Molecular cloning and tissue distribution of the ovine somatostatin receptor subtype 5: osst5.

The sheep is a valuable model to study growth hormone (GH) neuroregulation since its GH secretion pattern is close to that in humans and an integrated physiological approach is possible in this species. Somatostatin receptor subtype 5 (sst5) appears to be important in GH regulation but the ovine sst5 gene (osst5) has not yet been cloned. We report here the cloning of sst5 in that species. We screened a cDNA sheep library and isolated a 1.24 kb cDNA, which includes the whole coding region of osst5. The predicted protein consists of 367 amino acids exhibiting a putative seven transmembrane domain topology typical of G protein-coupled receptors. Nucleotide sequence comparisons with that of other species sst5 showed that osst5 displays 83.8, 81 and 79.7% homology with human, rat, and mice sst5, respectively. Southern blot analysis of ovine cortex DNA demonstrated that osst5 is encoded by a single gene. Osst5 transiently expressed in Chinese Hamster ovary (CHO) cells exhibit a high affinity for somatostatin-14. Reverse transcriptase-polymerase chain reaction (RT-PCR) studies demonstrated that osst5 mRNAs are present in pituitary, cortex, hypothalamus, hippocampus, colon and adrenal gland. The cloning of osst5 should provide a useful tool to study the mechanisms through which somatostatin inhibits hormone secretion in the sheep.

Hypothalamic mediated action of free fatty acid on growth hormone secretion in sheep.

Experimental data suggest that elevated FFA levels play a leading role in the impaired GH secretion in obesity and may therefore contribute to the maintenance of overweight. GH has a direct lipolytic effect on adipose tissue; in turn, FFA elevation markedly reduces GH secretion. This suggests the existence of a classical endocrine feedback loop between FFA and GH secretion. However, the FFA mechanism of action is not yet understood. The involvement of somatostatin (SRIH) is controversial, and in vitro experiments suggest a direct effect of FFA on the pituitary. In sheep it is possible to collect hypophysial portal blood and quantify SRIH secretion in hypophysial portal blood under physiological conscious and unstressed conditions. In this study we determined the effects of FFA (Intralipid and heparin) infusion on peripheral GH and portal SRIH levels in intact rams chronically implanted with perihypophysial cannula and in rams actively immunized against SRIH to further determine SRIH-mediated FFA effects on GH axis. Immediately after initiation of Intralipid infusion, we observed a marked increase in the FFA concentration (2160 +/- 200 vs. 295 +/- 28 nmol/ml; P < 0.01) as well as a significant decrease in basal GH secretion (1.8 +/- 0.1 vs. 2.5 +/- 0.3 ng/ml; P < 0.05) and a drastic reduction of the GH response to i.v. GH-releasing hormone injection (4.8 +/- 0.7 ng/ml in FFA group vs. 35.8 +/- 9.7 ng/ml in saline group; P < 0.01). No change in plasma insulin-like growth factor I levels was observed. During the first 2 h of infusion, the GH decrease observed was concomitant with a significant increase in portal SRIH levels (22.1 +/- .2 vs. 13 +/- 1.6 pg/ml; P < 0.01). In rams actively immunized against SRIH, the effect of FFA on basal GH secretion was biphasic. During the first 90 min of infusion, the decrease in GH induced by FFA was significantly blunted in rams actively immunized against SRIH (57 +/- 9% for immunized rams vs. 23.5 +/- 2.5% for control rams). This corresponds to the period of increased SRIH portal levels. After this first 90-min period, no difference was seen between control and immunized rams. Our results show that FFA exert their inhibitory action on the GH axis at both pituitary and hypothalamic levels, the latter mainly during the first 90 min, through increased SRIH secretion.