Biochemical changes that occur in the rat or mouse testis after a single subcutaneous injection of a sub-toxic dose of CdCl2 (2·2 μmol/100 g body wt) include decreases in the contents of DNA, RNA and in the activities of certain Zn2+-containing enzymes. The protein content is increased, although the rate of protein synthesis is reduced. Uptake of Cd2+ by the testis is small and does not cause the displacement of Zn2 +. In both species, the concentration of Zn2 + remains constant in the testis for a short time after the parenteral administration of Cd2 +, and then increases progressively. Initially, this increase appears to be due to the decrease in weight of the damaged testis, which is not accompanied by the loss of Zn2+. Later, when the testicular weight becomes constant, Zn2+ continues to accumulate to a level nine to ten times greater than in the normal testis.
In the male rat, subcutaneously injected Cd2+ accumulates to high levels and is retained in the liver and kidneys. In both of these organs also, the Zn2+ content increases with the uptake of Cd2+. Bound Cd2+ is present in the soluble components (cell sap) of these tissues as a single fraction, probably metallothionein. This Cd2+-binding protein also accumulates Zn2+ and is responsible for its increased uptake.
Competitive antagonism between the toxic Cd2+ cation and the essential Zn2 + ion may occur in the epididymis since, in this organ, the content of the latter cation decreases with the uptake of the former.
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