| The current studies aimed to determine the influence of GHRP-2 administration on the growth performance,nitrogenous metabolism,GH secretion of pig pituitary cells and the interaction of GHRP-2 with GHRH, SS on pituitary cells. The potential cell signal mechanism was also investigated. These studies provided theoretic basis for the development or utilization of GHRPs in practical condition.Trial 1: Swine pituitary cells were isolated from 1-3 days newly-born piglets and incubated into monolayer by plating culture method. Three incubation trials were conducted. In the first incubation, the isolated cells were treated by different doses of GHRP-2 including 0, 10-5, 10-6, 10-7, 10-8, 10-9,10-10,10-11 and 10-12 mol/L for 2 hours, respectively. In the second incubation, pituitary cells were treated by 10-6 Mol GHRP-2 for 5 to 180 minutes. In the last one, cells were continually treated with 10-6 Mol GHRP-2 for 6 times of one hour each. The results showed that 10-12～10-5 Mol GHRP-2 was able to stimulate the GH secretion of pituitary cells (P＜0.01), but no significant response.was observed for 10-12～10-8 Mol GHRP-2 group(P＞0.05). Compared to 10-12～10-8 Mol GHRP-2 treatment,GH secretion has a much more response to 10-7～10-5 Mol GHRP-2 (P＜0.01). 10-6 Mol GHRP-2 was the optimum level for GH secretion. Secretion peak of GH was reached within 5 minutes. GHRP-2 could promote the GH secretion at the first three treatments. GH secretion was reduced, however, for over four-time treatment of GHRP-2. It is conluded that GHRP-2 could promote the GH secretion of pituitary cells but it's depend on the dosage, time and frequency of the treatment.Trial 2: Swine pituitary cells were isolated in vitro and incubated into monolayer pituitary cells in the culture plate. The pituitary cells were then treated with GHRP-2,GHRH,SS,GHRP-2+ GHRH,SS+ GHRP-2,GHRH+SS and GHRP-2,GHRH,PMA,PMA + GHRP-2,GHRP-2+ GHRH for 2 hours,respectively; Pituitary cells were treated with GHRP-2,GHRH,GHRP-2 + GHRH for 2 hours prior to or after the incubation of cells for 24 hours with 10-6M PMA to delete the intracellular PKC. The results indicated that GHRP-2 and GHRH, independently or synergistically increased the GH secretion of the pituitary cells (P＜0.01), SS could absolutely inhibit the GH response to GHRH (P＜0.01), but no influence on the effects of GHRP-2. Synergistically,GHRP-2 and GHRH could promote the GH secretionGHRP-2,GHRH,PMA could stimulate the pituitary cells to secrete GH, and GH secretion has much more response to GHRP-2. While the interaction of GHRP, GHRH and PMA could extremely improve the secretion of GH (P＜0.01). The promoting effect of GHRH to GH secretion was not changed by PMA pre-treatment, but this response was significantly decreased (P＜0.01) compared with the PKC deletion group by PMA pretreatment. The synergistic effect of GHRP-2+GHRH was also lower than control group, however, the positive synergistic effect of GHRP-2+GHRH on GH secretion was also observed in PMA fortification group. It's concluded that GHRP-2 and GHRH could affect GH secretion of swine pituitary cells by different signal ways.Trial 3: Castrated male pigs at 25kgBW were selected and fixed with jugular vein catheter. 8 pigs were allotted to two groups, 4 pigs per group. In the first group, pigs were i.m injected GHRP-2 at 30μg/kgBW. 5ml blood was collected at the 0,15,30,45,60,90,120,180min and 6h,9h,12h aider the injection,respectively. The serum GH was determined by radioimmunoassay. The result indicated that the GH level in the treatment group was significantly increased at 30～90min (p＜0.01). The GH secretion peaked at 60 min and was comparable to control group within 120 min. Conclusion: GHRP-2 at 30μg/kgBW could promote the GH secretion; the mechanism for promoting effect of GHRP-2 to GH secretion should be further studied.Trial 4: Twenty DLY (Duroc×Landrace×York) pigs at (33±2) kg BW were chosen arid allotted to four groups of five pigs per group, and were i.m injected with GHRP-2 at 0,3,9,27μg/kgBW for 28 days,respectively. Metabolism experiment was conducted for 4 days at the third week. At the fourth week, the live back fat and longissimus muscle thickness were determined. The results indicated that in the first two weeks, the average daily gain of BW was dose-dependent. In the third week of continuous treatment, growth inhibition was observed, and the average daily gain returned to the control level at the fourth week, and the feed intake was not different among groups. The feed nitrogen digestibility of the four groups at the third week was 82.92%, 81.29%, 82.3% and 85.15%, respectively; the metabolic rate was 52.59%, 51.8%, 53.20% and 56.67%, respectively. Administration of GHRP-2 tended to reduce back fat thickness and increase longissimus muscle thielcness. It is concluded that average daily gain were improved, the F/G was lowered, but feed intake was not affected within two weeks of exogenous GHRP-2 administration (27μg/kgBW i.m). Continuous GHRP-2 administration caused growth inhibition in the third week. The back fat thickness was reduced and the Iongissimus muscle thickness was increased after the 28d continuous treatment, the mechanism for the growth promoting effect and the growth inhibition by continuous GHRP-2 treatment should be further studied.Pituitary cell incubation showed that GHRP-2 could stimulate GH secretion at dose and frequency dependence. GHRP-2 has a different signal pathway with GHRH,but been related to PKC. Injection of i.m GHRP-2 at 27μg/Kg BW could improve growth performance and feed conversion rate.Continously exogenous GHRP-2 administration inhibited GH secretion and growth performance,the potential mechanism need further investigation. |
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