Characteristics of calcium ion influx induced by human follicular fluid in individual human sperm.

The mechanism of Ca2+ influx into human spermatozoa was investigated. Human follicular fluid (hFF), which contains progesterone, stimulated an increase in the concentration of intracellular free Ca2+ ([Ca2+]i) as measured by fluorescence spectroscopy using Fura-2/AM. The technique allowed the measurement of [Ca2+]i in individual, motile spermatozoa, by improvement of the previous methods. The role of Ca2+ channels was investigated using selective blocking agents. Neither blockade of L-type channels with verapamil or lanthanum (La3+) nor blockade of N-type channels with omega-conotoxin affected the hFF-stimulated increase in [Ca2+]i. Blockade of T-type channels with either nickel (Ni2+) or amiloride significantly reduced the hFF-stimulated increase in [Ca2+]i and amiloride significantly decreased baseline [Ca2+]i. Incubation of sperm in Na(+)-free medium did not affect [Ca2+]i, indicating that Na+/Ca2+ exchange has no major role in Ca2+ influx. The results suggest that calcium influx induced by hFF occurs via T-type voltage-independent Ca2+ channels. The method described allows accurate measurement of [Ca2+]i in individual, motile human spermatozoa.