ABSTRACT

The effects of hypophysectomy and hormonal replacement therapy on insulin-like growth factor-I (IGF-I) mRNA in rat adipose tissue and adipocytes were studied. The effects of GH and IGF-I in vitro on IGF-I mRNA and IGF-I production were also studied in cultured rat adipocytes. Male rats were hypophysectomized at about 50 days of age and given replacement therapy with cortisol (400 μg/kg per day) and thyroxine (10 μg/kg per day). GH was given as a single i.v. or s.c. injection and also as a continuous s.c. infusion for 6 days. Epididymal fat pads were excised and used either for isolation of adipocytes or for determination of IGF-I mRNA in adipose tissue. A solution hybridization assay was used. The IGF-I mRNA content of adipocytes was analysed either immediately after isolation or after short-term (2–3 days) culture with or without GH or IGF-I. Hypophysectomy resulted in a marked decrease in IGF-I mRNA in both tissue and cells. Replacement therapy (in vivo) with cortisol and thyroxine alone had no effect, whereas additional treatment with GH caused a dose-dependent increase in IGF-I mRNA. IGF-I mRNA was also increased after a continuous s.c. infusion of GH. A single i.v. injection of GH (100 μg) resulted in an increase in IGF-I mRNA after approximately 2 h, with maximal levels around 6 h after the injection. In cultured adipocytes, addition of GH to the culture medium increased IGF-I mRNA in a dose-dependent manner and a marked increase was observed with a concentration of GH of 1 ng/ml. Addition of IGF-I (100 ng/ml) had no effect. The increase in IGF-I mRNA after addition of GH (100 ng/ml) was detectable after 3 h. The concentration of IGF-I in the culture medium was increased 24 h after the addition of GH. These results demonstrate that GH induces IGF-I mRNA in both adipose tissue and isolated fully differentiated adipocytes and that this increase in IGF-I mRNA results in increased IGF-I production.

Journal of Endocrinology (1991) 131, 139–145

It has been suggested, mainly by in vitro findings, that cardiovascular tissue in the spontaneously hypertensive rat (SHR) should be more prone to proliferate/hypertrophy than that of the Wistar-Kyoto rat (WKY). The present study tests the hypothesis that the tissue of the low-pressure compartment in SHR, being structurally similar to that of the WKY, shows an increased growth response due to activation of the GH-IGF-I system. An aortocaval fistula (ACF) was induced in 64 SHR and WKY male rats and 44 rats served as controls. They were all followed for 1, 2, 4 and 7 days after surgery. In separate groups of SHR (n=4) and WKY (n=3), central venous pressure was measured by telemetry recordings prior to opening of the fistula and for up to 16 h post-surgery. Systolic blood pressure was measured during the week post-surgery. The right ventricular (RV) and the caval vein IGF-I mRNA and RV IGF-I receptor and GH receptor mRNAs were quantitated by means of solution hybridisation assay. In rats with ACF the systolic blood pressure decreased, approximately 29% in SHR and 16% in WKY between 1 and 7 days post-surgery (P<0.05, n=5-6 in each group). SHR with ACF showed a transient elevation in central venous pressure vs WKY. Within the week following fistula induction both strains showed a similar, pronounced increase in RV hypertrophy. SHR with ACF showed a smaller, or even blunted, overall response with respect to activation of the GH-IGF-I system compared with WKY, the latter showing clear-cut elevation of gene expressions. Two days after shunt opening in SHR, RV and caval vein IGF-I mRNA increased by 57% and 108% (P<0.05 for both, n=5-6 in each group) respectively, and these expressions were then turned off, whereas RV GH receptor and IGF-I receptor mRNA expression remained unaffected compared with WKY rats. WKY rats showed on average a later and a greater response of GH-IGF-I system mRNA expression vs SHR. The present in vivo study suggests that the SHR requires less activation of the GH-IGF-I system for creating a given adaptive structural growth response.

ABSTRACT

Levels of mRNA for the insulin-like growth factor-I (IGF-I) in rat heart and skeletal muscle and its dependence on GH were investigated using a solution hybridization assay. Levels of IGF-I mRNA decreased following hypophysectomy, and replacement therapy with human GH (hGH) normalized heart and skeletal muscle levels. The stimulatory effect of hGH was dose-dependent, the lowest effective dose being 100 μg. A significant increase of IGF-I mRNA was observed 60 min after s.c. administration of 100 μg hGH and the maximum increase was apparent 6–12 h after hGH injection. Administration of 200 μg IGF-I or 11 μg insulin did not significantly change levels of IGF-I mRNA. The results show that GH regulates the level of IGF-I mRNA in rat heart and skeletal muscle and give further support to the hypothesis that locally produced IGF-I might be a local mediator for the direct stimulatory effect of GH on the growth and development of heart and skeletal muscle.

Journal of Endocrinology (1989) 120, 107–112

ABSTRACT

Expression of insulin-like growth factor I (IGF-I) mRNA and its dependence on GH was studied in rat epiphyseal growth plate by in-situ hybridization. Methodological aspects of the technique were first evaluated on fixed sagittal cryosections from 28-day-old rat epiphyseal growth plates to ascertain specific hybridization. In-situ hybridization was compared on sections from 10- and 28-day-old rats and the GH-dependence was studied in 35-day-old hypophysectomized rats. Expression of IGF-I mRNA was apparent in chondrocytes of the proliferative, hypertrophic and early degenerative zones of the growth plate of 28-day-old rats. The epiphyseal growth plate from 10-day-old rats showed a weak hybridization signal compared with 28-day-old rats. In contrast, the mesenchymal cells of the periosteum of 10-day-old rats showed a rather strong hybridization signal. Hypophysectomy resulted in a reduction in hybridization signal and cell number of the growth plate compared with 35-day-old age-matched normal rats. GH-Replacement therapy (200 μg human GH s.c. every 4 h for 24 h) resulted in partially restored expression of IGF-I mRNA.

The present study has shown that the IGF-I gene is expressed in the rat epiphyseal growth plate chondrocytes and that the expression is dependent on GH. The results support a paracrine/autocrine role of IGF-I for the expression of the stimulatory effect of GH on longitudinal bone growth.

Journal of Endocrinology (1990) 125, 67–74

Recent experimental data demonstrate cardiovascular effects of the GH secretagogues (GHSs) hexarelin and ghrelin, the proposed natural ligand for the GHS receptor. Moreover, specific cardiac binding sites for GHSs have been suggested. The aim of the present study was to investigate if the natural ligand ghrelin and synthetic GHS peptide hexarelin and analogues have direct effects on the cardiomyocyte cell line, H9c2. Hexarelin stimulated thymidine incorporation in a dose-dependent manner with significant responses at 3 micro M (147+/-3% of control, P<0.01) and elicited maximal effects at concentrations around 30 micro M. This activity was seen already after 12 h of incubation with a maximal effect after 18 h (176+/-9% of control, P<0.01). Ghrelin also had a significant stimulatory effect on thymidine incorporation (129+/-2% of control at 3 micro M and 18 h, P<0.05). The stimulatory effect on thymidine incorporation of hexarelin, Tyr-Ala-hexarelin, EP80317 and ghrelin was specific and no stimulatory effect was observed with the truncated GH-releasing peptide EP51389 or the non-peptidyl GHS MK-0677. In competitive binding studies, (125)I-labeled Tyr-Ala-hexarelin was used as radioligand and competition curves showed displacement with hexarelin, Tyr-Ala-hexarelin, EP80317 and ghrelin, whereas MK-0677 and EP51389 produced very little displacement at 1 micro M concentration, adding further support for an alternative subtype binding site in the heart compared with the pituitary. In conclusion, we have demonstrated a dose-dependent and specific stimulation of cardiomyocyte thymidine incorporation by natural and synthetic GHS analogues, suggesting increased cell proliferation and binding of GHS to H9c2 cardiomyocyte cell membranes. These findings support potential peripheral effects of GHS on the cardiovascular system independent of an increased GH secretion.