Effects of Ca-ATPase inhibitors on the intracellular calcium activity and motility of human spermatozoa

Although evidence suggests that high intracellular calcium activity ([Ca2+](i)) inhibits sperm motility, data concerning [Ca2+](i) within, or slightly above, the physiological range are sparse, particularly in mammalian sperm. We investigated inhibitors of the sarcoplasmic/endoplasmic reticulum Ca-ATPase (SERCA) and the plasma membrane Ca-ATPase with the objective of increasing the intracellular calcium ion activity in human spermatozoa to study its effect on motility and other functions. Thapsigargin (20 mumol/L) increased [Ca2+](i) from 140 +/- 7 nmol/L over a similar to2-min period to reach a plateau of 530 +/- 84 nmol/L (mean +/- SEM, n = 3, p < 0.05). In sperm suspended in calcium-free medium thapsigargin increased [Ca2+](i) from 13 +/- 3.3 to 35 +/- 7.5 nmol/L (p < 0.01), consistent with the release of calcium from intracellular stores. Cyclopiazonic acid (60 mumol/L) caused a transient decrease in [Ca2+](i). Quercetin, (200 mumol/L) caused a rapid increase in [Ca2+](i) to 1280 +/- 90 nmol/L, after which [Ca2+](i) fell quickly at first but then more slowly. Thapsigargin (20 mumol/L) caused similar to70% of sperm to acrosome react inless than or equal to5 min, but once acrosome reacted, many sperm died over the next 30 min. Lower concentrations of thapsigargin caused fewer acrosome reactions but were less toxic. Both thapsigargin and quercetin caused rapid dose-dependent decreases in sperm motility. The results are consistent with high [Ca2+](i) in the range observed in caput epididymal or cryopreserved spermatozoa inhibiting motility, but might be confounded by other events following the acrosome reaction.