Growth hormone responses to growth hormone-releasing hormone and clonidine in dogs with Cushing's syndrome

Pulsatile secretion pattern of growth hormone in dogs with pituitary-dependent hyperadrenocorticism

The amplitude and frequency of growth hormone (GH) secretory pulses are influenced by a variety of hormonal signals, among which glucocorticoids play an important role. The aim of this study was to investigate the pulsatile secretion pattern of GH in dogs in which the endogenous secretion of glucocorticoids is persistently elevated, i.e. in dogs with pituitary-dependent hyperadrenocorticism (PDH). Blood samples for the determination of the pulsatile secretion pattern of GH were collected at 10-min interval between 08:00 and 14:00 h in 16 dogs with PDH and in 6 healthy control dogs of comparable age. The pulsatile secretion patterns of GH were analyzed using the Pulsar program. GH was secreted in a pulsatile fashion in both dogs with PDH and control dogs. There was no statistical difference between the mean (+/-S.E.M.) basal GH level in dogs with PDH (0.7+/-0.1 microg/l) and the control dogs (0.6+/-0.1 microg/l). The mean area under the curve (AUC) for GH above the zero-level in dogs with PDH (4.6+/-0.6 microg/l per 6 h) was significantly lower than that in the control dogs (7.3+/-1.0 microg/l per 6 h). Likewise, the mean AUC for GH above the base-level in dogs with PDH (0.6+/-0.1 microg/l per 6 h) was significantly lower than that in the control dogs (3.7+/-1.0 microg/l per 6 h). The median GH pulse frequency in the dogs with PDH (2 pulses/6 h, range 0-7 pulses/6 h) was significantly lower (P = 0.04) than that (5 pulses/6 h, range 3-9 pulses/6 h) in the control group. The results of this study demonstrate that PDH in dogs is associated with less GH secreted in pulses than in control dogs, whereas the basal plasma GH concentrations were similarly low in both groups. It is discussed that the impaired pulsatile GH secretion in dogs with PDH is the result of alterations in function of pituitary somatotrophs and changes in supra-pituitary regulation.

An improved radioimmunoassay for the determination of canine growth hormone based on commercially available reagents

Determination of canine growth hormone (cGH) is of importance when evaluating the growth hormone status in dogs, for instance when diagnosing pituitary dwarfism and acromegaly. At present, analysis for the cGH content in canine blood has mainly been restricted to specialized laboratories which have had the capacity to develop suitable methods. We therefore formulated the objective of the present study to be the development of a generally accessible method utilizing commercially available reagents. We describe a radioimmunoassay (RIA) for the determination of cGH which was based on reagents developed for the determination of porcine growth hormone (pGH), thereby drawing benefit from the similarity between growth hormones from the two species. The RIA method was characterized by satisfying precision and accuracy parameters: intra- and inter-assay coefficients of variation (CV%) were 4-27% and 4-20%, respectively, depending on the cGH concentration of the samples. Recovery of added cGH and parallelism between standard curve and sample dilutions were acceptable, thus indicating identity between the calibrator system and the material under investigation (i.e. plasma samples). When measuring individual plasma samples or samples taken during clonidin response tests from normal dogs, dogs suffering from pituitary dwarfism and from dogs suffering from acromegaly, the results were comparable with previous findings.