A K+-selective cGMP-gated ion channel controls chemosensation of sperm.

. The signalling pathway downstream of the synthesis of cGMP by a guanylyl cyclase is ill-defined. In particular, the ion channels that are involved in Ca 2+ influx and their mechanisms of gating are not known 4 . Using rapid voltage-sensitive dyes and kinetic techniques, we record the voltage response that is evoked by the chemoattractant in sperm from the sea urchin Arbacia punctulata. We show that the chemoattractant evokes a brief hyperpolarization followed by a sustained depolarization. The hyperpolarization is caused by the opening of K + -selective cyclic-nucleotide-gated (CNG) channels in the flagellum. Ca 2+ influx commences at the onset of recovery from hyperpolarization. The voltage threshold of Ca 2+ entry indicates the involvement of low-voltage-activated Ca v channels. These results establish a model of chemosensory transduction in sperm whereby a cGMP-induced hyperpolarization opens Ca v channels by a ‘recovery-from-inactivation’ mechanism and unveil an evolutionary kinship between transduction mechanisms in sperm and photoreceptors. Using fast mixing and flash photolysis techniques, we were able to record optically the voltage response from intact motile sperm of the sea urchin Arbacia punctulata. We have chosen the fluorescent dye di-8-ANEPPS as a potentiometric probe due to its rapid response time, its exclusive staining of plasma membrane and its electrochromic mechanism of voltage sensing. Depolarization and hyperpolarization shift the emission spectrum of di-8-ANEPPS by a few nanometres to lower and higher wavelengths, respectively, by a mechanism known as electrochromism 5,6 (see