GROWTH-HORMONE SECRETION IN THE HORSE - UNUSUAL PATTERN AT BIRTH AND PULSATILE SECRETION THROUGH TO MATURITY

A heterologous radioimmunoassay was developed and validated for the me asurement of horse GH in plasma. It utilized recombinant-derived bovin e GH as the radiolabelled ligand, a guinea-pig anti-porcine GH serum a s first antibody and pituitary-derived horse GH as standard. Cross-rea ctivites were high with all of the pituitary and recombinant-derived G H preparations tested (49-140%) and very low (<0.3%) with horse FSH, L H and prolactin. A synthetic analogue of GH-releasing factor(1-29) sti mulated the expected pattern of GH release in foals. Plasma GH concent rations in foals were low at birth (<20 ng/ml) but rose sharply to a d efinite 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 lev els again by 60-100 min post partum. GH secretion was clearly pulsatil e 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 i ntervals for 7-8 h. Basal levels and pulse amplitudes were higher in f oals than in adults and pulse frequency was higher in stallions than i n mares (3-5 pulses/8 h vs 1-2 pulses/8 h). Pulsatile secretion was fu rther characterized in one mare by simultaneous sampling of jugular ve in and pituitary cavernous sinus blood. Peak GH concentrations in cave rnous sinus blood draining the pituitary gland were more than tenfold higher than the corresponding peak concentrations in peripheral circul ation. The patterns of GH release in the horse therefore appear to be similar to those reported in other species with the exception of the l ow values at birth followed by the dramatic rise and fall in concentra tions during the first hour post partum.