Figure 5.

NCKX activities in sea urchin sperm. (A) Na⁺-dependent Ca²⁺ efflux. Sperm were loaded with fura-2 in NaFSW. For measurements, 50 μl fura-2-loaded sperm were diluted into 1.5 ml of NaFSW, and after 2 min an equal volume of NaFSW or ASW was added as indicated by the arrow. Rapid efflux of Ca²⁺ occurred in ASW. (B) K⁺-dependent Ca²⁺ influx. Sperm were loaded with fura-2 in K⁺ and divalent cation free artificial seawater (KDFSW) and diluted into Li⁺/K⁺ solution (480 mM LiCl/10 mM KCl/1 mM EGTA/20 mM Hepes, adjusted to pH 8 with 1 M N-methyl-d-glucamine), or Li⁺ solution (490 mM LiCl/1 mM EGTA/20 mM Hepes, pH 8 with N-methyl-d-glucamine) as indicated. Ten micromolar CaCl₂ was added at the arrow. More Ca²⁺ entered the cells in the 10 mM K⁺ sample, showing that suNCKX can run in reverse mode. Data are from a representative experiment repeated seven times.