Inhibin : its role in the regulation of the pituitary-testis axis.

The endocrine and exocrine functions of the male gonads, the testes, are regulated by gonadotrophic hormones which are secreted by the pituitary gland. Two separate gonadotrophic hormones have been recognized: luteinizing hormone (LH) which influences Leydig cell function, and follicle-stimulating hormone (FSH) which affects the function of the seminiferous tubules. The secretion of gonadotrophins is stimulated by a hypothalamic factor, luteinizing hormone-releasing hormone (LH-RH) and can be inhibited by steroid hormones which are secreted by the testes. The existence of another hormone which is produced by the seminiferous tubules in the testis and also influences the pituitary gland in males has been suspected for many years (Mottram & Cramer, 1923). This principle appeared to be a non steroidal factor and has been called "inhibin" by McCullagh (1932). The reality and significance of the inhibin concept has been much debated and has received increasing attention during the past ten years (see chapter 2). A schematic diagram on the interactions between stimulating and inhibiting substances in the regulation of the hypothalamic-pituitary-testicular axis is shown in figure 1.1. The aim of the investigations, described in this thesis was to examine the regulation of inhibin production in and inhibin secretion from testicular tissue and to investigate the physiological significance of this protein hormone in male reproduction.