Slow calcium oscillations in human spermatozoa

We have used single-cell imaging to investigate intracellular Ca²⁺ signalling in human spermatozoa stimulated with progesterone (3 µM). In approx. 9% of cells progesterone caused the activation of slow repetitive [Ca²⁺]_i (intracellular Ca²⁺ concentration) oscillations, with a period of 1–4 min, which persisted for the duration of recording (20–30 min). Pretreatment with nifedipine, which blocks T- and L-type voltage-operated Ca²⁺ channels in spermatogenic cells, did not modify the characteristics of the oscillations, but reduced the proportion of cells in which they were observed. Stimulation with Bay K 8644 or FPL64176 induced [Ca²⁺]_i oscillations in 5–10% of cells, but their frequency was low (period, 4–5 min). Application of valinomycin (1 µM) to clamp membrane potential at E_K (equilibrium potential for potassium) did not modify activity in oscillating cells, showing that plasma membrane potential and activation of voltage-operated conductances are not involved in the mechanism by which sperm [Ca²⁺]_i oscillations are generated.