Insulin-like growth factor I in the dog: a study in different dog breeds and in dogs with growth hormone elevation

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Abstract
A radioimmunoassay (RIA) devised for the measurement of human insulin-like growth factor I (IGF I) was employed for the measurement of canine IGF I. Canine IGF I was extracted from plasma specimens by gel chromatography. Columns were eluted with 1 M acetic acid and the fractions representing the 55 to 85% bed volume were pooled, lyophilized and reconstituted with assay buffer. Serial dilutions of canine IGF I from both normal and acromegalic dogs when added to the RIA system gave a similar displacement pattern of human [125I]IGF I as the one obtained by the addition of unlabeled human IGF I. The dose-response curve obtained by canine IGF I paralleled the one obtained by human IGF I. Logit-log transformation and least squares fitting resulted in straight line fitting of the standard curve between 0.039 and 5 ng IGF I added per tube. The within-assay coefficient of variation (CV) was 16.7% and the between-assay CV was 21.8%. Plasma IGF I concentrations in normal dogs appeared to be a function of body size. The concentrations were 36 .+-. 27 ng/ml in Cocker Spaniels, 87 .+-. 33 ng/ml in Beagles, 117 .+-. 34 ng/ml in Keeshonds and 280 .+-. 23 ng/ml in German Shepherds (mean .+-. SEM [standard error of the mean]). The mean IGF I level in a group of dogs with growth hormone (GH) elevation was 700 .+-. 90 ng/ml. Though this group of dogs comprised both small and large dogs, the mean IGF I level significantly differed from the one found in German Shepherds, the largest breed studied (P < 0.01). IGF I levels in dogs with GH elevation were similarly elevated in both dogs exhibiting acromegaly and dogs exhibiting GH-diabetes, but no signs of acromegaly. In dogs with GH elevation, drop in GH levels was associated with a significant drop in IGF I levels.