Regulation of fatty acid metabolism and gluconeogenesis by growth hormone and insulin in sheep hepatocyte cultures. Effects of lactation and pregnancy

Towards Understanding the Direct and Indirect Actions of Growth Hormone in Controlling Hepatocyte Carbohydrate and Lipid Metabolism

Growth hormone (GH) is critical for achieving normal structural growth. In addition, GH plays an important role in regulating metabolic function. GH acts through its GH receptor (GHR) to modulate the production and function of insulin-like growth factor 1 (IGF1) and insulin. GH, IGF1, and insulin act on multiple tissues to coordinate metabolic control in a context-specific manner. This review will specifically focus on our current understanding of the direct and indirect actions of GH to control liver (hepatocyte) carbohydrate and lipid metabolism in the context of normal fasting (sleep) and feeding (wake) cycles and in response to prolonged nutrient deprivation and excess. Caveats and challenges related to the model systems used and areas that require further investigation towards a clearer understanding of the role GH plays in metabolic health and disease are discussed.

Buffering agent-induced lactose content increases via growth hormone-mediated activation of gluconeogenesis in lactating goats

Dairy goats are often fed a high-concentrate (HC) diet to meet their lactation demands; however, long-term concentrate feeding is unhealthy and leads to milk yield and lactose content decreases. Therefore, we tested whether a buffering agent is able to increase the output of glucose in the liver and influence lactose synthesis. Eight lactating goats were randomly assigned to two groups: one group received a HC diet (Concentrate : Forage = 6:4, HG) and the other group received the same diet with a buffering agent added (0.2 % NaHCO(3), 0.1 % MgO, BG) over a 19-week experimental period. The total volatile fatty acids and lipopolysaccharide (LPS) declined in the rumen, which led the rumen pH to become stabile in the BG goats. The milk yield and lactose content increased. The alanine aminotransferase, aspartate transaminase, alkaline phosphatase, pro-inflammatory cytokines, LPS and lactate contents in the plasma significantly decreased, whereas the prolactin and growth hormone levels increased. The hepatic vein glucose content increased. In addition, pyruvate carboxylase (PC), phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6PC) expression in the liver was significantly up-regulated. In the mammary glands, the levels of glucose transporter type 1, 8, 12 as well as of sodium-glucose cotransporter 1 increased. Cumulative buffering agent treatment increased the blood concentrations of glucose via gluconeogenesis and promoted its synthesis in the liver. This treatment may contribute to the increase of the milk yield and lactose synthesis of lactating goats.

Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects

In evolutionary terms, GH and intracellular STAT 5 signaling is a very old regulatory system. Whereas insulin dominates periprandially, GH may be viewed as the primary anabolic hormone during stress and fasting. GH exerts anabolic effects directly and through stimulation of IGF-I, insulin, and free fatty acids (FFA). When subjects are well nourished, the GH-induced stimulation of IGF-I and insulin is important for anabolic storage and growth of lean body mass (LBM), adipose tissue, and glycogen reserves. During fasting and other catabolic states, GH predominantly stimulates the release and oxidation of FFA, which leads to decreased glucose and protein oxidation and preservation of LBM and glycogen stores. The most prominent metabolic effect of GH is a marked increase in lipolysis and FFA levels. In the basal state, the effects of GH on protein metabolism are modest and include increased protein synthesis and decreased breakdown at the whole body level and in muscle together with decreased amino acid degradation/oxidation and decreased hepatic urea formation. During fasting and stress, the effects of GH on protein metabolism become more pronounced; lack of GH during fasting increases protein loss and urea production rates by approximately 50%, with a similar increase in muscle protein breakdown. GH is a counterregulatory hormone that antagonizes the hepatic and peripheral effects of insulin on glucose metabolism via mechanisms involving the concomitant increase in FFA flux and uptake. This ability of GH to induce insulin resistance is significant for the defense against hypoglycemia, for the development of "stress" diabetes during fasting and inflammatory illness, and perhaps for the "Dawn" phenomenon (the increase in insulin requirements in the early morning hours). Adult patients with GH deficiency are insulin resistant-probably related to increased adiposity, reduced LBM, and impaired physical performance-which temporarily worsens when GH treatment is initiated. Conversely, despite increased LBM and decreased fat mass, patients with acromegaly are consistently insulin resistant and become more sensitive after appropriate treatment.

Pregnancy impairs ketone body disposal in late gestating ewes: Implications for onset of pregnancy toxaemia

The impact of pregnancy on ketone body disposal during a hyperketonaemic clamp was examined by tracer isotope dilution techniques in seven 12 h fasted sheep in three reproductive states, in the dry non-gestating period, late in gestation and during early lactation. After a sampling period of 60 min under basal conditions a DL-BHB racemate solution was continuously infused intravenously for 3 h at rates of 14.3-24.3 micromol/(kg min) to elevate the D-BHB concentration in blood plasma to values between 5 and 7 mmol/l. Two separate experiments were carried out with the same sheep in each of the three reproductive states. During pregnancy three ewes were pregnant with a single lamb and four ewes carried twins. Maximal D-BHB turnover rates fell significantly in late gestation by 26% relative to early lactation and by 22% when compared with the dry non-pregnant state. Reduction of maximal D-BHB disposal rate during late gestation was accompanied by a significant 297% (p<0.005) and a non-significant 49% increase in the basal BHB concentration in blood, a non-significant 10% and 4% decrease in the rate constant of D-BHB turnover and a non-significant 24% and 13% rise in the incremented increase of D-BHB concentration per unit D-BHB infusion, relative to the dry and the lactating period, respectively. Induction of hyperketonaemia significantly lowered NEFA and glycerol concentrations in blood by 67% and 57%, respectively, compared to the pre-infusional concentrations. The magnitude of this effect was the same in all reproductive states and was explained as a direct antilipolytic action of D-BHB on adipose tissue. It is concluded that the reduced ability of the late gestating ewe to utilize D-BHB promotes hyperketonaemia. Since hyperketonaemia exerts several adverse effects, e.g. on energy balance and glucose metabolism it appears that the impairment of ketone bodies disposal in late pregnancy facilitates development of pregnancy toxaemia, especially in ewes carrying twins.

Prepartal Plane of Nutrition, Regardless of Dietary Energy Source, Affects Periparturient Metabolism and Dry Matter Intake in Holstein Cows

Previous research in our laboratory showed that dietary fat supplementation during the dry period was associated with decreased peripartum hepatic lipid accumulation. However, fat supplementation decreased dry matter (DM) intake and thereby confounded results. Consequently, 47 Holstein cows with body condition scores (BCS) < or = 3.5 at dry-off were used to determine whether source or amount of energy fed to dry cows was responsible for the decreased hepatic lipid content. Moderate grain- or fat-supplemented diets [1.50 Mcal of net energy for lactation (NE(L))/kg] were fed from dry-off (60 d before expected parturition) to calving at either ad libitum (160% of NE(L) requirement) or restricted (80% of NE(L) requirement) intakes. Postpartum, cows were fed a single lactation diet for ad libitum intake and performance was measured for 105 d. Prepartum intakes of DM and NE(L) were significantly lower for feed-restricted cows as designed. During the first 21 d postpartum, previously restricted cows had higher intakes of DM and NE(L). Body weights and BCS were lower prepartum for restricted cows but groups converged to similar nadirs postpartum. Restricted-fed cows had lower concentrations of glucose and insulin and increased concentrations of NEFA in plasma during the dry period. Peripartum NEFA rose markedly for all treatments but were higher postpartum for cows previously fed ad libitum. Plasma concentrations of NEFA and BHBA remained lower in cows restricted-during the dry period. Postpartum concentrations of total lipid and triglyceride in liver were lower in cows previously feed-restricted. Across dietary treatments, activity of carnitine palmitoyltransferase (CPT) in hepatic mitochondria was lowest at - 21 d, highest at 1 d, and decreased at 21 and 65 d relative to parturition. The activity of CPT at d 1 tended to be higher for previously feed-restricted cows; thereafter, CPT activity declined more rapidly than in cows fed ad libitum. Nutrient intake during the dry period had more pronounced effects on peripartal lipid metabolism and DMI than did composition of the prepartum diet.

Metabolic profiles and bile acid extraction rate in the liver of cows with fasting-induced hepatic lipidosis

This study was designed to monitor lipid profile in the portal and hepatic blood of cows with fasting-induced hepatic lipidosis, and to compare the results with those in the jugular blood. The work was also carried out to investigate bile acid (BA) in these vessels, and further to investigate BA extraction rate in the liver. Five cows were equipped with catheters in the portal, hepatic and jugular veins (day 0), fasted for 4 days (day 1-day 4) and then refed (day 5-day 11). Before morning feeding, blood was sampled before, during and after fasting from the catheterized vessels. In the portal blood, the concentration of non-esterified fatty acids (NEFA) showed a progressive increase and at day 5 there was an approximate twofold rise. Increased NEFA concentrations were also found similarly in the other two veins. At day 5, beta-hydroxybutyrate (BHBA) in the portal, hepatic and jugular blood rose to 197, 190 and 186% of the pre-fasting value, respectively. However, the concentrations of NEFA and BHBA in the three veins gradually returned to pre-fasting concentration during the refeeding period. Compared with the pre-fasting value at day 0, the content of liver triglyceride (TG) increased significantly at day 5 (P < 0.01). In the liver, the hepatic extraction rate of BA dropped from 3.1 times pre-fasting to 2.2 times during fasting. There were no significant differences in the concentrations of glucose, TG, total cholesterol, cholesterol esters, free cholesterol and phospholipids. The results of the current study show that metabolic alterations occur in the portal, hepatic and jugular veins during induction of hepatic lipidosis in cows, and mostly metabolites, with exception of BA concentration, run parallel. The decreased BA extraction rate in the liver of fasted cows was considered to reflect hepatic cell impairment caused by TG accumulation. Hopefully, the findings, at least in part, contribute to the explanation of the pathophysiology of hepatic lipidosis in dairy cows.

The role of insulin, glucagon, dexamethasone, and leptin in the regulation of ketogenesis and glycogen storage in primary cultures of porcine hepatocytes prepared from 60 kg pigs

A study was conducted to elucidate hormonal control of ketogenesis and glycogen deposition in primary cultures of porcine hepatocytes. Hepatocytes were isolated from pigs (54-68 kg) by collagenase perfusion and seeded into collagen-coated T-25 flasks. Monolayers were established in medium containing fetal bovine serum for 1 day and switched to a serum-free medium for the remainder of the culture period. Hepatocytes were maintained in DMEM/M199 containing 1% DMSO, dexamethasone (10(-6) or 10(-7) M), linoleic acid (3.4 x 10(-5) M), and carnitine (10(-3) M) for 3 days. On the first day of serum-free culture, insulin was added at 1 or 100 ng/ml and glucagon was added at 0, 1, or 100 ng/ml. Recombinant human leptin (200 ng/ml) was added during the final 24 h; medium and all cells were harvested on the third day. Concentrations of acetoacetate and beta-hydroxybutyrate (ketone bodies) in media and glycogen deposition in the cellular compartment were determined. Ketogenesis was highly stimulated by glucagon (1 and 100 ng/ml) and inhibited by insulin. In contrast, glycogen deposition was stimulated by insulin and attenuated by glucagon; high insulin was also associated with a reduction in the ketone body ratio (acetoacetate:beta-hydroxybutyrate). High levels of dexamethasone stimulated ketogenesis, but inhibited glycogen deposition at low insulin. Culture of cells with leptin for 24 h, over the range of insulin, glucagon, and dexamethasone concentrations had no effect on either glycogen deposition or ketogenesis. These data suggest that while adult porcine hepatocytes are indeed sensitive to hormonal manipulation, leptin has no direct influence on hepatic energy metabolism in swine.

Differential regulation of porcine hepatic IGF-I mRNA expression and plasma IGF-I concentration by a low lysine diet

The influence of dietary lysine on hepatic insulin-like growth factor-I (IGF-I) gene expression and plasma IGF-I level was investigated. Two male 6-wk-old pigs from each of six litters were used. Each littermate was assigned to one of two diets, control or low lysine (LL), that were isoenergetic and similar in protein content and provided 14.3 MJ digestible energy/kg for both diets, 185 g protein/kg for the control diet and 180 g protein/kg for the LL diet. The control diet contained all essential amino acids in the recommended amounts, including 11.5 g lysine/kg. The LL diet was similar but contained only 7 g lysine/kg. Pigs were pair-fed these diets for 3 wk. Growth rates and feed efficiencies of pigs fed the LL diet were significantly lower than those of pigs fed the control diet (P < 0.01). Plasma IGF-I levels in pigs fed the LL diet were 52% lower than in those fed the control diet (P < 0.01), and the LL group also had lower plasma IGF-binding protein-3 (IGFBP3) levels (P < 0.05). Despite the strikingly lower plasma IGF-I in pigs fed the LL diet, hepatic IGF-I mRNA abundance did not differ between the two treatment groups. We conclude that the reduction in plasma IGF-I caused by reduced dietary lysine may have been due in part to suppression of post-transcriptional events in IGF-I expression. The lower plasma IGFBP3 in pigs fed the LL diet suggests that increased clearance rates of circulating IGF-I may have been involved in this response.

Effects of hyperinsulinaemia under euglycaemic condition on liver fat metabolism in dairy cows in early and mid-lactation

The objective was to examine the effects of insulin under euglycaemic conditions on liver long chain fatty acids (LCFA) metabolism with special focus on the aetiology of hepatic lipidosis in early lactation. A 4-day hyperinsulinaemic-euglycaemic clamp (clamp) was conducted on four dairy cows starting in weeks 4 and 17 postpartum. Insulin was infused continuously (1 microg/kg BW per h) and a 50% glucose solution was infused to maintain euglycaemia. Liver biopsies were taken 6 days before, the last day of, and 5 days after the clamp, and blood samples were taken in the same period. In the liver tissue, the relative triglyceride content decreased (P < 0.01) and the glycogen content increased (P < 0.0001) in response to the clamp. Hepatic in vitro palmitate oxidation capacity was lowest during the clamp period and could be explained by a significant decrease in incomplete oxidation (ketogenesis) (P < 0.04) and a tendency to a decreased complete oxidation of palmitate (P < 0.10). Plasma non-esterified fatty acids concentration was decreased during the clamp in early lactation (P < 0.05) but there was no effect on the mid-lactation clamp. The present study shows that increased insulin under euglycaemic conditions seems to depress hepatic LCFA oxidation capacity. However, in terms of preventing hepatic lipidosis, the anti-lipolytic effect of insulin on adipose tissue, which results in decreased mobilization of and hence hepatic load with LCFA, appears more important.

Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition

The key regulatory enzyme of phosphatidylcholine (PC) synthesis, CTP:phosphocholine cytidylyltransferase (CT), is known to be activated in vitro by translocation from soluble to particulate fractions of the cell. In the present study the periparturient cow was chosen as a model to investigate whether translocation of CT can contribute to the regulation of PC synthesis in vivo. Between parturition and 1.5 weeks post-partum, the cytosolic CT activity in the liver of the adult animal decreased 1.9-fold, and this correlated with a 1.8-fold increase in microsomal CT activity. At that time, microsomal CT activity started to decline again whereas the cytosolic activity rose concomitantly until both activities reached their pre-partum values at 8 weeks post-partum. The activities of soluble and membrane-bound CTP:phosphoethanolamine cytidylyltransferase (ET), the analogous enzyme in the CDP-ethanolamine pathway, did not change significantly throughout this period. Whereas hepatic PC concentrations declined until about 2 weeks post-partum and thereafter gradually returned to pre-partum levels, the PC levels in very-low-density-lipoproteins, started to rise 2 weeks after the partus reaching a maximum of 219% of the original value at 8 weeks post-partum. These results strongly suggest that there is a reversible redistribution of CT between cytosol and membranes in a physiologically relevant animal model, supporting the concept that translocation of CT is occurring in vivo.

Effects of fatty acids and hormones on fatty acid metabolism and gluconeogenesis in bovine hepatocytes

Primary cultures of hepatocytes were used to study the effects of extracellular oleate concentration and hormones on fatty acid metabolism and gluconeogenesis. Rates of oleate uptake and oxidation to acid-soluble products varied linearly as oleate concentrations increased (0.1 to 2 mM), but rates of triglyceride accumulation varied quadratically. Insulin increased the proportion of oleate that was esterified by 22% without affecting the formation of acid-soluble products. Cells incubated with 2 mM [1-(14)C]oleate for 24 h eliminated 9.6% of the labeled intracellular lipid as acid-soluble products in the following 24 h when no oleate was present during depletion and eliminated 7.7% when 2 mM oleate was present. Insulin reduced labeled triglyceride depletion by 49%. Gluconeogenesis from [2-(14)C] propionate was depressed by 24%, and formation of acid-soluble products was increased by 46% in cells infiltrated with lipid because of previous exposure to 2 mM oleate for 45 h. Rates of gluconeogenesis from propionate were reduced 23% when 2 mM oleate was present during the 3-h period that gluconeogenesis was measured, and the effect was not modified by lipid infiltration. Lipid infiltration influenced hepatic function, and insulin regulated hepatic triglyceride concentration.

Nutrition during the dry period and hepatic lipid metabolism of periparturient dairy cows

From dry-off until 7 d before expected parturition date, 30 Holstein cows were fed a low energy diet (control) to maintain body condition or high energy diets (high fat or isocaloric high grain) to attempt to increase body condition. After parturition, all cows were fed a common lactation diet. Body condition was unchanged for cows fed control or high grain diets, but was decreased for cows fed the high fat diet because DMI was decreased. Cows fed the high fat diet during the dry period had decreased triglyceride accumulation in the liver at 1 d postpartum, which was accompanied by lowered plasma NEFA, increased peroxisomal beta-oxidation of palmitate by liver homogenates, and decreased esterification of palmitate by liver slices. During the last 3 wk prepartum, cows fed the high fat diet had lower glycogen and higher acid-soluble carnitine in liver, higher concentrations of NEFA and bST in plasma, higher ratio of triiodothyronine to thyroxine in plasma, and lower concentrations of insulin and thyroxine in plasma than did cows fed the high grain diet. After parturition, cows fed the high fat diet during the dry period had decreased percentage and production of milk fat; differences among treatments were not significant for DMI, body condition, milk production, or milk protein. Periparturient lipid metabolism was altered significantly in cows fed a high fat diet during the dry period, although diet composition and nutrient intakes were confounded.

Decreased sensitivity of very-low-density lipoprotein secretion to the inhibitory effect of insulin in cultured hepatocytes from lactating rats

Hepatocytes were prepared from 10-11-day lactating rat dams and from lactating dams which had been weaned for periods of either 1-2 days or 7 days. Hepatocytes from each group were cultured for periods of up to 48 h in a chemically defined medium. Compared with those from the 7-day weaned animals, hepatocytes from the lactating rats were resistant to the inhibitory effects of insulin on the secretion of very-low-density lipoprotein (VLDL) triacylglycerol (TAG). These differences persisted for up to 48 h in culture. Hepatocytes from the 1-2 day weaned animals remained relatively insulin-resistant in this respect. Similar differences in the response to insulin were not observed for the secretion of VLDL apolipoprotein B. TAG production increased and ketogenesis decreased in the hepatocytes from the lactating compared with those from the 7-day weaned rats. Insensitivity of the liver to the normal effects of insulin on the secretion of VLDL TAG may arise from a need to maintain an adequate flux of hepatic lipids to the lactating mammary gland in order to meet the large demand for milk-fat production.

The influence of medium composition on the maintenance of cytochrome P-450, glutathione content and urea synthesis: a comparison of rat and sheep primary hepatocyte cultures

Rat and sheep primary hepatocytes have been cultured in four different medium formulations: Williams' E, Chee's, Medium 199 and Modified Earle's. The total cytochrome P450 content, intracellular concentration of reduced glutathione, rate of urea synthesis and total protein content of cultures of cells from both species in each medium have been determined. Modified Earle's and Chee's medium proved to be the most favourable formulations for the culture of rat hepatocytes. After 48 h, cells cultured in Modified Earle's had significantly more cytochrome P450 and a significantly greater rate of urea synthesis than cells in any other medium. After 6 days in culture the difference in cytochrome P450 levels between rat hepatocytes in Chee's medium and those in Modified Earle's medium was abrogated. The difference in the rate of urea synthesis between rat hepatocytes cultured in each of these two media was shown to be more dependent on the medium in which the cells were maintained during the period of urea synthesis measurement than on the medium in which the cells had been previously cultured. Sheep hepatocytes cultured in Chee's medium ruptured and died within 24 h. Apart from this, sheep cells were less sensitive to changes in medium formulation than were rat hepatocytes. The initial plating efficiency was lower in sheep cells. Total cytochrome P450 content was the most discriminatory of the four parameters for evaluating the status of rat hepatocyte cultures. However, urea synthesis may be the most useful parameter for assessment of hepatocyte function in hybrid liver devices such as bioartificial liver support systems where access to the cells during operation of the device is restricted.

Comparison of growth hormone-releasing factor and somatotropin: lipid and glucose metabolism in dairy cows

Lactating, primiparous Holstein cows were continuously infused for 63 d with either recombinant growth hormone-releasing factor or recombinant bST or were not infused (control). The releasing factor and bST treatments reduced fat synthesis from acetate and lipoprotein lipase activity in omental adipose tissue. Also, both treatments increased hormone-sensitive lipase activity per gram of adipose tissue and reduced fatty acid esterification per milligram of protein in adipose tissue. Protein concentration in adipose tissue with both treatments was greater than that of the control, although the protein concentration was less in adipose tissue from cows treated with the releasing factor than from those treated with bST. In mammary tissue, releasing factor increased fat synthesis from acetate more than did bST treatment, but percentages of fat or short-chain fatty acids in milk were not different between treatments, nor were differences significant between treatments and control for fatty acid esterification or lipoprotein lipase activity in mammary tissue or gluconeogenesis and fatty acid esterification in liver. We conclude that protein concentration was lower in omental adipose tissue of cows treated with releasing factor than that in tissue of cows treated with bST, although both treatments similarly suppressed synthesis of fat from acetate and activity of lipoprotein lipase.

Lipid absorption and transport in ruminants

The objective of this paper is to review new insights on the biological mechanisms of absorption and transport of lipid in ruminants, especially the modern concepts and analytical methods used in studies on structural properties and intravascular and tissue metabolism of lipoproteins and their factors of variation. The intestinal absorption of lipids (including long-chain fatty acids) is detailed, and variations in the qualitative and the quantitative aspects of absorption with diet composition, especially for high fat diets, are presented. Also, structural properties and distribution characteristics of lipoprotein classes in different lymphatic and blood vessels are compared across several animal species. Physicochemical and hydrodynamic properties of the lipoprotein particles and their apolipoprotein moieties are given for the main classes of lipoproteins. Finally, lipoprotein metabolism is discussed in relation to development and physiological, nutritional, and hormonal status. Intravascular metabolism of lipoproteins, including the role of lipolytic enzymes and lipid transfer proteins, is presented. Characteristics of the intestinal and hepatic synthesis of lipoproteins and apolipoprotein fractions are compared, especially through experiments stimulating the hepatic secretion of triglyceride-rich lipoproteins. Different methods of measurement of lipoprotein tissue uptake or secretion in ruminants are discussed.

Etiology of lipid-related metabolic disorders in periparturient dairy cows

Plasma NEFA concentrations increase prior to and at parturition, resulting in increased fatty acid uptake by the liver, fatty acid esterification, and triglyceride storage. Liver triglyceride concentration increases four- to fivefold between d 17 prior to calving and d 1 following calving. Increases in liver triglyceride following calving do not appear to be dramatic. Severity of fatty liver 1 d postpartum is correlated negatively with feed intake 1 d prepartum. Export of newly synthesized triglyceride as very low density lipoprotein occurs slowly in ruminants and is a major factor in the development of fatty liver. Nutritional strategies to minimize the elevation in plasma NEFA prior to calving results in lower liver triglyceride at calving. Fatty liver probably precedes clinical spontaneous ketosis. Liver triglyceride to glycogen ratio may be used to predict susceptibility of cows to ketosis. Consequently, strategies to reduce liver triglyceride at calving may decrease incidence of ketosis. Research to determine methods to reduce fatty acid delivery to the liver or to enhance hepatic export of very low density lipoprotein near calving is warranted. Identification of the cause for the slow rate of assembly and secretion of hepatic very low density lipoprotein in ruminants will be required to assess the feasibility of increasing export of very low density lipoprotein.

Characterization of sheep hepatocytes in primary culture

1. Primary cultures of isolated sheep hepatocytes were used to characterize metabolic functions of liver: gluconeogenesis, ureagenesis and protein synthesis. The rates of all three metabolic activities were linear over a 20 hr culture period. 2. Hepatocytes in the presence of glucagon increased the synthesis of urea by approx 30% (P < 0.05) and increased release of glucose into the medium by 60% (P < 0.05). 3. In the absence of insulin, significantly more (35%; P < 0.05) glucose was released in the medium than in the presence of insulin. 4. Results help evaluate the primary culture of sheep hepatocytes as an appropriate experimental model to study nutritional and hormonal regulation of liver in the ruminant species.