade

Following the intraarterial infusion of 3H-adenine to isolated perfused canine subcutaneous adipose tissue, its adenine nucleotides are labelled. A continuous release of radioactivity, comprised of non-nucleotide material, was observed. The rate of this release was markedly enhanced by sympathetic nerve stimulation. The major components of the enhanced release appeared to be inosine and adenosine. Adrenergic alpha-receptor blockade (phentolamine or Hydergin) abolished the enhanced nucleoside release, while glycerol release was enhanced. The release of radioactivity was decreased during mechanical blood flow reduction and enhanced afterwards. However, the magnitude of this enhancement of release after clamp was much less than following nerve stimulation. The results suggest that adenosine or a closely related compound is released from canine subcutaneous adipose tissue by sympathetic nerve stimulation and that the release is related to adrenergic alpha-receptor stimulation. Since adenosine is a potent inhibitor of catecholamine induced lipolysis in this tissue the possibility of a regulatory role must be considered.

Inhibition by acidosis of adenosine 3',5'-cyclic monophosphate accumulation and lipolysis in isolated rat fat cells

Lipolysis and cyclic AMP accumulation were studied in isolated rat fat cells at normal (7.4) and decreased (7.0, 6.6) pH. Acidosis inhibited lipolysis and cyclic AMP accumulation due to NA non-competetively. Maximal lipolysis (3 muM NA) was inhibited by 25% at pH 7.0 and by 61% at pH 6.6 Cyclic AMP accumulation 5 min after 3 muM NA was inhibited by 57% at pH 7.0 and by 83% at pH 6.6. Between 10 and 60 minutes of incubation NA-stimulated lipolysis was linear at pH 7.4, whereas a progressively increasing inhibition was seen at lower pH. The FFA production was inhibited to the same degree as glycerol production by acidosis. The fraction of FFA associated with the cells was the same at all pHs. Thus, we have no evidence that acidosis inhibits lipolysis via accumulation of FFA intracellularly. NA-induced accumulation of 3H-cAMP from 3H-ATP, endogenously formed by prelabelling the cells with 3H-adenine, was inhibited by acidosis both in the presence and absence of theophylline in the incubation medium (by 48 and 44% respectively at pH 7.0 and by 74 and 68% at pH 6.6). Cyclic nucleotide phosphodiesterase in homogenates of fat cells was inhibited by decreasing the pH, whether measured at high or low substrate concentrations. Basal adenylyl cyclase activity in a cell membrane fraction from fat cells was affected to a minor degree, while NA-stimulated activity was inhibited by decreased pH. The response to 3 muM NA at pH 6.6 was inhibited by 43% relative to control. The results show that acidosis inhibits NA-induced cyclic AMP accumulation by interfering with the formation, rather than the inactivation of the nucleotide. Since NA-induced lipolysis is a cyclic AMP-mediated process it is suggested that at least part of the antilipolytic effect of acidosis is due to inhibition of cyclic AMP formation.

Cyclic AMP-dependent and independent inhibition of lipolysis by adenosine and decreased pH

NA-stimulated lipolysis and cAMP formation in isolated rat fat cells is inhibited by acidosis. In the present report we have examined the quantitative relationship between lipolysis and cAMP formation at normal and reduced pH and the possible involvement of adenosine, an endogenous inhibitor of cAMP formation. Adenosine antagonized cAMP accumulation and to a considerably lower degree lipolysis, effects potentiated by acidosis. Theophylline, an antagonist of adenosine effects, stimulated lipolysis and cAMP-accumulation, and potentiated responses to NA. Adenosine deaminase (ADA) had theophylline-like effects. Acidosis inhibited lipolysis and cAMP accumulation induced by ADA and theophylline to a larger extent than those induced by NA. It is suggested that adenosine modulates fat cell cAMP production and may contribute to the antilipolytic effect of acidosis. There was a curvilinear relationship between cAMP elevation and glycerol production in fat cell suspensions, which was different at pH 7.4 and at pH 6.6. The amount of cAMP needed for half-maximal activation of lipolysis increased from 1.3 (pH 7.4) to 3.1 pMol X 10(-5) cells (pH 6.6). The maximal glycerol production was reduced from 1 300 to 900 nMol X 10(-5) cells. The antilipolytic effect of acidosis is apparently due partly to an inhibition of cAMP formation and partly to inhibition of subsequent step(s) in the activation sequence.

Indomethacin-induced increase in noradrenaline turnover in some rat organs

1 The effect of a prostaglandin synthesis inhibitor, indomethacin, on noradrenaline turnover rate in various rat tissues was determined from the product of the endogenous noradrenaline concentration and of the rate constant of (-)-[3H]-noradrenaline decline after injection of the labelled amine in tracer doses. 2 Treatment of the rats with indomethacin (5 mg/kg p.o. five times during 2.5 days) increased noradrenaline turnover rate 32-36% in submandibular gland, spleen and heart, and 4-17% in epididymal and subcutaneous adipose tissue. 3 The extrapolated [3H]-noradrenaline content at time 0 was greater in heart and adipose tissue of indomethacin-treated animals than in controls, while it was not significantly changed in spleen and salivary gland. 4 There was no apparent relationship between tissue concentrations of intravenously injected [14C]-indomethacin and the effect of indomethacin on noradrenaline uptake and turnover rate in the different tissues. 5 Indomethacin treatment did not affect monoamine oxidase and catechol O-methyl-transferase activities in the different tissues. 6 The results are consistent with the hypothesis that indomethacin increases noradrenaline turnover in the rat by blockade of a locally operating feed back inhibition of transmitter release by prostaglandins. However, additional effects, such as an increased impulse traffic, cannot be ruled out.

Some metabolic consequences of the anaphylactic reaction in the rabbit

The administration of egg albumin to rabbits sensitized to this antigen caused marked increases in the arterial concentration of lactate, glucose and glycerol, but no change in the arterial FFA level. Antigen administration had no effect in non-sensitized rabbits. Phentolamine (3 mg/kg) or propranolol (1 mg/kg) did not significantly alter the responses to egg albumin in sensitized rabbits. Noradrenaline or sympathetic nerve stimulation decreased blood flow but caused no significant change in lipolysis in rabbit epigastric adipose tissue in situ. It is therefore questionable if catecholamines are the major cause of the observed metabolic consequences of the anaphylactic reaction in the rabbit. These metabolic events, i.e. increased lactate levels, lipolysis, and reesterification of fatty acids, are similar to those reported during hemorrhagic or endotoxin shock in dogs, in spite of specied-differences and the difference in the genesis of the shock.

Role of pre- and postjunctional inhibition by prostaglandin E2 of lipolysis induced by sympathetic nerve stimulation in dog subcutaneous adipose tissue in situ

1. Canine subcutaneous adipose tissue was isolated and autoperfused in situ after labelling of the noradrenaline stores by (3)H-(-)noradrenaline.2. Prostaglandin E(2) (10-200 ng/ml) increased blood flow and glucose uptake, and caused a dose-dependent inhibition of lipolysis induced by sympathetic nerve stimulation (4 Hz). The actions of exogenous prostaglandin E(2) are therefore similar to those of prostaglandin E(1) in this tissue. There were no consistent effects of prostaglandin E(2) on the vasoconstriction or on the (3)H-noradrenaline overflow induced by nerve stimulation.3. Phenoxybenzamine (1.5-2 mg i.a.) caused a 5-fold increase in (3)H-noradrenaline overflow and a 95% reduction of the vasoconstrictor response to nerve stimulation. The lipolytic response was similar to that of the control. Prostaglandin E(2) (100-200 ng/ml) administered after phenoxybenzamine caused a 90% inhibition of lipolysis, while the vasoconstrictor response was enhanced to about 50% of control. Prostaglandin E(2) inhibited (3)H-noradrenaline overflow by about 50% but it was still larger than that of the control.4. It is suggested that exogenous prostaglandin E(2) inhibits lipolysis induced by sympathetic nerve stimulation mainly by a postjunctional action in canine subcutaneous adipose tissue.