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A heterologous radioimmunoassay was developed and validated for the measurement of horse GH in plasma. It utilized recombinant-derived bovine GH as the radiolabelled ligand, a guinea-pig anti-porcine GH serum as first antibody and pituitary-derived horse GH as standard. Cross-reactivites were high with all of the pituitary and recombinant-derived GH preparations tested (49–140%) and very low (<0·3%) with horse FSH, LH and prolactin. A synthetic analogue of GH-releasing factor(1–29) stimulated the expected pattern of GH release in foals.

Plasma GH concentrations in foals were low at birth (<20 ng/ml) but rose sharply to a definite and, in most cases, very large peak (18–195 ng/ml) during the first 30–40 min post partum, followed by a steady decline to basal levels again by 60–100 min post partum. GH secretion was clearly pulsatile in all older foals tested (2 weeks, 1 month and 4 months of age) and in six adults (three mares and three stallions), all bled at 15-min intervals for 7–8 h. Basal levels and pulse amplitudes were higher in foals than in adults and pulse frequency was higher in stallions than in mares (3–5 pulses/8 h vs 1–2 pulses/8 h). Pulsatile secretion was further characterized in one mare by simultaneous sampling of jugular vein and pituitary cavernous sinus blood. Peak GH concentrations in cavernous sinus blood draining the pituitary gland were more than tenfold higher than the corresponding peak concentrations in peripheral circulation. The patterns of GH release in the horse therefore appear to be similar to those reported in other species with the exception of the low values at birth followed by the dramatic rise and fall in concentrations during the first hour post partum.

Journal of Endocrinology (1993) 138, 81–89

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