The accumulation of the lipophilic cation radiolabeled triphenylmethylphosphonium (TPMP') was utilized to determine the resting membrane potential across the plasma membrane ($) on human sperm. Washed sperm were suspended and incubated in low-K+ and high-K+ medium and allowed to take up the cation to a steady state (that is, 20 min at 37°C). The data were transformed according to concentration using the intracellular volume and with these values inserted in the Nernst equation we obtain $.When the concentration of K+ was low in the medium, the $ was -69 + 2 mV, the $ was temperature-and pH-dependent. The sperm were incubated in the same conditions in the presence of testosterone, estradiol, and progesterone. The addition of these hormones induced the decrease in $ by about 29%, 32%, and 40%, respectively. On the other hand, capacitation of spermatozoa was induced by the addition of calcium and the ionophore A23 187 and in these conditions the II. decreased 50%. The action of the hormones on sperm membrane was accompanied by some steroid-dependent structural modification promoting the $ decrease. This effect plays an important role in the capacitation, preparing it for sperm penetration in the ova.
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