Camp. Biochem. Physiol. Vol. 9lC, No. 2, pp. 619-622, 1988 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Printed in Great Britain Pergamon Press plc CLENBUTEROL STIMULATED INCREASE OF PLASMA FREE FATTY ACIDS IN RESERPINIZED PIGS* C. Y. HuQ, J. NOVAKOFSKI~ and H. J. MERSMANN$$II tDepartment of Animal Sciences, University of Illinois, Urbana, IL 61801, USA and fRoman L. Hruska U.S. Meat Animal Research Center, U.S. Dept of Agriculture, Clay Center, NE 68933, USAT zyxwvutsrqponmlkjih (Received 12 November 1987) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Abstract-l. Previous studies indicate the /I-adrenergic agonist, clenbuterol, does not stimulate porcine adipose tissue lipolysis or CAMP concentration in vitro but increases plasma free fatty acid concentrations when infused, implying an indirect mechanism in vivo. 2. One indirect mechanism is the release of endogenous catecholamines to increase adipose tissue lipolysis and raise plasma free fatty acids. 3. In pigs treated with reserpine to deplete endogenous catecholamines, clenbuterol infusion increased plasma free fatty acids concentration suggesting that this increase in viva did not result from release of endogenous catecholamines. INTRODUCTION Feeding selected /I-adrenergic agonists such as clen- buterol to animals raised for meat production im- proves overall efficiency by decreasing carcass fat content and concomitantly increasing carcass muscle mass (Ricks et al., 1984). The mode of action(s) of these p-adrenergic agonists is not understood. Be- cause porcine adipose tissue lipolysis is stimulated by some /I-adrenergic agonists in vitro (Mersmann, 1984) the decrease in carcass fat may result from direct stimulation of lipolysis. Clenbuterol does not stimulate lipolysis in vitro or increase cyclic AMP concentration in porcine adipose tissue incubated with clenbuterol (Hu et aI., 1987; Mersmann, 1987). However, in pigs infused intravenously with clen- buterol, the concentration of plasma free fatty acids (FFA) is elevated two to three-fold (Mersmann, 1987). The lack of effect of clenbuterol on porcine adipose tissue lipolysis in vitro coupled with the increase in plasma FFA concentration in uivo suggest that clenbuterol acts through indirect mechanisms in vivo and not directly with the adipose tissue /J-adren- ergic receptor to increase CAMP concentration and subsequently activate hormone sensitive lipase. One possible indirect mechanism is the release of endogen- ous catecholamines that in turn stimulate adipose tissue lipolysis. The objective of this study was to test *A preliminary report of this data was presented; Fedn Proc. Fedn Am. Sots exp. Biol. 45,613 (1986) abstract no. 2666. §Present address: Animal Science, Oregon State University, Corvallis, OR 97331. //Author to whom reprint requests should be addressed. TMention of trade name, proprietary products or specific equipment does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that may also be suitable. this hypothesis by measuring the effect of infused clenbuterol on plasma FFA concentration in pigs previously treated with reserpine to deplete endogen- ous catecholamine stores. MATERIALS AND METHODS Pigs (Sus domesticus) were crossbred (l/4 Landrace, l/4 Laree White. 114 Chester White. 114 Yorkshire) females and castrated males. Because of the young age (8813 weeks), sex was not considered a factor in the experiments. Pigs were fed ad libitum a corn-soybean meal based diet containing about 18% protein from weaning at 4 weeks of age. At the time of experiment, pigs weighed 20-36 kg. Six pigs were used in each of four treatment groups (24 total pigs). The four treatment groups consisted of pigs injected intraperitoneally with vehicle at - 24 hr (group A), with reserpine at - 24 hr (group B), with reserpine at -48 hr and -24 hr (group C) and because reserpine caused gastrointestinal emptying, pigs fasted for 24 hr (group &not injected). Reserpine was dissolved at 4mg/ml in a vehicle of 3% acetic acid-IO% propylene glycol-10% ethanol (Curtis and Rogler, 1970) was sterilized by filtration (0.45 pm filter) and was injected at 1 mg/kg body wt per injection. Within each treatment group, three pigs were infused with clenbuterol dissolved in saline (0.9% NaCl containing 40 pg ascorbate/ml) whereas the other three pigs were infused with saline. One pig from the -24 hr reserpine group (group B) did not respond to clenbuterol and was not included in the data set and one pig from the -48, -24 hr reserpine group (group C) died before infusion, On the day of infusion of saline or clenbuterol, pigs were anesthetized with Na thiamylal and maintained with 67% N,O: 33% 0, plus intravenous (ear vein) succinyl choline. The pigs were artificially respired. Saline was infused (ear vein) until the initiation of clenbuterol infusion (zero time) or about 60min from time of anesthesia. Saline pigs were infused continuously with saline. Clenbuterol was serially diluted with saline and infused continuouslv. beginning at zero time, and at sequentially increased cdncen~ation~ of 0.01, 0.033, 0.1, 0.33, 1.0 and 3.33pg/kg body wt min with each concentration infused for 45 min. Arterial blood 619