Comparison of metabolism of free fatty acid by isolated perfused livers from male and female rats

Comparative changes with age of the fasting response in circulating ketone bodies, glucose and insulin and oral glucose tolerance test in the rat

1. Body weight loss in 48 hr fasted rats decreased with age. 2. Blood glucose and plasma RIA-insulin levels correlated negatively and positively respectively with body weight in fed rats. Fasting produced a greater fall in blood glucose and a smaller decrease in RIA-insulin in young than in old rats. 3. Blood ketone bodies correlated negatively with body weight after 48 hr fasting. 4. In oral glucose tolerance tests, blood glucose rose more in adult and old rats than in prepuberals when both fed and fasted. RIA-insulin levels rose more in prepuberals than in older rats when fed but not when fasted. 5. Changes in body composition and reduced insulin sensitivity with age are discussed.

Alterations of plasma HDL lipids and apolipoproteins in female rats treated with ethynyl estradiol

Female Sprague-Dawley rats (160-200 g body weight) were injected subcutaneously daily for 14 days with 1, 5, 15, 50 or 100 micrograms ethynyl estradiol in sesame oil/kg body weight, or with oil alone. Food consumption was restricted to 15 g/day, and animals were fasted overnight before exsanguination. Concentrations of plasma cholesteryl ester decreased with increasing dosage of ethynyl estradiol, as a result of decreases in HDL cholesteryl ester. Concentrations of plasma unesterified cholesterol were not altered by treatment; the tendency for a decrement in the HDL cholesterol was not statistically significant. Plasma and HDL phospholipid also tended to decrease with increasing dose of ethynyl estradiol. Plasma levels of apolipoprotein A-I increased on treatment with 5 micrograms ethynyl estradiol/kg but were diminished at a dose of 50 micrograms ethynyl estradiol/kg. The ultracentrifugally isolated high density lipoproteins were delipidated and the distribution of apolipoproteins was characterized by SDS-polyacrylamide gel electrophoresis and esoelectric focusing. The proportion of apolipoprotein E in HDL was depressed, while apolipoprotein A-I was increased and apolipoprotein C unchanged by treatment with ethynyl estradiol. Clearly, the alterations in plasma HDL lipid levels resulting from treatment with ethynyl estradiol were accompanied by distinct changes in composition of the apolipoproteins of HDL, and a biphasic response dependent on dose of the drug. A possible mechanism for the diminution in the proportion of HDL apolipoprotein E may be the enhanced removal of the apolipoprotein E-rich (HDL1) subfraction of the HDL from the circulation on treatment with ethynyl estradiol.

Postnatal development and sex differences in hepatic phosphatidate phosphohydrolase activity in the rat

The mechanism leading to the difference in hepatic triacylglycerol metabolism between female and male rats was investigated by studying the ontogeny of hepatic soluble phosphatidate phosphohydrolase activity in feeding animals of both sexes. A sevenfold increase occurred within 12 hr of birth, returning to the adult level during the third postnatal day. The changes in enzyme activity were followed by similar changes in hepatic triacylglycerol concentrations. A sex difference was observed only in the adult rats, where the enzyme activity in the livers of feeding female rats was about 25% higher than that in the feeding males. The effects of gonadectomy and sex steroids were studied in a separate series of experiments on fasting animals. The activity of the soluble enzyme was 65% higher in the intact female rats than in the males, and that of the microsomal enzyme 130% higher. The activity ratio between the soluble and microsomal enzyme in the male rats was 4.3 on a liver wet weight basis with the methods used. Gonadectomy increased the soluble and microsomal activities by 25% and 80% respectively within 6 wk in the male rats. The soluble and microsomal activities were still at the same control levels 2 wk after the gonadectomy, the subcutaneous implants of testosterone or estradiol resulting in 10-fold increases in plasma hormone levels had no effects on these enzyme activities, although testosterone caused 50% decrease in the hepatic triacylglycerol concentration. These data indicate that, if hormonally mediated, the postnatal increase in phosphatidate phosphohydrolase activities is not related to sex steroids and also suggest that the basis of the sex difference in hepatic soluble phosphatidate phosphohydrolase activity remains to be established.

Lack of effect of somatostatin on the glucagon-induced alterations of hepatic metabolism of [1-14C]oleate

Livers from normal fed male rats were perfused in a recycling system in vitro. Glucagon was infused in varying quantities to give final concentration in the cell-free perfusate of 4.9 . 10(-10)-4.9 . 10(-7) M after 3 h of perfusion, assuming no degradation of the hormone. Where indicated, cyclic somatostatin was infused simultaneously to give a final concentration of 3.0 . 10(-6) M. In the absence of somatostatin, glucagon at a concentration as low as 4.9 . 10(-10) M increased the release of glucose and increased ketogenesis, but impaired the synthesis and release of perfusate triacylglycerol and very low density lipoprotein lipids. Somatostatin did not affect these actions of glucagon. Somatostatin alone, however, did reduce the output of very low density lipoprotein. It is suggested that the alteration of fatty acid metabolism by somatostatin in vivo results from modulation of pancreatic glucagon secretion, not from interference by somatostatin of the action of glucagon on the liver.

NADP-dependent dehydrogenases in rat liver parenchyma. III. The description of a liponeogenic area on the basis of histochemically demonstrated enzyme activities and the neutral fat content during fasting and refeeding

The activities of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase(6PGDH), malic enzyme (ME) and isocitrate dehydrogenase (ICDh) were investigated with optimized histochemical methods (Rieder it al 1978), and the activity of 3-hydroxybutyrate dehydrogenase (3HBDH) and neutral fat content with conventional techniques in the liver of male rats under the following experimental dietary conditions: (A) Fasting for 0, 12 and 84h; (B) 84-h fasting followed by refeeding with a low-fat, high-carbohydrate diet for 6 h and for 2, 3, 5, 7, 11 and 14 nights; (C) refeeding with standard diet for 5 nights; (D) low-fat high-carbohydrate diet for 7 an 14 nights. The activities of G6PDH, 6PGDH and ME decreased slightly during fasting primarily in zone 1 and increased dramatically on refeeding with a low-fat, high-carbohydrate diet. This activity increase was confined mainly to zone 3 during the first 3 days and was accompanied by a deposition of neutral fats that began in zone 3 and progressed to zone 1. Neutral for accumulation was maximal after 3 nights, with a uniform accumulation of large droplets in all the hepatocytes; this was followed by a release that started in zone 3 and proceeded in a periportal direction. On the other hand, G6PDH, 6PGDH and ME attained their maximum activities after 5 amd 7 nights of low-fat diet, the activities being nearly homogeneously distributed over the liver acinus in a few cases. Subsequently the activities fill mainly in zone 1, causing the activity patterns and levels to approach those of the animals in group (D). In contrast to this, the activity of ICDH increased during fasting principally in zone 1, so that the otherwise steep activity gradient in favor of zone 3 lessened. Refeeding led at first to a fall of activity below the initial value, but later the normal distribution pattern was restored. The activity of 3HBDH showed a behavior similar to that of ICDH. The findings are discussed with reference to the functional heterogeneity of the liver parenchyma, and the existence of a liponeogenic area in zone 3 is proposed.

Hepatic metabolism of [1-14C]oleate in pregnancy

Livers from normal ad libitum fed nonpregnant and 20 day pregnant rats were perfused in vitro. Uptake and utilization of [1-14C]oleate were measured while the concentrations of free fatty acids in the erythrocyte-free perfusate was maintained at a steady-state level (mean 0.42 +/- 0.03 (S.E.) mM). Pregnancy increased the incorporation by the isolated liver of [1-14C]oleate into perfusate triacylglycerol and cholesteryl esters and into hepatic triacylglycerol, phospholipid, diacylglycerol and cholesteryl esters. The conversion of [1-14C]oleate by the livers into ketone bodies and CO2 was depressed by pregnancy. With pregnancy, the output of the very low density lipoprotein triacylglycerol, cholesterol, cholesteryl esters and phospholipid was increased significantly by the perfused liver. The relative molar composition of the lipid components of the very low density lipoproteins, however, was not altered by gestation, suggesting that the number of particles secreted was increased by pregnancy. The total output of glucose was decreased by livers from pregnant rats. It may be concluded from these data that livers from pregnant rats in late gestation channel fatty acid ([1-14C]oleate) preferentially into products of esterification rather than into products of oxidation.

The metabolism of oleic acid by the perfused rat liver in experimental diabetes induced by antiinsulin serum

The metabolism of varying quantities of oleic acid was examined in isolated perfused livers from normal fed rats and from animals made diabetic by pretreatment with guinea pig antiinsulin serum (AIS). The data presented reemphasize the fact that the quantity of free fatty acid (FFA) coming to the liver is a necessary, but not the most important, factor affecting the subsequent metabolism of the FFA. Rates of ketogenesis and output of triglyceride and the terminal concentration of hepatic triglyceride were proportional to uptake of FFA in certain concentration ranges. For equal rates of uptake of FFA, ketogenesis was greater, and the quantity of triglyceride secreted or accumulated within the liver was less, with livers from diabetic animals than with livers from normal animals. In confirmation of previous data, the liver was observed to have a maximal capacity to secrete triglyceride. Triglyceride accumulated in livers from normal-fed and diabetic animals only when uptake of FFA was more than sufficient to saturate the secretory process. Since proportionately more FFA was catabolized by livers from AIS treated animals, greater uptake of FFA was required to produce maximal rates of output of triglyceride and accumulation in livers from diabetic than from normal animals. Rates of ketogenesis by livers from normal fed animals increased minimally with increasing uptake of FFA (up to 1.0 mM free fatty acid). Even when uptake increased considerably with FFA concentrations of approximately 2.5 mM, rates of ketogenesis by livers from normal animals were less than half those of livers from diabetic rats, and maximal rates were not achieved by the normal controls. It is evident that changes in hepatic metabolism of FFA in the intact diabetic animal result from simultaneous alterations of supply of FFA and hormonally induced metabolic changes in the liver. Moreover, although hepatic secretion and accumulation of triglyceride is greater in isolated perfused livers from normal rats than from diabetic animals when the livers are exposed to equal quantities of FFA, the diabetic livers can accumulate more triglyceride, secrete more triglyceride, and oxidize more FFA to ketone bodies than can the normal under conditions in which considerably more substrate is available to the diabetic rather than to the normal livers. These differences might also be expected to occur in the acutely insulin deficient intact animal, in which changes in hormonal status and substrate (FFA) availability occur simultaneously, and might, in part, explain the ketonemia, hypertriglyceridemia, and hepatic steatosis often observed in vivo.

Effects of dibutyryl adenosine 3',5'-monophosphate on hepatic metabolism of free fatty acids

The effects of dibutyryl cyclic adenosine 3',5'-monophosphate (Bu2cAMP) on metabolism of free fatty acids by perfused livers from normal fed male rats were investigated. In one group of experiments, Bu2cAMP was added to the medium and infused at a constant rate to maintain concentrations of 0, 0.4, 1.0, 4.0, or 10. 0 X 10(-5) M nucleotide in the perfusate plasma, assuming the nucleotide was not metabolized by the liver. Oleic acid was infused as the complex with albumin at the rate of 124.3 mumoles/hr. Uptake of free fatty acid by the liver was identical in all groups. Production of ketone bodies, however, increased, and output of triglyceride decreased with increasing concentration of Bu2cAMP. The nucleotide also stimulated output of glucose. Maximal effects were observed when the concentration of Bu2cAMP was approximately 2-3 X 10(-5) M. The output of very low density lipoproteins, as judged by flotation in the zonal ultracentrifuge, was also diminshed by the nucleotide. In other experiments, 1-14C-oleate was infused (120.8 mumoles/hr) along with 2 X 10(-5) M Bu2cAMP, and the disposition of 14C into CO2, ketone bodies, and esterified lipids was evaluated. Bu2cAMP depressed the proportion of 1-14C-oleate converted to triglyceride and increased the fraction converted to ketone bodies and CO2. Not only was ketogenesis stimulated, but a larger proportion of the ketone bodies was derived from exogenous fatty acid.

Hepatic secretion and turnover of serum phosphatidylcholine-cholesterol acyltransferase in male and female rats

The activity of serum phosphatidylcholine-cholesterol acyltransferase (LCAT), output of the enzyme by the perfused rat liver, and the effect of pretreatment with colchicine on LCAT were studied in male and female rats. It was observed that: 1. Serum LCAT activity in the female exceeded that of the male in fasted animals, whereas in fed animals, LCAT activity was higher in the male than the female. With both sexes, however, serum LCAT activity in fed animals was greater than that in fasted animals. Data are presented which suggest that the observed sex differences were due to concentration and/or composition of the substrate rather than to differences in the serum concentration of the enzyme. 2. The release of LCAT by perfused livers from fasted female rats exceeded that of the male animals. The output of LCAT was inhibited by pretreatment (male) with colchicine, which suggests that hepatic secretion of LCAT is dependent on vesicular transport. 3. The decay of serum LCAT activity in vivo following injection of colchicine was more rapid in fasted female rats than in male animals. These observations lead us to postulate that the turnover rate of LCAT is higher in female rats than in male animals.