Clenbuterol analogues show no relationship between Beta2-receptor binding in bovine skeletal muscle and urinary nitrogen excretion in female rats.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 18 CLENBUTEROL ANALOGUES SHOW NO RELATIONSHIP B ETWEEN BETA2-RECEPTOR BINDING IN BOVINE SKELETAL MUSCLE AND URINARY NITROGEN EXCRETION IN FEMALE RATS G-G. PEG*, D-B. LINDSAY+, M.L. MA !ITXEWS* and M-N, SIL&ENCE* Clenbuterol causes fat breakdown, and directs nutrients towards muscle growth, The drug i s known to have a i n a larger, leaner carcass, stimulatory effect on B,-adrenoreceptors. However, it is debated whether these receptors mediate the anabolic effects of clenbuterol in muscle (Maltin et al. We report on the relationship between B, -receptor affinity in bovine 1987). skeletal muscle, and anabolic potency in rats (as reflected by a urinary nitrogen (N) bioassay), for clenbuterol and five structural analogues. resulting Cell membrane fragments containing B, -receptors were prepared from the longissimus dorsi muscle of a Friesian bullock using the method described by Jones et al. 1979. 1251-Iodocyanopindolol (ICYP) was used to validate the Breceptor preparation (Motulsky and Insel 1982). 'Receptors were incubated in the presence of ICYP and with increasing concentrations of B-agonist. Binding displacement curves were analysed using iterative non-linear regression to yield equilibrium dissociation constants (K,) for each drug. Changes in body protein metabolism in clenbuterol-treated rats are accounted for solely by changes in skeletal muscle (Maltin et al. 1987) and are reflected by a fall in urinary (N) excretion. Female Wistar rats were housed individually in metabolism cages for 48 h, and fed a standard powdered laboratory diet mixed with varying doses of drug. Urine was collected over the 48 h period and analysed for N concentration. Six treated rats and 6 controls were used to test each dose of drug. Dose-response curves comprised between 5 and 12 points. The maximun decrease in urinary N achieved was 74%. Estimated values (37% fall in urinary N output) are presented. ED5o Table 1 Comparison between B,-receptor affinity (K,,mmol/l) in bovine skeletal muscle and relative potency in lowering urinary N output (ED,,ug/kg SW) over 48h in female rats using 5 clenbuterol analogues The results show no direct relationship between B,- receptor binding affinity and the ability of the test compounds to reduce urinary nitrogen N. If the latter measurement reflects the anabolic potency of clenbuterol analogues as believed, then these data support the hypothesis that the anabolic effects of clenbuterol-like compounds in skeletal muscle are not mediated directly by the stimulation of B,-receptors. The high relative potency of the keto-derivative, may provide a lead in future research to discover the true mechanism of action of clenbuterol. JONES, L-R., BESCH, H-R. Jr., FLEMING, J-W., McCONNAUGHEY, MM. and WATANABE, A.M. (1979). J. Biol. Chem. 254: 530. MALTIN, C.A., DELDAY, MI., HAY, SM., SMITH, F.G. and REEDS, P.J. (1987). -Biosci. Rep. 7: 51. MOTULSKY, H.J. and INSEL, P.A. (1982). New. Eng. J. Med. 307: 18. + CSIRO Div Tropical Animal Production, P-0, Box 5545, Rockhampton, Qld. 4702. 537