Cellular signaling during chemotaxis in sea urchin sperm. The signaling events are initiated by binding of the ligand to the receptor GC. The rise of cGMP opens K+-selective cyclic nucleotide-gated channels (CNGK). The ensuing hyperpolarization activates a voltage-dependent Na+/H+ exchanger (NHE) and a pacemaker channel (HCN). The resulting alkalization by NHE and depolarization by HCN opens a sperm-specific Ca2+ channel (CatSper). Ca2+ is exported by a Na+/Ca2+-K+ exchanger (NCKX).