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Comparative labelling of rat epididymal spermatozoa by intratesticularly administered ⁶⁵ZnCl₂ and [³⁵S]cysteine

Summary. Spermatozoa of rats injected intratesticularly with 20 µCi ⁶⁵ZnCl₂ and 10 µCi [³⁵S]cysteine were collected from the caput and cauda of the epididymis at 2, 6, 10, 14, 18, 22 and 28 days after injection. The highest specific activities with respect to each isotope were observed in spermatozoa from the caput on Day 10. Maximal levels in spermatozoa from the cauda were obtained on Days 14 and 18 for ³⁵S and Day 18 for ⁶⁵Zn. Estimation of the relative behaviour of ⁶⁵Zn and ³⁵S by calculation of ⁶⁵Zn/³⁵S ratios suggests that: (1) ³⁵S associated with spermatozoa arrived in the epididymis slightly in advance of ⁶⁵Zn and was therefore probably incorporated more readily into proteins of very late spermatids; (2) approximately 60% of ⁶⁵Zn was lost from spermatozoa and 75% from isolated sperm heads during transit from caput to cauda, assuming total retention of ³⁵S; and (3) retention of ⁶⁵Zn by the seminiferous epithelium was superior to that of [³⁵S]cysteine, as indicated by increasing ⁶⁵Zn/³⁵S ratios following the days of peak specific activity in both caput and cauda epididymidal spermatozoa. Only small percentages of either isotope were recovered in isolated sperm heads, suggesting that the primary sites of labelling were in the sperm tail.

Superior retention of ⁶⁵Zn by testis was confirmed by increasing ⁶⁵Zn/³⁵S ratios in individual fractions of testicular homogenates between 2 and 10 days after injection. In addition, both isotopes appeared to be transferred from the testis cytosol to particulate material during this period.