Assessment of endogenous growth hormone pulsatility in gelded yearling horses using deconvolution analysis

Hypothesis/objectives: Defining normal Growth Hormone (GH) secretory dynamics in the horse is necessary to understand altered GH dynamics related to issues like welfare and disease. Animals and methods: Twelve healthy yearlings and two mature Standardbreds were used to quantify GH secretion. Endogenous GH half-life was determined after administration of 1.0 microg/kg BW GH releasing hormone (GHRH). Exogenous GH half-life was determined after administration of 20 microg/kg BW recombinant equine GH (reGH) with and without suppression of endogenous GH secretion by somatostatin infusion (50 microg/m(2)/h). Pulse detection algorithm (Cluster) as well as deconvolution analysis was used to quantify GH secretory dynamics based on GH concentration-time series sampled every 5 min from 22:00 till 06:00 h. In addition, reproducibility, impact of sampling frequency and influence of altering initial GH half-life on parameter estimates were studied. Results: Mean endogenous GH half-life of 17.7 +/- 4.4 (SD) min and mean exogenous half-life of 26.0 +/- 2.9 min were found. The mean number of GH secretion peaks in 8 h was 12 +/- 3.2. Ninety-nine percent of the total amount of GH secreted occurred in pulses, basal secretion was 0.012 +/- 0.014 microg/L/min and half-life was 8.9 +/- 2.6 min. Compared with a 5-min sampling frequency, 20- and 30-min sampling underestimated the number of secretory events by 45% and 100%, respectively. Conclusions: The deconvolution model used was valid to GH time series in Standardbreds. As in man, the equine pituitary gland secretes GH in volleys consisting of multiple secretory bursts, without measurable intervening tonic secretion. The required GH sampling frequency for the horse should be around 3 min. Clinical relevance: Defining normal GH secretory dynamics in the horse will make it possible to detect alterations in the GH axis due to pathophysiologic mechanisms as well as abuse of reGH.