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The Physiology of Sea-Urchin Spermatozoa : The Dilution Effect in Relation to Copper and Zinc
1 Department of Zoology, University of Cambridge, and the Marine Station, Millport
1. When a suspension of sea-urchin semen in sea water is diluted, the total O2 consumed and the rate of O2 uptake per spermatozoon are greater than before dilution. This is the Dilution Effect, first described by Gray (1928 a). A further investigation of this phenomenon has been made, using the semen of Echinus esculentus.
2. The form of the O2 uptake-time curve of such suspensions varies according to the ratio semen: sea water.
3. In very dense suspensions (d 
5 x 109 sperm/ml.), the apparent low O2 uptake per unit quantity of spermatozoa is mainly due to inadequate O2 saturation of the lower layers of the suspension, in which the spermatozoa are virtually unable to respire. Such suspensions are not suitable for experiments in Warburg or Barcroft manometers.
4. In dense suspensions (4 x 108 < d 
109 sperm/ml.) the Dilution Effect was observed when sea water was added to the suspension, in such proportions that the sperm density was only reduced by a factor of 1.14. No Dilution Effect occurred when isotonic ‘Analar’ NaCl was added in the same proportions. On the basis of the small dilution involved, and the negative results with ‘Analar’ NaCl, it is concluded that the Dilution Effect is not exclusively due to the sperm having more space to move after dilution and therefore being able to expend more energy.
5. The Dilution Effect occurs when isotonic ‘Analar’ NaCl containing 1 p.p.m. CuCl2 is added to a dense sperm suspension, in the above proportions. Stimulation of O2 uptake is proportional, over certain limits, to the amount of CuCl2 added. Both CuCl, ZnCl2, and to a lesser extent CuSO2, have similar effects.
6. In a particular experiment, egg-water and Cu salts were about equally efficacious in increasing the O2 uptake of a sperm suspension.
7. The characteristic decline in the O2 uptake of dense suspensions can be partly reversed by the addition of Cu salts.
8. In dilute suspensions (d < 4 x 108 sperm/ml.), there is no Dilution Effect when sea water or CuCl2 are added in the above proportions. On the other hand, the decline in O2 uptake can be partly arrested by the addition of Cu salts.
9. Diethyldithiocarbamate markedly inhibits the O2 uptake of unwashed sea-urchin spermatozoa diluted with sea water. The inhibition does not take place in the presence of added CuCl2. Cu may therefore have a catalytic function in the metabolism of sea-urchin semen.
10. The possibility that Cu lack is responsible for the reduced O2 uptake of spermatozoa in dense suspensions is considered.
Submitted on September 14, 1949
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