The role of citrate in determining the activity of calcium ions in human semen.

The concentrations of calcium and citrate in 52 samples of human semen obtained from 10 donors were 11.07 +/- 0.41 (mean +/- SEM) and 27.2 +/- 1.62 mM, respectively, and were strongly correlated (r = + 0.79). The pH of the semen was 7.65 +/- 0.069. All 3 parameters varied significantly between the donors (P less than 0.001 in each case), but the activity of calcium ions measured with a calcium electrode was 0.24 +/- 0.022 mM (n = 45) and did not vary significantly between donors. The expected activity of calcium ions was calculated on the hypothesis that the chelation of calcium by citrate could explain the discrepancy between the concentration and the activity of the ion. The result was 0.57 +/- 0.043 mM (n = 52). It was concluded that chelation by citrate could be the most important factor in determining the low activity of Ca++ in semen.