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Comparison of Plasma Free-Fatty-Acid and Blood-Glycerol Concentrations with Measurement of Lipolysis in Porcine Adipose Tissue In Vitro

U.S. Department of Agriculture^,2,3,, Clay Center, NE 68933

Abstract

Rates of adipose tissue lipid metabolism in vitro are often measured to evaluate the function in vivo of metabolic pathways and thus appraise the accretion or loss of depot fat. This study directly addressed the comparison of degradative metabolism in vitro and in vivo. The concentrations of plasma free-fatty-acids and blood-glycerol as putative representatives of lipolysis in vivo and the lipolytic rate in adipose tissue in vitro obtained at the time of blood sampling were both measured in the same pig. Concentrations of plasma free-fatty-acids and blood-glycerol were increased or decreased by infusion of the norepinephrine analog, isoproterenol or by infusion of the adrenergic antagonist, propranolol, respectively. Although lipolytic rates and sensitivity to isoproterenol in vitro changed during some acute hormonal manipulations of the pig, the modulation in vitro was usually small relative to the large changes observed in plasma free-fatty-acid and blood-glycerol concentrations. Some of the subtle changes in vitro may reflect biological responses to hormone infusion, e.g., desensitization of the response to adrenergic agonists, but the magnitude of rate changes in vitro negates prediction of the rates in vivo from rates in vitro. Extrapolation of lipolytic rates in vitro and several adipose tissue anabolic rates obtained from the literature indicate the improbability for prediction of rates and degree of fat accretion in pigs from metabolic rates in vitro.

¹ I thank D. L. Ochsner for execution of these experiments and Dr. M. D. MacNeil for consultation regarding data presentation. I also acknowledge the assistance of T. W. Acton for use and care of swine and B. E. Larsen and S. V. Reece for handling and care of swine. M. A. Stones and M. M. Bierman very capably provided secretarial assistance.

² Agr. Res. Serv., Roman L. Hruska U.S. Meat Anim. Res. Center.

³ Mention of a trade name, proprietary product or vendor does not constitute a guarantee or warranty by the USDA and does not imply its approval of the exclusion of other products or vendors that may be suitable.

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