In growing pigs, blocking somatostatin with a vaccine raised their natural growth hormone (GH) levels and made them respond more consistently to a GH‑releasing peptide (GRF‑1‑29). Fasting also boosted GH, likely by lowering somatostatin and/or increasing GRF signals.

Three experiments were conducted with growing pigs actively immunized against a protein-conjugated somatostatin (SRIF) in Freund's adjuvant. In the first experiment, blood from 24-week-old pigs (seven immunized and eight control) was sampled at 20-min intervals for 6 h to evaluate basal GH concentrations. The animals were then injected iv with porcine GH-releasing factor (GRF)-(1-29)NH2 (10 micrograms/kg). Before GRF stimulation, immunized animals had higher (P less than 0.05) baseline mean GH levels (2.6 vs. 1.4 ng/ml) and area under the GH curve (AUC; 1632 vs. 779 ng/min.ml); they also had higher AUC after GRF administration (4268 vs. 1972 ng/min.ml). In a second experiment eight immunized and eight control pigs were injected iv four times at 90-min intervals with porcine GRF (10 micrograms/kg). Control pigs responding to the first injection did not respond to the second and third, and those responding to the second did not respond to the first, third, and fourth, indicating a decreased responsiveness that was longer than 3 h post-GRF response in control pigs. SRIF-immunized pigs had a more consistent GH response to the GRF injections. Overall, a reduced response was observed after the second and the fourth injections in immunized pigs, although five and six of eight animals had a GH peak response higher than 10 ng/ml during these periods. In a third experiment, effects of fasting, GRF, and SRIF immunization were studied. Immunization and fasting had their own positive effects on serum GH levels. Immunization increased baseline mean GH levels (5.0 vs. 2.2 ng/ml) and total AUC before (2318 vs. 1073 ng/min.ml) and after (1886 vs. 910 ng/min.ml) iv GRF stimulation (10 micrograms/kg) compared to controls. Fasting increased the mean baseline GH level (4.5 vs. 2.6 ng/ml), and it increased AUC before exogenous GRF stimulation (2009 vs. 1392 ng/min.ml). In conclusion, SRIF in pigs seems to be a potent GH-governing factor, since, when inhibited, baseline mean GH levels increase, and a consistent response to GRF is observed. Fasting could increase GH concentrations by different ways: decreasing SRIF release and increasing GRF release or modifying the sensitivity of the somatotrophs to both factors.