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INTRODUCTION |
Neonatal growth is generally believed to be independent of somatotropin (ST)5. Provision of exogenous ST to neonatal pigs fails to alter weight gain or bone development (Veum et al. 1997
) or amino acid utilization (Harrell et al. 1994). However, some evidence suggests that the ability of ST to inhibit lipid deposition in adipose tissue has already developed by birth. In genetically obese fetal (Hoffman et al. 1983) and newborn (Buonomo and Klindt 1993) pigs, the circulating ST concentration was lower than in normal pigs, while the capacity of de novo fatty acid synthesis was higher in pre-obese than in control fetuses (Hausman et al. 1991). Hypophysectomized fetal lambs (Stevens and Alexander 1986) and pigs (Hausman et al. 1995) exhibited increased fat deposition in adipose tissue that was abolished by ST treatment. Kasser et al. (1983) proposed that high ST levels are responsible for inhibiting fetal pig adipose tissue lipogenesis by suppressing insulin action. Consistent with this idea, ST abolished insulin-stimulated fatty acid synthesis from acetate in cultured adipose tissue from newborn lambs (Vernon 1982).
Abundant evidence exists that ST regulates lipid deposition in adipose tissue of growing animals by decreasing de novo fatty acid synthesis. In growing pigs, ST decreases glucose incorporation into fatty acids in adipose tissue in vivo (Dunshea et al. 1992b) and into total lipids in vitro (Walton and Etherton 1986). ST may also regulate lipid deposition through affecting exogenous lipid uptake by adipose tissue. Lipoprotein lipase (LPL) is the major enzyme responsible for regulating the uptake of circulating triglycerides for storage in adipose tissue. ST decreases the activity of LPL in rat and human adipose tissue both in vitro (Murase et al. 1981; Ottosson et al. 1995) and in vivo (Barber et al. 1992; Richelsen et al. 1994), as well as bovine adipose tissue in vivo (Liesman et al. 1995). No previous studies have examined ST effects on LPL activity in porcine adipose tissue in vitro. Additionally, ST may regulate lipid deposition through affecting lipid mobilization (lipolysis) in adipose tissue. However, the effects of ST on lipolysis are somewhat controversial. Direct lipolytic effects of ST were demonstrated in isolated perifused rat adipocytes (Sengupta et al. 1981), cultured 3T3-F442A adipocytes (Dietz and Schwartz 1991) and newly-differentiated rat (Wabitsch et al. 1996a) and human (Wabitsch et al. 1996b) adipocytes in culture. In contrast, several studies were unable to detect a direct lipolytic effect of ST in vivo (Dunshea et al. 1992a) or in vitro (Walton and Etherton 1986) in growing pigs, or in vitro in sheep (Borland et al. 1994), or in lactating cows (Boisclair et al. 1997, Houseknecht and Bauman 1997).
The objective of this study was to determine if ST regulates lipid deposition in neonatal pig adipose tissue through affecting lipid synthesis, LPL activity and/or lipolysis.
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MATERIALS AND METHODS |
Hormones and reagents.
Porcine recombinant ST was donated by Fort Dodge Vaccine (Princeton, NJ). Porcine insulin was purchased from Eli Lilly & Co. (Indianapolis, IN). Fatty acid-free bovine serum albumin (BSA) was purchased from the Intergen Company (Purchase, NY). Type I collagenase was purchased from Worthington Biochemicals (Freehold, NJ). D-[U-14C]-glucose and 3H-triolein were purchased from Du Pont NEN (Boston, MA). Medium 199 (M199) was purchased from Gibco, BRL (Grand Island, NY), containing 25 mmol/L HEPES buffer, 25 mmol/L NaHCO3, L-glutamine and Earle's Salts. All other reagents were purchased from the Sigma Chemical Co. (St. Louis, MO).
Animals.
Pigs were acquired from the Cook College Swine Farm (New Brunswick, NJ). Neonatal pigs were nursing and growing pigs were fed a corn-soybean meal diet with <4% calculated fat. Four neonatal (2.9 ± 0.1 kg, 7 d of age, 2 males:2 females) and four growing (17.0 ± 1.4 kg, 60 ± 3 d of age, 2 males:2 females) crossbred pigs (Hampshire × Yorkshire × Landrace) were killed with an overdose of acepromazine maleate and ketamine HCl. The dorsal skin was cleaned with iodine and alcohol prior to collection of subscapular adipose tissue. Tissue subsamples were immediately placed in 37°C M199 containing 10 mg/L gentamicin for transfer to the laboratory. All procedures were approved by the Rutgers University Animal Care and Use Committee (New Brunswick, NJ).
Tissue culture.
Tissue culture followed the procedure described by Fried and Zechner (1989). In a sterile hood, tissue was minced into 5-10 mg pieces and washed with warm sterile saline. Tissue fragments (300-500 mg/15 mL M199 + 10 mg/L gentamicin) were cultured with or without ST (4.5 nmol/L) in the absence or presence of insulin (7 nmol/L). Porcine ST was dissolved in a buffer composed of 25 mmol/L NaHCO3 and 25 mmol/L NaCO3 in saline (pH 9.6), and diluted in M199. Culture was maintained at 37°C under an atmosphere of 5% CO2. After 4 or 24 h of culture, 1 mL of medium and 30-40 mg tissue from each dish were collected and stored at 
80°C for later analysis of glycerol and LPL activity (see below).
Adipocyte isolation.
Adipocytes were isolated using the procedure of Rodbell (1964) as modified by Honnor et al. (1985). Briefly, tissue from three dishes in each treatment was pooled and washed with warm sterile saline. Fresh or cultured adipose tissue was digested with collagenase for 1 h in modified Krebs-Ringer bicarbonate albumin buffer (KRBA) (pH = 7.4, containing 4% BSA, 5 mmol/L glucose). This buffer was supplemented with 100 nmol/L N6-phenylisopropyladenosine (PIA) for glucose metabolism studies or 200 nmol/L adenosine for lipolysis experiments. Adenosine or its analog, PIA, suppresses adenyl cyclase activity and hence basal lipolysis allowing for increased reproducibility of adipocyte lipolytic rates (Honnor et al. 1985), and minimal cell lysis (Fried, S. K., unpublished observation). Adipocytes were isolated by gentle centrifugation (<500 × g for 1 min). Cells were washed by removing buffer using a plastic catheter attached to a syringe and adding fresh collagenase-free buffer. After three such washes, the cells were resuspended with fresh buffer with PIA (glucose metabolism studies) or adenosine (lipolysis studies) (1:4, vol/vol).
Adipocyte incubations.
Glucose incorporation was determined by incubating cells with 14C-glucose (DiGirolamo et al. 1993). Briefly, 0.5 mL of cell suspension (1-2 × 106 cells) was added to a plastic vial with 0.5 mL of KRBA containing 0.25 µCi (9.25 kBg) 14C-glucose. The final concentration of glucose was 5 mmol/L. Incubations were carried out in triplicate. Vials were gassed with 95% O2 and 5% CO2, sealed with a cap, and placed in an oscillating 37°C water bath for 2 h. A preliminary study showed no consistent effect of acutely adding insulin or ST to adipocyte incubation on glucose incorporation. Thus, subsequent studies were carried out in the absence of hormones. Total lipid in the adipocytes was extracted with heptane and glucose incorporation into total lipid was determined. Incorporation of 14C-glucose into fatty acids and glyceride-glycerol was separated after saponification in ethanolic KOH, acidification and heptane extraction (DiGirolamo et al. 1974). Adipocyte size and number were determined according to the method of DiGirolamo et al. (1971). Both our preliminary study and the study of Etherton and Chung (1981) showed that glucose incorporation into lipid in adipocytes increased linearly with time and cell number, providing validation of our glucose incorporation assay.
During lipolysis experiments, isolated adipocytes were incubated for 2 h in KRBA containing 40 g/L BSA and 5 mmol/L glucose in the presence of 4 mg/L adenosine deaminase (ADA), with or without 100 nmol/L PIA.
Glycerol release.
Glycerol accumulation in culture medium or incubation medium was determined by an enzymatic/fluorometric method (Edens et al. 1993). Briefly, samples were deproteinized using 0.65 mol/L perchloric acid and neutralized with imidazole. The reaction mixture of glycerokinase (EC 2.7.1.30), glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) and ATP was added to the deproteinized sample. Glycerol was quantified according to the fluorescence of NADH after adding NAD.
Heparin-releasable LPL activity.
Heparin-releasable LPL activity was measured as described by Fried and Kral (1987). Briefly, samples of adipose tissue were incubated at room temperature with 5000 u/L heparin for 45 min. This eluate was then assayed for LPL activity using the glycerol-stabilized 3H-triolein emulsion. 1 U of LPL activity was defined as catalyzing the release of 1 µmol free fatty acid (FFA) per hour. Taskinen et al. (1980) originally reported that it is possible to measure heparin-releasable LPL activity in fragments of frozen adipose tissue. Our preliminary data showed no significant difference between the values from fresh and liquid nitrogen frozen samples. Additionally, our values from growing pigs using previously frozen tissue were comparable to those of growing pigs measured by Mersmann (1998) using fresh tissue.
Statistics.
Values were expressed as mean ± SEM. Data were first analyzed by a three-way analysis of variance (ANOVA) using age as a grouping factor and ST and insulin as repeated measures. Because the interaction with age was not significant, the effects of hormones × time within each age group were compared by two-way ANOVA for hormone effects. When F values for main effects or interactions were significant, individual means were compared with Bonferroni's t-test (two-tailed). The effect of age on basal glucose metabolism was tested using an independent t-test. Probabilities <0.05 were considered statistically significant.
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RESULTS |
Glucose metabolism in adipocytes from fresh vs. cultured adipose tissue.
Walton and Etherton (1986) demonstrated that culture with insulin maintains the capacity of adipose tissue from growing pigs for glucose metabolism. Initial experiments were conducted to verify that this culture system was suitable for adipose tissue of neonatal pigs, and further, to demonstrate that the metabolic activity of adipocytes isolated from cultured adipose tissue could be studied. Isolated adipocytes were prepared from adipose tissue of neonatal or growing pigs immediately after removal from the animal and after 24 h of culture with insulin. In adipocytes prepared from fresh adipose tissue, glucose incorporation was about 9-fold greater in growing pigs [315 ± 11 nmol/(106 cells·2 h)] than in neonatal pigs [35 ± 4 nmol/(106 cells·2 h); P < 0.001)]. When expressed relative to cell surface area to account for the difference in fat cell size between groups, glucose incorporation was only about 2.6-fold greater in growing pigs than in neonatal pigs. After culture with insulin for 24 h, rates of glucose incorporation into lipid by both growing and neonatal pig adipocytes were maintained at values observed in freshly-isolated adipocytes [growing: 338 ± 51 nmol/(106 cells·2 h) and neonatal: 31 ± 3 nmol/(106 cells·2 h)]. These data show that culture with insulin for 24 h maintained the capacity of the adipocytes for glucose metabolism and that the culture system was suitable for studies of ST action.
Culture of adipose tissue with ST decreased lipid synthesis from glucose in adipose tissue from neonatal and growing pigs.
After culture with ST for 24 h, with or without insulin, lipid synthesis, as determined by glucose incorporation into total lipid, significantly decreased in both age groups (Fig. 1A; P < 0.01). ST inhibited glucose incorporation into both fatty acid and glyceride-glycerol components of total lipid (Fig. 1B and 1C). Culture with ST alone decreased glucose incorporation into fatty acids by 78 ± 3% (P < 0.01) in neonatal pig adipose tissue and by 79 ± 3% (P < 0.01) in growing pig adipose tissue. Culture with insulin alone increased glucose conversion to fatty acids in both groups (P < 0.01). Culture with ST and insulin, as compared to insulin alone, decreased glucose incorporation into fatty acids by 43 ± 9% (P < 0.05) in neonatal pig adipose tissue and by 33 ± 3% (P < 0.05) in growing pig adipose tissue. Culture with ST alone also decreased glucose incorporation into glyceride-glycerol by 52 ± 3% (P < 0.05) in neonatal pig adipose tissue. However, ST did not significantly decrease glucose conversion to glyceride-glycerol in growing pig adipose tissue. In the presence of insulin, culture with ST decreased glucose incorporation into glyceride-glycerol by 25 ± 10% (P < 0.05) in neonatal pig adipose tissue and by 34 ± 3% (P < 0.05) in growing pig adipose tissue.
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| Fig 1.
Glucose incorporation into A) total lipid, B) fatty acids, C) glyceride-glycerol by adipocytes from neonatal (7 d) or growing (60 d) pig adipose tissue cultured for 24 h with somatotropin (ST) (4.5 nmol/L) in the absence or presence of insulin (7 nmol/L). Data were analyzed by two-way analysis of variance. Main effects of ST and insulin were significant for all variables in both age groups. Individual means were compared by Bonferroni t-test. Bars (mean ± SEM; n = 4) with different superscripts within an age class differed significantly at P < 0.05.
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To determine the time needed for ST effects on glucose metabolism, adipose tissue was cultured with or without ST in the absence or presence of insulin for only 4 h. Glucose incorporation into total lipid, fatty acids and glyceride-glycerol was not significantly altered by treatments in either group (data not shown).
Culture of adipose tissue with ST decreased heparin-releasable LPL activity in adipose tissue from neonatal and growing pigs.
Heparin-releasable LPL activity in the cultured tissue is shown in Table 1. After culture with ST for 24 h, heparin-releasable LPL activity was decreased by 58 ± 14% (P < 0.01) in neonatal adipose tissue and by 69 ± 10% (P < 0.01) in growing pig adipose tissue. With insulin present, ST decreased LPL activity by 75 ± 4% in neonatal pig adipose tissue (P < 0.01) and by 89 ± 6% in growing pig adipose tissue (P < 0.01). Insulin alone significantly increased LPL activity in the 24 h of culture (P < 0.01). However the effect of insulin totally disappeared in the presence of ST (ST × insulin interaction, P < 0.01). The effect of ST was also significant (P < 0.05) by 4 h of culture in both groups, but the effect of insulin was not.
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Table 1.
Heparin-releasable lipoprotein lipase (LPL) activity in neonatal (7 d) or growing (60 d) pig adipose tissue cultured for 4 or 24 h with or without somatotropin (ST) (4.5 nmol/L) in the absence or presence of insulin (7 nmol/L)1
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Culture of adipose tissue with ST increased lipolysis in adipose tissue from neonatal and growing pigs.
To characterize whether ST affected lipolysis during culture, glycerol release into the culture medium was measured. There was no effect of ST after only 4 h of culture in either group (data not shown). However, after 24 h of culture with ST, glycerol accumulation in the medium (Table 2) increased by 120 ± 32% in neonatal pig adipose tissue (P < 0.01) and by 247 ± 56% (P < 0.01) in growing pig adipose tissue. With insulin present, culture with ST for 24 h increased glycerol accumulation in the medium by 44 ± 17% in neonatal pig adipose tissue, (P < 0.05) and 76 ± 16% in growing pig adipose tissue (P < 0.01). Culture with insulin alone for either 4 h (data not shown) or 24 h (Table 2) significantly increased glycerol release in both groups.
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Table 2.
Glycerol release from neonatal (7 d) or growing (60 d) pig adipose tissue cultured for 24 h with or without somatotropin (ST) (4.5 nmol/L) in the absence or presence of insulin
(7 nmol/L)1,2
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We also assessed the lipolytic capacity of isolated adipocytes prepared from adipose tissue cultured for 24 h with or without ST. Adipocytes were incubated in the presence of ADA, i.e., under ligand free-conditions, and also with a maximally-inhibitory concentration of PIA. Lipolytic rate in the presence of ADA alone was higher in adipocytes isolated from adipose tissue cultured with ST compared to the control tissue (P < 0.05) in both age groups (Table 3). However, PIA inhibited lipolysis in adipocytes from both control and ST-treated cultures to similarly low levels (Table 3).
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Table 3.
Glycerol release from adipocytes isolated from neonatal (7 d) and growing (60 d) pig adipose tissue cultured with or without somatotropin (ST) (4.5 nmol/L) for 24 h1,2,3
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DISCUSSION |
The present data demonstrate that ST directly regulates endogenous lipid synthesis and exogenous lipid uptake as well as lipid mobilization in neonatal porcine adipose tissue. In vitro, culture of neonatal adipose tissue with ST for 24 h decreased de novo fatty acid synthesis and LPL activity. ST inhibited de novo fatty acid synthesis even in the presence of insulin and antagonized the stimulatory effect of insulin on LPL. These effects of ST required prolonged treatment; they were not apparent after 4 h of culture. In addition, ST increased spontaneous lipolysis during culture and increased basal lipolytic rates in isolated adipocytes from ST-treated adipose tissue that were incubated in the presence of ADA to remove adenosine in the medium.
The magnitude of alterations in adipose tissue metabolism by ST treatment was similar in neonatal and in growing pigs. Therefore, in the neonatal pig, as in growing pigs, ST has the ability to directly influence lipid deposition in adipose tissue. The effect of ST on glucose conversion to glyceride-glycerol was relatively greater in neonatal than in growing pig adipose tissue. Since lipid is a major component of the neonatal diet, the ability of ST to decrease LPL activity, as well as to decrease esterification, may both play important roles in regulating nutrient partitioning during the neonatal period.
Previous in vitro studies of ST effects on adipose tissue (Walton and Etherton 1986) only reported data on glucose incorporation into total lipid and did not distinguish between fatty acids and glyceride-glycerol. The present results show that, in the absence and presence of insulin, ST decreased glucose conversion to fatty acids, and had relatively less of a suppressive effect on conversion to glyceride-glycerol. Thus, the main effect of ST is a decrease in fatty acid synthesis. Larger effects of ST on fatty acid than glyceride-glycerol synthesis were previously shown by studies of growing pigs in vivo (Dunshea et al. 1992b). In adipose tissue from neonatal but not growing pigs, ST significantly decreased glucose conversion to glyceride-glycerol in the absence of insulin. Thus, ST would be expected to have a marked effect on esterification of fatty acids in neonatal adipose tissue under conditions of low insulin. In the presence of insulin, ST decreased glyceride-glycerol synthesis in adipose tissue from both age groups. Therefore, under hyperinsulinemic conditions, the reduced capacity of the adipocytes for glucose conversion into glyceride-glycerol after ST treatment may also play a role in decreasing esterification of fatty acids. Overall, our data indicate that ST not only inhibits basal lipid synthesis, but may also antagonize long-term effects of insulin to increase lipid synthetic capacity in the neonatal pig, as has been shown in the growing pig (Walton and Etherton 1986). In agreement with the present results, adipose tissue explants from 15-19-d-old pigs that were administered ST in vivo showed decreased lipogenesis (Harrell et al. 1996), and administration of ST to young pigs (10-25 kg) reduced lipid deposition (Harrell et al. 1997). In contrast to our in vitro results, however, the response was not as great as in older pigs. Thus, in vivo factors undoubtedly modulate the direct effects of ST on adipocytes.
The suppressive effect of ST on fatty acid synthesis may have been due to decreases in the activities of the fatty acid-synthesizing enzymes, such as acetyl-CoA carboxylase and fatty acid synthase (Harris et al. 1993), and/or due to decreases in the mass of glucose transporter proteins, such as GLUT4 (as reviewed by Etherton et al. 1993). Administration of ST may lower enzyme activity by decreasing mRNA relative abundance, as shown for fatty acid synthase (Mildner and Clarke 1991, Donkin et al. 1996), and acetyl-CoA carboxylase (Liu et al. 1991). In our study, the ability of ST to inhibit lipid synthesis required more than 4 h to become fully manifest. This lag may reflect the time required for ST to decrease the expression of lipogenic genes or, more likely considering the relatively long half life of these enzymes, components of the signalling pathway involved in ST action.
The ability of adipocytes from growing pigs to synthesize lipid was greater than adipocytes from neonatal pigs. When expressed per 106 cells, glucose incorporation was about 9-fold greater in growing pigs than in neonatal pigs. When expressed relative to cell surface area (to account for the difference in fat cell size between groups), glucose incorporation was only about 2.6-fold greater in growing pigs than in neonatal pigs. Given that the surface area of the adipocyte in growing pigs (2670 ± 135 µm2) was larger than that in the neonate (985 ± 68 µm2), more glucose transporters may have been available, allowing more glucose to be transported into the adipocytes. Thus, the capacity of neonatal adipocytes for glucose metabolism is not entirely explained by the difference in fat cell size and undoubtedly relates to developmental factors such as the high-fat diet during the neonatal period. These results are consistent with those in rat adipose tissue in which the lipogenic pathway is very low during the suckling period and increases considerably after weaning on a high-carbohydrate diet (Tsujikawa and Kimura 1980). Since lipid is a major component of the neonatal diet, it is not necessary to synthesize much fatty acid, suggesting that it would be more important for ST to regulate exogenous lipid uptake by adipose tissue during the neonatal period.
LPL is the major enzyme responsible for hydrolysis of triglycerides in chylomicrons and very low density lipoproteins to provide FFA for tissue utilization or storage. ST markedly reduced LPL activity at 4 and 24 h of culture in both neonatal and growing pigs. By 24 h of culture with ST, activity was decreased over 50%, suggesting that the activity of LPL could limit the uptake of circulating triglycerides into adipose tissue after ST treatment. Our results also show that the long-term stimulatory effect of insulin on LPL activity was totally inhibited by ST, suggesting that ST may regulate LPL activity through antagonizing the stimulatory effect of insulin. The suppressive effect of ST on LPL activity was observed previously in rat and human adipose tissue both in vitro (Murase et al. 1981, Ottosson et al. 1995) and in vivo (Barber et al. 1992, Richelsen et al. 1994), as well as bovine adipose tissue in vivo (Liesman et al. 1995). In contrast, Kramer et al. (1993) elevated ST by daily injection in finishing pigs for 26 d, with no effect on LPL activity in adipose tissue. This lack of effect may have been due to the fact that results were expressed per mg protein which would not account for the decrease in fat cell size in the ST-treated group (smaller fat cells have less protein per cell and therefore have more cells per mg protein).
We found that culture of adipose tissue from both neonatal and growing pigs with ST stimulated lipolysis, as determined by glycerol release during culture. This effect of ST was not detectable by 4 h, but was apparent after 24 h. Thus, the effect of ST is chronic rather than acute. Because we did not analyze the time course in detail, we cannot be certain of the magnitude of the effect of ST on glycerol accumulation in the medium during culture. Our limited time course data indicate that lipolysis slowed from 4 to 24 h of culture in both control and ST-treated cultures. Nevertheless, it was clear that culture with ST consistently increased the amount of glycerol accumulating in the medium over 24 h, indicating an activation of lipolysis under the conditions in our cultures. The conclusion that ST has a chronic lipolytic effect in neonatal, as well as growing pig, adipocytes is strengthened by the finding that culture with ST also increased rates of spontaneous lipolysis measured in acute incubation of adipocytes under carefully controlled conditions.
Our finding that culture with ST increases the rate of spontaneous lipolysis is consistent with previous reports in cultured 3T3-L1 adipocytes (Dietz and Schwartz 1991) and newly-differentiated rat (Wabitsch et al. 1996a) and human adipocytes (Wabitsch et al. 1996b) in culture. Consistent with these findings, Doris et al. (1996) found a small increase in basal lipolysis in adipose tissue removed from sheep treated in vivo with ST. We show here for the first time that chronic (24 h) exposure to ST also increases lipolysis in adipocytes from neonatal pigs. The effect of ST on lipolysis was detectable under ligand-free conditions, but not when rates of lipolysis were suppressed by incubation of adipocytes with a high concentration of PIA, an adenosine receptor agonist. Thus, the higher rates of lipolysis observed during culture with ST may reflect the fact that the endogeneous adenosine concentration within the tissue fragments was relatively low. Previous studies showed that ST decreased sensitivity to the antilipolytic effects of adenosine (Doris et al. 1994, Doris et al. 1996, Lanna et al. 1995). Taken together, these results indicate that ST increases lipolysis by increasing the capacity for spontaneous lipolysis (under ligand-free conditions) as well as by decreasing sensitivity to submaximally inhibiting concentrations of adenosine. Thus, the increased glycerol accumulation in the medium detected during culture with ST suggests that there are submaximally inhibiting concentrations of adenosine present in adipose tissue fragments during culture. The lack of effect of ST on lipolysis in previous studies (Walton and Etherton 1986; Borland et al. 1994) are probably explained by differences in the incubation conditions. Additional studies are needed to assess the effect of ST on sensitivity to submaximal concentrations of adenosine and to investigate interactions among ST and other hormones (insulin, glucocorticoids, catecholamines) in the regulation of lipolysis. It will be particularly important to assess the interactions between ST and glucocorticoids that increase responsiveness to ST (Vernon and Finley 1986, Walton et al. 1986).
Variable results have been obtained in studies of in vivo ST administration to animals. Doris et al. (1996) and Harrell et al. (1997) observed a small increase in serum FFA after ST administration to pigs and sheep, respectively. However, daily administration of ST for 7 d to growing pigs (Dunshea et al. 1992a) did not alter plasma glycerol concentration or glycerol or FFA rate of appearance. This may be explained by the method of ST delivery, since lipolytic effects of ST were only observed in humans in vivo when the hormone was administered in a pulsatile manner to mimic the physiological situation (Cersosimo et al. 1996). Alternatively, in vivo stress (Boisclair et al. 1997) may also confound the results.
The lipolytic effect of ST in vivo may also be mediated by increased response to catecholamine stimulation of lipolysis, as demonstrated in pigs (Wray-Cahen et al. 1987), cattle (McCutcheon and Bauman 1986), and sheep (Doris et al. 1996). In future studies, it will be important to determine whether ST affects the neonatal adipocyte response to paracrine, neural and endocrine regulators of lipolysis in vitro.
Our data also show that culture with insulin for either 4 or 24 h increased glycerol release from adipose tissue in both groups. Similar to our results, Smith et al. (1976) and Walton and Etherton (1986) also observed that a long-term culture with insulin increases lipolysis in human and porcine adipose tissue, respectively. The mechanism to explain this insulin effect is not known, but it may relate to the increased rate of glucose metabolism in the presence of insulin (Smith et al. 1976). This chronic effect of insulin to increase spontaneous rates of lipolysis is mechanistically distinct from this hormone's acute antilipolytic effect. In fact, in preliminary experiments we found that insulin was able to acutely inhibit lipolysis in adipocytes prepared from adipose tissue of growing pigs that had been cultured with or without ST (though sensitivity may be decreased in cells previously cultured with ST).
In summary, our results show for the first time that, in the neonatal pig, ST regulates adipose tissue metabolism. ST inhibited both endogenous lipid synthesis and limits exogenous lipid deposition by decreasing LPL activity. In addition, ST stimulates lipid mobilization, further diminishing net lipid deposition. All of these actions of ST may play important roles in nutrient partitioning during the neonatal period.
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Abstract
Introduction
Abstract
