Figure 5: Signalling events underlying the ‘off responses’.

(a) Signalling pathway underlying chemotaxis of A. punctulata sperm. The balance of synthesis by the chemoreceptor guanylyl cyclase (GC) and hydrolysis by PDE determines intracellular cGMP concentration. cGMP levels set the membrane potential of the cell via a cyclic nucleotide-gated K⁺-selective channel (CNGK). When sperm hyperpolarize, intracellular alkalization via a Na⁺/H⁺ exchanger (sNHE) and depolarization via hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels prime the sperm-specific Ca²⁺ channel (CatSper) to open. When cGMP synthesis ceases, the cell quickly depolarizes, more CatSper channels open and a strong steering ‘off response’ takes place. (b) Relative changes in [Ca²⁺]_i (ΔF/F₀; top) and relative changes in membrane potential V_m detected with a ratiometric dye (ΔR; bottom) of sperm suspensions on release cGMP by light pulses of 180, 500 or 1,000 ms duration. Release of cGMP results in a rapid hyperpolarization and depolarization of sperm followed by a rise in [Ca²⁺]_i (‘on response’). When photolysis ceases, a second Ca²⁺ signal takes place (‘off response’).