Purified Ovine Growth Hormone Releasing Factor: Effects on Growth Hormone Secretion and Pituitary Cyclic Nucleotide Accumulation

Abstract

Growth hormone releasing factor (GRF) was purified from ovine hypothalami on a Sephadex G-25 column. Three fractions from one of the major protein peaks induced the release of radioimmunoassayable growth hormone (GH) from rat pituitaries incubated in vitro, but did not induce any appreciable release of other anterior pituitary hormones. The kinetics of GH release from rat pituitaries included an initial phase with an exponential release of hormones, a subsequent phase with a constant rate of release, and a terminal declining rate of release seen with high GRF doses. The minimally effective dose of this GRF preparation which stimulated GH secretion in vitro contained 1.5 µg of protein. No change in pituitary cyclic AMP was observed following the exposure of the explants to 3 or 6 µg of GRF for 10 min, but proportional increases in pituitary cyclic GMP were observed at this time period. Although the synthetic decapeptide, Val-His-Leu-Ser-Ala-Glu-Glu-Lys-Glu-Ala, has been shown to deplete bioassayable pituitary GH, it did not stimulate the release of radioimmunoassayable GH nor did it substantially elevate the pituitary cyclic AMP content at the dose levels and time intervals tested. The synthetic decapeptide did not alter pituitary cyclic GMP content after 10 or 60 min of incubation. These observations further substantiate that GRF capable of releasing radioimmunoassayable GH can be isolated from hypothalamic tissue. Preliminary studies of the mechanism of action of GRF indicate that cyclic GMP rather than cyclic AMP plays a central role in mediating the neurohormonal control of GH release.