Our results reveal that the characteristics (affinity, specificity) of GH binding to crude testicular homogenates are identical to those described in rainbow trout crude hepatic preparations (this study; ) and in hepatic or testicular membrane preparations (this study; ). Binding was specific to GH with little or no affinity for other hormones. Other studies also showed that PRL does not bind to GH-R in teleosts, suggesting that GH and PRL have distinct binding sites in fish. It should also be noted that our preparations could not be contaminated by an extracellular circulating GH-BP, as no GH-BP could be found in rainbow trout. The apparent Ka obtained in our study are also in the same range as those described using homologous radioreceptor assays in the ovary, in central nervous system, in gills of the same species, and in the liver of other fish. The apparent dissociation constant of the GH-R (0.2 nM) may be considered to be high compared to the plasma level measured in this experiment (0.03 nM). However, it has been shown that the plasma GH level in rainbow trout shows episodic fluctuations that can lead to a transient high level of GH, which could efficiently bind to GH-R. buy antibiotics online
Functional GH-R have not yet been reported in the testis of other species. Our data are in agreement with previous observations of low levels of GH-R/GH-BP mRNA in the testis of birds and mammals, as well as with the effect of GH treatment on testicular steroidogenesis and IGF-I mRNA expression in mammals and teleosts.