The effects of epinephrine on guinea pig placental glycogen metabolism and on cellular cyclic AMP

Some structural studies on the galactose-containing polysaccharide from bovine placenta

Polysaccharides were extracted from 8-month-old placenta with aqueous HgCl2. The protein-free material was purified by selective precipitation with Cetavlon in the presence of sodium borate at pH 8.5 and was homogeneous on molecular-sieve chromatography, electrophoresis, and on treatment with Concanavalin A. The preparation contained galactose and glucose as principal monosaccharides with 5 per cent of hexosamines. Methylation studies suggested that D-gluco and D-galactopyranosyl units may be constituents of glucan and galactan respectively which form a molecular aggregate that does not dissociate during the fractionation procedures. After treatment of the fraction with beta-amylase, the proportion of glucose in the polysaccharide diminished, indicating the presence of (1-->4)-linked alpha-D-glucopyranosyl residues. Also, when the fraction was treated with a crude protease having glucosidase activity a residual alpha-D-galactopyranan was isolated and found to contain non-reducing end-groups (30.0 per cent), 3-O-(39.5 per cent) and 3,6-di-O-substituted (30.5 per cent) units. The structure of the galactan was not modified according to methylation data, on removal of the glucosyl component. The polysaccharide fraction (pH 8.5 Cetavlon), isolated from bovine placenta, thus contains a glycogen-like material associated with a galactan as molecular aggregate. This galactan has not been previously recognized in bovine placenta and its occurrence in this organ supports the hypothesis that galactose-containing polysaccharides are involved in foetal development.

Mobilization of placental glycogen in diabetic rats

Since glycogen accumulates in the placenta in diabetes and does not appear to be susceptible, in general, to the effect of fasting, the capacity of catecholamines to elicit glycogenolysis was investigated in non-diabetic and diabetic rats. Injection of epinephrine or isoproterenol caused a decrease in placental glycogen within 20 min in non-diabetic, 20 day pregnant rats, in association with the rise in serum glucose and lactate. Incubation with isoproterenol induced glycogenolysis in placental slices from non-diabetic and diabetic rats, nearly commensurate with lactate production. This effect of isoproterenol was concentration dependent and of similar magnitude in non-diabetic and diabetic rat placentas. Glucagon was ineffective in inducing placental glycogenolysis in vivo or in vitro. Protracted stimulation of the catecholamine receptor by the administration of cholera toxin effected a pronounced decrease in placental glycogen, percentagewise higher in diabetic than non-diabetic rats. These results show that placental glycogen is amenable to mobilization by hormonal stimuli effecting phosphorylase activation.

Effect of exposure to cold and fasting on the placental glycogen and triglyceride content in the rat

The effect of exposure to cold (+2 degrees C, 3 and 24 h) and fasting (1, 2 and 3 days) on placental glycogen and triglyceride content was investigated in rats pregnant for 20 days. The stimuli did not affect the level of glycogen in the placenta. The level of triglycerides remained unchanged in the rats exposed to cold. It rose after only one day of fasting and then levelled out on the 2nd and 3rd days. Fasting, but not exposure to cold produced hypoglycemia and elevation of the plasma free fatty acids level. We conclude that activation of the adrenergic system during exposure to cold does not interfere with the glycogen and triglyceride content of the placenta. Prolonged hypoglycemia also does not affect the placental glycogen level although it increases the accumulation of neutral fat.

The structure of placental glycogen

Glycogen was purified from human term placenta and its structural features investigated. The beta-amylolysis limit and average chain lengths indicated that some degradation of the glycogen had occurred prior to its extraction. The sedimentation coefficient distribution of the purified glycogen showed that it contained a significant proportion of aggregated material. Diffusion coefficient measurements allowed calculation of the molecular weight distribution. The placental glycogen contained a significant proportion of high molecular weight material, although not as much as liver or skeletal muscle glycogens. Because the high molecular weight glycogen of liver and skeletal muscle is associated with the lysosome it is likely that this is also true of the large placental glycogen. Lysosomal glycogen is degraded hydrolytically to glucose and so placental glycogen may be involved in fetal glucose homeostasis.

Induction of glycogenolysis in cultured Ewing's sarcoma cells by dopamine and beta-adrenergic agonists

This study describes hormonal regulation of glycogen metabolism in Ewing's sarcoma cells. 3H-Glycogen synthesized in cultured Ewing's sarcoma WE-68 cells from 3H-glucose was hydrolyzed in a concentration-dependent manner by various catecholamines. The order of potency for the glycogenolytic effects of catecholamines was isoproterenol greater than or equal to dopamine greater than norepinephrine greater than epinephrine. The concentrations giving half-maximal effectiveness (EC50) were about 2 x 10(-8) M, 3 x 10(-8) M, 8 x 10(-8) M, and 5 x 10(-7) M for isoproterenol, dopamine, norepinephrine, and epinephrine, respectively. Higher concentrations of each of the catecholamines were necessary to elicit EC50 stimulation of cyclic AMP production in Ewing's sarcoma cells. Glycogenolysis induced by dopamine was blocked by chlorpromazine, a dopamine D1-receptor antagonist, but not by haloperidol, a dopamine D2-receptor antagonist. The glycogenolytic action of norepinephrine was markedly reduced by propranolol, a beta-adrenoreceptor antagonist, and was not affected by yohimbine, an alpha-adrenoreceptor antagonist. In addition, chlorpromazine also antagonized the glycogenolytic response to norepinephrine. Dibutyryl cyclic AMP, 3-isobutyl-1-methylxanthine, and the diterpene forskolin were also found to induce 3H-glycogen hydrolysis. Our data indicate that catecholamines exert their glycogenolytic effects in Ewing's sarcoma cells by stimulation of cyclic AMP formation via beta-adrenergic receptors and dopamine D1-receptors.