Relaxin-like factor (RLF), now mainly known as insulin-like factor 3 (INSL3), is essential for testis descent during fetal development; however, its function in the adult testis is still being elucidated. As a major step toward understanding the as-yet-unknown function of INSL3 in boars, this study aimed to develop a time-resolved fluoroimmunoassay for boar INSL3, characterize the dynamics of INSL3 expression during development, and demonstrate the expression of the INSL3 hormone–receptor system in the testis. All samples were collected from Duroc boars. The sensitivity of the assay system established was 8.2 pg/well (164 pg/ml), and no cross-reactivity with other hormones, such as porcine relaxin, was observed. Circulating INSL3 was shown to increase progressively during development. INSL3 secreted from the Leydig cells was released not only into the blood circulation but also into the interstitial and seminiferous compartments in sufficient concentrations. A testicular fractionation study revealed that its receptor RXFP2 transcripts were expressed mainly in testicular germ cells. In addition, INSL3 bound to the germ cell membranes in a hormone-specific and saturable manner. These results reveal that INSL3 secreted into the interstitial compartment from the Leydig cells is transported into the seminiferous compartments, where its receptor RXFP2 is expressed mainly in the germ cells to which INSL3 binds, suggesting that INSL3 functions as a paracrine factor on seminiferous germ cells.
Itaru Minagawa, Dai Sagata, Ali Mohammed Pitia, Hiroshi Kohriki, Masatoshi Shibata, Hiroshi Sasada, Yoshihisa Hasegawa and Tetsuya Kohsaka
Hideyuki Takahashi, Yohei Kurose, Muneyuki Sakaida, Yoshihiro Suzuki, Shigeki Kobayashi, Toshihisa Sugino, Masayasu Kojima, Kenji Kangawa, Yoshihisa Hasegawa and Yoshiaki Terashima
The present study was conducted to investigate roles of ghrelin in glucose-induced insulin secretion in fasting- and meal-fed state in sheep. Castrated Suffolk rams were fed a maintenance diet of alfalfa hay cubes once a day. Hyperglycemic clamp (HGC) was carried out to examine glucose-induced insulin response from 48 to 53 h (fasting state) and from 3 to 8 h (meal-fed state) after feeding in Experiment 1 and 2 respectively. Total dose of 70 nmol/kg body weight of d-Lys3-GHRP6, a GH secretagogue receptor 1a (GHS-R1a) antagonist, was intravenously administered at 0, 60, and 120 min after the commencement of HGC. In the fasting state, the ghrelin antagonist significantly (P < 0.01) enhanced glucose-induced insulin secretion. In the meal-fed state, i.v. administration of synthetic ovine ghrelin (0.04 μ g/kg body weight per min during HGC) significantly (P < 0.05) enhanced glucose-induced insulin secretion. d-Lys3-GHRP6 treatment suppressed ghrelin-induced enhancement of the insulin secretion. In conclusion, ghrelin has an inhibitory and stimulatory role in glucose-induced insulin secretion via GHS-R1a in fasting- and meal-fed state respectively.