Short communication: growth hormone response to somatostatin-28 and growth hormone-releasing factor in dairy heifers

In a attempt to investigate the role fo somatostatin-28 (SRIF-28) in modulating growth hormone (GH)-releasing factor (GRF)-induced GH release in dairy cattle, we assigned Holstein heifer calves (n = 12) to receive 0, 10, and 20 microg of SRIF-28/100 kg of BW (0, 10, 20) in conjunction with administration of 3 microg of SRIF-28/100 kg ofBW. Administration of 10 or 20 microg of SRIF-28 reduced GRF-induced GH release, because areas under the response curves during 30 min after GRF administration were less when cows received those dosages compared with receiving 0. Some evidence of a dose response was observed because serum GH concentrations increased from 15 to 20 min after GRF administration when animals received 10 microg but continued to decrease when animals received 20 microg of SRIF-28. We conclude that, in contrast to studies with SRIF-14, pituitary GH release is very sensitive to inhibitory effects of SRIF-28 when used in as little as a fivefold molar excess. Additional experimentation is necessary to determine whether equimolar concentrations of SRIF-28 are effective in altering GRF-induced GH release. Further, these results give rise to speculation that SRIF may be involved in physiological mechanisms mediating well-documented genetic influences on GH concentrations in dairy cattle.