Role of FSH, numbers of FSH receptors and testosterone in the regulation of inhibin secretion during the seasonal testicular cycle of adult rams.

The regulation of inhibin secretion has not been elucidated fully in male ruminants. The aim of this study was to determine the relative importance of FSH and testosterone concentrations, and FSH receptors, in the control of secretion of immunoactive inhibin in rams. In Expt 1, temporal changes in hormone concentrations and testicular FSH binding were determined for two groups of rams (n = 4) kept under opposite, alternating 4 month periods of long (16 h light:8 h dark) and short (8 h light:16 h dark) days. Testicular biopsies (1-2 g) were collected when the testes were regressed, redeveloping, redeveloped and regressing. In Expt 2, separate groups of rams (n = 4) kept under natural photoperiod (latitude 45 degrees 48 minutes N) were designated as controls or passively immunized (for 3 weeks) with sufficient oestradiol antiserum to increase testosterone secretion without altering LH and FSH; this was done when the testes were regressed (non-breeding season) and redeveloped (breeding season). In both groups of rams (Expt 1), 'seasonal' increases in FSH concentrations began a few weeks earlier than did increases in inhibin concentrations. FSH reached maximum concentrations during testicular recrudescence, whereas numbers of FSH receptors in the testis and circulatory inhibin concentrations did not reach peak values until the testes were fully developed. Numbers of FSH receptors per testis, but not FSH concentration, were positively correlated (r = 0.65) with inhibin concentrations across the four stages of the testicular cycle. Near the end of testicular recrudescence early in the breeding season (Expt 2), relatively high FSH concentration was associated with increased abundance of FSH receptor mRNA (90%) and number of receptors (45%) in the testis and increased inhibin concentrations (50%), compared with when the testes were regressed. Moderate, physiological increases in testosterone secretion in immunized rams did not affect inhibin in either season. These results indicate that: (i) FSH stimulation of immunoactive inhibin secretion by Sertoli cells as testes recrudesce is via increases in secretion (early) and cognate receptors (late); (ii) FSH upregulates the synthesis of its own receptor late in recrudescence; and (iii) the positive correlation (r = 0.70) observed between circulatory testosterone and immunoactive inhibin does not reflect a causal relationship.

Cadmium in caribou (Rangifer tarandus) kidneys: speciation, effects of preparation and toxicokinetics.

Caribou kidney is a major source of cadmium (Cd) in the traditional diets of many aboriginal communities in the Arctic. In order to characterize the risk of Cd exposure, we studied the speciation of Cd in caribou kidneys and how, it can be affected by food preparation. Cd in caribou kidneys was bound to metallothionein (MT) (40%) high molecular weight proteins (50-500 kDa) (30%) and existed as free ions (10%). There was no change in Cd concentrations after the samples were baked at 350 degrees C for 30 min but the MT level was significantly decreased. Food preparation also denatured the high molecular weight proteins, resulting in a significant increase of free Cd. The toxicokinetics of Cd in caribou kidney was studied using a rodent model. Four groups (n = 6) of adult Sprague-Dawley rats were fed with rat chow with the addition of one of the following: 10% veal kidney with 0 microg/g Cd, 10% veal kidney spiked with CdCl2 to make up to 66 microg Cd; 10% caribou kidney containing 20 microg/g Cd and 10% caribou kidney containing 66 microg/g Cd. After 14 days, Cd accumulation increased with dose but less than 1% of the dosed Cd accumulated in the liver and kidney of rats. The percentage of Cd accumulated in the liver was higher in the group fed with caribou kidney than that dosed with spiked veal kidney. Our results suggest that caribou kidney is a major source of Cd exposure among the Arctic populations but moderate consumption may be considered unavoidable.