Glucagon-induced desensitization of broiler adipocyte lipolysis

Fasting and Glucagon Stimulate Gene Expression of Pyruvate Dehydrogenase Kinase 4 in Chickens

The excessive accumulation of body fat has become a serious problem in the broiler industry. However, the molecular mechanisms underlying the regulation of lipid metabolism-related genes in broiler chickens are not fully understood. In the present study, we investigated the role of glucagon on the expression of lipid metabolism-related genes in chicken white adipose tissue (WAT). Four hours of fasting significantly increased plasma levels of free fatty acid in broiler chickens. The mRNA levels of adipose triglyceride lipase (ATGL) and pyruvate dehydrogenase kinase 4 (PDK4) in abdominal WAT significantly increased by fasting, whereas the mRNA levels of diacylglycerol O-acyl-transferase homolog 2 (DGAT2) and peroxisome proliferator-activated receptor-γ (PPARγ) significantly decreased. The results suggest that fasting stimulates lipolysis and suppresses adipogenesis and re-esterification of TG in chicken WAT. Glucagon significantly increased the mRNA levels of PDK4 in chicken primary adipocytes, whereas there were no significant changes in the mRNA levels of ATGL, DGAT2, and PPARγ. Our findings suggest that glucagon upregulates PDK4 expression and may stimulate lipolysis without affecting the expression of ATGL in chicken WAT.

Down-regulation of glucagon receptors on the surface of broiler adipocytes

A radioreceptor assay that specifically measures the affinity and concentration of glucagon receptors on the surface of broiler adipocytes was used to determine whether down-regulation of glucagon receptors is involved in the mechanism whereby glucagon induces desensitization of its ability to acutely stimulate lipolysis. Adipocytes from abdominal fat of 42- to 49-d-old female broilers were preincubated in vitro with 0 to 8 nM glucagon for 0 to 24 h before removal of glucagon by washing and assessment of lipolysis during a subsequent 1 h incubation at 37 C and binding of ([125I]iodo-Tyr10) glucagon during an ensuring 6 h incubation at 12 C. In Experiment 1, pretreatment of adipocytes with 0 to 8 nM glucagon for 24 h reduced lipolysis 70% with a preincubation dose of glucagon (ED50) of 0.4 +/- 0.04 nM, whereas glucagon binding was decreased 75% with an ED50 of 0.7 +/- 0.1 nM. In Experiment 2, pretreatment of adipocytes with 5.4 nM glucagon for 0 to 2 h decreased lipolysis 60% with a time of preincubation with glucagon (T1/2) of < 1 min, whereas glucagon binding was reduced 40% with a T1/2 of 2 +/- 0.6 min. In Experiment 3, detailed analysis of lipolysis and glucagon binding to adipocytes pretreated with 5.4 nM glucagon for 5 min, 2 h, or 24 h revealed that sensitivity of adipocytes to acute stimulation of lipolysis by glucagon and the number of glucagon receptors on the adipocyte surface were decreased by glucagon pretreatment. The assay concentration of glucagon (EC50) for glucagon stimulation of lipolysis, a measure of adipocyte sensitivity, increased 0.8-fold at 5 min, 1.6-fold at 2 h, and 4.9-fold at 24 h of preincubation. The number of low-affinity glucagon receptors decreased 12% at 5 min, 20% at 2 h, and 50% at 24 h of preincubation with glucagon. Throughout this study, decreased glucagon binding was closely associated with reduced lipolysis, implicating down-regulation of cell-surface glucagon receptors in the mechanism whereby glucagon induces desensitization of its ability to acutely stimulate lipolysis in broiler adipocytes.

Metabolic responses of the turkey hen (Meleagris gallopavo) to an intravenous injection of chicken or porcine glucagon

The purpose of this study was to compare the metabolism of chicken and porcine glucagon in the turkey hen. Six hens each were infused with either porcine or chicken glucagon (16.6 micrograms/kg body weight). Blood samples were obtained at intervals pre- and post-infusion and analyzed for glucagon, glucose, insulin, free fatty acids, thyroxine (T4) and triiodothyronine (T3). The half-life (T 1/2) of porcine (4.5 min) and chicken glucagon (5.5 min) were similar. A 13% increase in glucose concentrations occurred within 10 min and remained elevated (22%) for 2 hr. A concomitant increase in insulin was noted, though not significant from pre-injection levels. A 50% increase in free fatty acids occurred in 2 min, reaching a zenith at 10-15 min post-treatment, and returned to baseline by 2 hr. Significant (P < 0.05) decreases in the circulating concentrations of both T3 and T4 hormones were observed within 5-10 min post-glucagon treatment and remained suppressed for the duration of sampling. These data indicate that the hormonal and metabolic responses of female turkeys to mammalian and avian glucagon are similar.

Prolonged in vitro exposure of broiler adipocytes to somatostatin enhances lipolysis and induces desensitization of antilipolysis

Adipocytes isolated from abdominal fat of female broilers and maintained in primary culture were used to characterize acute and chronic effects of somatostatin (SRIF) on lipolysis and antilipolysis and to determine whether desensitization and cross-regulation phenomena are involved in the regulation of adipocyte metabolism by SRIF and glucagon. Acute exposure of adipocytes to SRIF resulted in a dose-dependent inhibition of basal and glucagon-stimulated lipolysis. The potency and extent of lipolysis inhibition by SRIF were inversely related to the dose of glucagon used to stimulate lipolysis. Preincubation of adipocytes with SRIF induced a dose-response and time-dependent increase in the set-point of lipolysis. Adipocyte sensitivity to glucagon was not altered by SRIF pretreatment. In contrast, preincubation with SRIF reduced adipocyte sensitivity and maximal responsiveness to SRIF. The pattern and extent of antilipolysis attenuation were dependent on the dose and time of preincubation with SRIF as well as on the concentration of glucagon used to acutely stimulate lipolysis. In two experiments, the attenuation of antilipolysis induced by SRIF pretreatment was observed in the absence of enhanced lipolysis, whereas in one experiment enhanced lipolysis was observed in the absence of attenuated antilipolysis. These results indicated that persistent activation of antilipolysis by SRIF increased the set-point of lipolysis by sensitizing (i.e., cross-regulating) lipolysis and by desensitizing antilipolysis.

Triiodothyronine attenuates somatostatin inhibition of broiler adipocyte lipolysis

A study with broiler adipocytes in culture was undertaken to determine whether triiodothyronine (T3) potentiates lipolysis by increasing glucagon binding, by attenuating inhibition of lipolysis, or both. Fat cells isolated from abdominal fat were preincubated with T3 for .5 to 24 h before removal of T3 by washing and measurement of lipolysis. Preincubation of adipocytes with T3 enhanced (P < .05) basal as well as glucagon-stimulated lipolysis in a dose-response and time-dependent manner. Enhancement of lipolysis was maximal in the presence of 15 to 150 nM T3. Potentiation of lipolysis by 150 nM T3 was evident at 4 and 24 h but not at .5 h of pretreatment. Overall, T3 enhancement of lipolysis stimulated by submaximal doses of glucagon was similar in magnitude to enhancement of lipolysis stimulated by a maximal dose of glucagon but was greater (P < .05) than its enhancement of basal lipolysis. Pretreatment of adipocytes with T3 did not alter (P > .05) binding of 125I-glucagon to cell-surface receptors. When fat cells were preincubated with 150 nM T3 for 24 h, the ability of somatostatin to inhibit basal and glucagon-stimulated lipolysis was reduced (P < .05). Thus, prolonged exposure of adipocytes to T3 did not increase lipolytic sensitivity to glucagon or binding of glucagon to cell-surface receptors. However, T3-treated adipocytes exhibited enhanced basal lipolysis, enhanced lipolytic responsiveness to glucagon, and attenuated inhibition of lipolysis by somatostatin.