IGF-I acts as a local proliferation and maturation factor for chondrocytes in the growth plate. However, the expression of different alternative IGF-I mRNA classes in the growth plate has not been characterized. Using quantitative reverse transcription PCR, the abundance of each alternative IGF-I mRNA class in resting, proliferative and hypertrophic chondrocytes was measured in rat costochondral growth plates. Class 1Ea mRNA was the most abundant IGF-I transcript overall and was highly expressed in proliferative chondrocytes at 2 and 4 weeks of age; by 6 weeks, the majority of 1Ea mRNA expression had shifted to hypertrophic chondrocytes. Class 1Eb mRNA was the second most abundant transcript and its distribution was uniform across all the cell types at 2 weeks of age. The expression pattern changed with increasing age such that at 6 weeks a gradient existed with hypertrophic chondrocytes expressing higher levels of 1Eb than resting chondrocytes. Class 2Ea mRNA was constitutively expressed at low levels across the growth plate at all ages, while class 2Eb mRNA expression was negligible. The distribution of total IGF-I mRNA also shifted across growth plate cell types as the animals aged from 2 to 6 weeks. These findings suggest that IGF-I class 1 mRNA plays the predominant role in the maturation of the growth plate.

Elevated growth hormone (GH) concentrations suppress reproductive function in a variety of species, although it is unclear whether GH directly suppresses reproductive performance, or whether GH activates other pathways to achieve these effects. The ovine metallothionein 1a-ovine GH (oMt1a-oGH) transgenic mouse has been used to model the effects of GH on both body composition and reproductive function. A recent report has documented increased leptin levels in obese oMt1a-oGH mice. Given the importance of leptin in modulation of the reproductive endocrine axis, as well as the reports documenting reduced leptin signal transduction in animals with elevated leptin levels, we hypothesized that high leptin concentrations in response to elevated GH would reduce fertility. To determine the effects of high circulating leptin levels on the reproductive endocrine axis, we assessed hypothalamic neuropeptide Y (NPY) and GnRH expression. At weaning, oMt1a-oGH transgenic (TG) and wild-type (WT) female mice were allocated to one of four treatment groups: oMt1a-oGH females chronically expressing the transgene (TG ON); oMt1a-oGH females expressing the transgene from 3 to 8 weeks of age (TG ON/OFF); WT females receiving the transgene stimulus from 3 to 8 weeks of age (WT ON/OFF); and WT females never receiving the transgene stimulus (WT OFF). Eight-week-old females were housed with males for a 2-week period, after which females were isolated from males and allowed to carry pregnancies to term. Body and gonadal fat pad (GFP) weights, along with plasma leptin concentrations, estrous cyclicity, pregnancy rate and litter characteristics, were recorded for each female. Chronic expression of the oMt1a-oGH transgene resulted in larger leaner mice, and inactivation of the transgene produced obese females. Pregnancy rate was reduced in TG ON females when compared with all other groups, and infertility was associated with elevated leptin levels. In addition, high leptin levels were associated with increased NPY expression, suggesting reduced leptin-signaling capacity, which may contribute to suppression of the reproductive axis in oGH animals.