[Postnatal development of hormonal relations in the hypothalamo-hypophyseal axis in bulls]. / Postnatální vývoj hormonálních vztahu v ose hypotalamus-hypofýza-varle u býka.

A review of basic knowledge of endocrine events associated with the development of bovine male reproductive organs during early postnatal ontogenesis and infantile, prepuberal, puberal and adult periods is presented. Changes in the dynamics of release of GnRH, LH, FSH, testosterone and other clinically significant androgens, oestrogens and inhibin, including their interactions in the hypothalamus-hypophysis-testicle axis are described. Attention is paid to the relationships between reproductive hormones and the most relevant morphological and physiological changes in bovine male reproductive organs during the postnatal development, particularly the multiplication of Leydig cells, the differentiation and multiplication of Sertoli cells, testicular growth and initiation of spermiogenesis.

Average testosterone levels in bulls after GnRH stimulation at breeder bull rearing stations and A. I. stations.

Base average testosterone levels and testosterone levels in 60 and 90 min after GnRH administration were determined in bulls at five breeder bull rearing stations at their age of 13 months and at nine A. I. stations at their age of 17-34 months. Significant increases in testosterone levels in 60 and 90 min after GnRH administration were the responses of all groups of bulls. There were differences between the average testosterone levels typical of the various BRSs and AISs. Highly significant correlation coefficients 0.344 for base levels and 0.583 for induced levels were calculated from a comparison of the testosterone levels determined at BRSs at the bull's age of 13 months and of those recorded after a six-month stay of the same bulls at AISs. A decrease in testosterone levels was also observed.

[Testosterone production after in vitro stimulation of testicular cells with gonadotropins in bulls]. / Produkce testosteronu po stimulaci varletních bunek býku gonadotropinem in vitro.

The response of bulls' testicular cells to the stimulation by serum gonadotropin ad us. vet. (Bioveta, Ivanovice in Haná) was investigated in in vitro tests. The testicular tissue was sliced, collagenase-split and the loosened cells were stimulated by gonadotropin concentrations of 7.8 to 250 I U per l; control samples without gonadotropin treatment were used for comparisons. The testicular cells responded to the increasing stimulation by higher production of testosterone.