PARADOXICAL INCREASES OF CIRCULATING NONESTERIFIED FATTY-ACIDS IN SOMATOTROPIN TREATED CATTLE UNDERGOING MILD DISTURBANCES

Effects of various doses of bovine somatotropin (bST) on plasma concen trations of nonesterified fatty acids (NEFA) were studied in a 14-d La tin square with six Holstein heifers. Animals were given daily injecti ons of excipient or bST at 12:00 p.m. and fed twice daily at 7:00 a.m. and 7:00 p.m. On Day 14, plasma NEFA remained low through the day exc ept around the 7:00 p.m. feeding when they were substantially elevated . The elevation was significantly greater in bST-treated animals and c orrespond to the excitement of the animals in anticipation of the even ing feeding To further investigate this phenomenon, a second experimen t was conducted in which nine growing Holstein steers were fed hourly and received either daily intramuscular (i.m.) injection of excipient or bST (120 mg/kg BW) for 15 d in a crossover design. Daily profiles o f NEFA were obtained under undisturbed conditions or concurrently with intensive handling. Although no elevations could be detected in any c ase in control animals, bST caused a substantial rise in NEFA concentr ation only when animals were subjected to intensive handling. This sug gested that NEFA peaks noted in bST-treated heifers in the first exper iment resulted from increased ability of adipose tissue to respond to adrenergic stimulation associated with the anticipation of feeding. Co nsistent with this hypothesis. plasma NEFA concentrations in bST-treat ed steers were increased to a greater extent during a challenge involv ing i.v. injection of epinephrine. This amplification of adipose tissu e response by bST must be considered when conducting intensive studies . Even the minimal excitment associated with blood sampling can confou nd the results regarding lipid mobilization, and this may have contrib uted to the notion that ST is a lipolytic hormone. (C) Elsevier Scienc e Inc. 1997.