Evidence that bovine growth hormone treatment increases the rate of extrathyroidal 5'-monodeiodinase activity in cattle.

This study examined the influence of bovine growth hormone (bGH) on liver and kidney thyroxine-5'-monodeiodinase activity (TMA) in growing beef cattle. In a preliminary trial (trial 1), tissue samples were obtained at slaughter from two placebo-injected and two bGH-injected (29.2 IU/day for 14 days before slaughter) Hereford heifers (398 kg avg slaughter wt), with one heifer on each treatment fed at either 1.0 or 2.0 times maintenance energy requirement. In a second trial, tissue and plasma samples were obtained from six placebo-injected and six bGH-injected (29.2 IU/day for 19 days) Hereford steers (331 kg avg slaughter wt) fed at 1.8 times maintenance energy requirement. In a third trial, liver tissue and plasma samples were obtained from 14 Angus X Hereford steers (270 kg avg wt) fed either an 8% protein, 1.96 Mcal-metabolizable energy/kg diet (8 steers) or a 14% protein, 2.67 Mcal-metabolizable energy/kg diet (6 steers) in association with acute administration of bGH. Half the steers in each group were given two injections per steer of either placebo or 37.8 IU bGH at 24-hr intervals and slaughtered 24 hr after the second injection. Units of tissue TMA in all trials were measured at slaughter; one unit defined as 1 ng T3 generated/hr/mg protein during incubation with T4 (1.3 fM). In trial 1, TMA in liver and kidney was 2.86 and 1.21 times greater, respectively, for bGH than for placebo treatments. In trial 2, units of TMA in liver homogenates were higher (P less than .05) for bGH (4.14) than for placebo (2.83) treatments and higher (P less than .02) in liver microsomal preparations for bGH (32.7) than for placebo (27.3) treatments. Units of TMA in kidney homogenates were also higher (P less than .10) for bGH (1.48) than for placebo (.87) treatments and higher (P less than .02) in kidney microsomal preparations for bGH (23.0) than for placebo (16.4) treatments. Following acute injection of bGH (trial 3), units of TMA in liver homogenates were higher (P less than .01) for bGH (2.5) than for placebo (1.8) treatments. Plasma T4 (70.0 vs 73.5 ng/ml) and T3 (1.08 vs 1.18 ng/ml) concentrations appeared slightly higher in bGH-treated steers (trial 2) and increased by 20 and .5 ng/ml, respectively, (P less than .01) within 6 to 12 h after acute injection of bGH (trial 3). Concurrently, plasma TSH concentration increased following acute GH injection (trial 3).(ABSTRACT TRUNCATED AT 400 WORDS)