GROWTH HORMONE, INSULIN, PROLACTIN, GLUCOSE AND MONONUCLEAR LEUKOCYTE INSULIN BINDING RESPONSES TO CLONIDINE IN RAM LAMBS

Concentrations of growth hormone, insulin, prolactin and glucose were measured in blood taken frequently, over a 12-h period on three occasions (phases 1, 2 and 3) during a 72-d growth trial, from Suffolk (SU) and Strain 2 (ST) (a synthetic Finnish Landrace, Suffolk, Shropshire line) lambs treated continuously with either sterile H 2 O (control) or a low dose (5–8 μg kg −1 d −1 ) of clonidine (LC). Prior to slaughter the specific binding of insulin to mononuclear leukocytes (MNL) was measured in SU and ST control and LC lambs and in SU and ST lambs treated for 72 d with a high dose (HC) of clonidine (24–41 μg kg −1 d −1 ). Clonidine was administered using osmotic pumps implanted subcutaneously at 28-d intervals. Overall mean and baseline concentration of serum prolactin increased in SU and decreased in ST lambs in response to LC. Serum insulin increased over time in SU control lambs and decreased over time in ST control lambs. Serum insulin increased with LC treatment but the effect appeared earlier in ST lambs. Growth hormone (GH) profile characteristics were similar for control ST and SU lambs in phase 1 and GH overall mean, baseline and peak frequency decreased over time. The decrease was most pronounced in ST lambs. The low dose of clonidine reduced GH concentration and this effect was most pronounced in phases 2 and 3 for SU lambs and in phases 1 and 2 for ST lambs. Insulin specific binding to MNL at low insulin concentrations was threefold greater in SU than ST control lambs. Treatment with LC and HC depressed insulin specific binding to MNL at low insulin concentrations in SU lambs but the opposite effect was observed in ST lambs. The results suggest that the breed effect on endocrine responses to clonidine may underlie the previously reported effect of breed on carcass responses to clonidine. Key words: Clonidine, insulin, growth hormone, prolactin, receptors