As ghrelin effects are suspected to be mediated by a

As ghrelin effects are suspected to be mediated by a heterogeneous population of G-protein-coupled-receptors [2], [21], other so far unknown subtypes of ghrelin receptors differing from the classical GHSR-1a may additionally exist that might also mediate ghrelin effects on the heart. Thus, the real clinical impact of the detected changes in ghrelin and GHSR-1a expression cannot fully be elucidated at this moment and further studies are needed to address these issues.
Introduction Ghrelin, a 28-amino PFI 3 octanoylated peptide, was recently isolated from the stomach of rat and identified as the endogenous ligand for the growth hormone secretagogue receptor, GHSR [1]. GHSs are a family of small synthetic peptide or non-peptide molecules that stimulate the release of growth hormone from the anterior pituitary via a specific seven-transmembrane G protein-coupled receptor [2]. Two GHSR subtypes generated by alternative splicing of a single gene have been identified: the full-length type 1a receptor and the truncated type 1b [3]. GHS-R1a is the functionally active, signal transduction form of the receptor. In contrast, GHS-R1b lacks transmembrane domains 6 and 7 and is unable to bind a ligand or transduce a signal. Like ghrelin, the ghrelin receptor also has a widespread distribution in various tissues, suggesting multiple paracrine, autocrine and endocrine roles for ghrelin. Expression of the ghrelin receptor, GHSR-1a, has been detected in the pituitary, thyroid, pancreas, stomach, heart, lung, adrenal cortex, immune system, adipose tissue and human breast carcinoma [4], [5], [6]. Gaytan et al. [7] reported that GHS-R1a peptide expression paralleled follicular development, with stronger immunostaining in the granulosa and theca cell layers of healthy human antral follicles. Moreover, expression of ghrelin in the ovary has been demonstrated in steroidogenically active luteal cells and intersticial hilus cells. This suggests a potential relationship between GHSR-1a expression, follicular growth and regulation of ovarian production. In addition, ghrelin and GHSR-1a expression were detected in reproductive tissues including placenta [8], ovary [9] and testis [10]. To date, we have demonstrated that ghrelin is secreted from ovarian follicles, that it increases estradiol secretion by modifying aromatase activity and that it acts as an anti-apoptotic and proliferation stimulate factor in prepubertal pig ovaries [11]. Moreover, we have shown that ghrelin stimulates local, ovarian growth hormone (GH) secretion and cooperates with GH in ovarian function [12].
Materials and methods
Results
Discussion The present study is the first to demonstrate the presence of the functionally active form of the ghrelin receptor, GHSR-1a, in the pig ovary, using RT-PCR and Western blot. The distribution of ghrelin and its functional receptor, GHSR-1a, have been shown in many tissues, including ovary [4]. Gaytan et al. [7] used an immunohistochemical approach to characterize in detail the expression and pattern of cellular location of ghrelin and GHSR-1a in the cyclic human ovary. Ovarian expression of GHSR-1a protein showed a wider pattern of tissue distribution, with a detectable specific signal in oocytes as well as somatic follicular cells; luteal cells from young, mature, old and regressing corpora lutea; and interstitial hilus cells. Strong GHSR-1a immunoreactivity was detectable in granulosa and theca layers of growing follicles also [7]. This suggested a potential relationship between GHSR-1a expression and follicular growth, Sirotkin et al. [19] reported that both ghrelin and GHSR-1a mRNA were expressed in fragments of chicken ovarian tissue. Moreover, they revealed three splice variants of GHSR-1a: full-length type 1a transcript (cGHS-R1a), a second variant termed cGHSR-1a, and a third GHSR-1a isoform termed cGHSRtv. The presence of GHSR-1a peptide in ovarian follicles suggests a potential regulatory role of locally produced ghrelin in the control of follicular development. Our recently published data demonstrated that ghrelin was secreted by ovarian follicles [20]. Measuring the concentration of ghrelin in follicular fluid, the follicular wall and conditioning medium, we noted that the level of ghrelin in the follicular fluid was the sum of the amounts in the follicular wall and the culture medium. We hypothesized, then, that the concentration of ghrelin noted in the follicular fluid was not the result of blood infiltration but of local, ovarian production. The fact that GHSR-1a is present in the ovary, together with the fact that ghrelin is produced locally in the follicles suggests its direct action on the ovary.