Calcium buffering capacity of human seminal plasma: the role of EGTA in stimulating sperm motility.

The role of EGTA in stimulating human sperm motility (reported in Lancet i: 460-461, 1984) was investigated by measuring the calcium buffering capacity of human seminal plasma. Human seminal plasma contains 9.5 +/- 1.1 mM (mean +/- SE) calcium of which 0.16 +/- 0.01 mM only exists as free Ca2+. The free Ca2+ concentration was not changed by the addition of either 1 mM CaCl2 or 1 mM EGTA. The ability of seminal plasma to bind calcium ions was determined by adding varying amounts of CaCl2. Scatchard analysis of the results indicates the presence of high amounts of high-affinity (Kd1 = 3.4 +/- 0.2 microM, Bm1 = 11.5 +/- 1.5 mM) and low-affinity (Kd2 = 0.55 +/- 0.04 mM, Bm2 = 30.4 +/- 1.7 mM) Ca2+-complexing agents. The low-affinity Ca2+-binding substance may be citrate. These results seem to suggest that human seminal plasma has a high Ca2+ buffering capacity and the stimulation of sperm motility by EGTA ought to be mediated via a mechanism other than the reduction of the free calcium concentration in semen.