Fig. 6.

Sequential activation of RetGC isozymes by GCAPs in rods via Ca²⁺ feedback. GCAP1 has lower affinity for Ca²⁺ and therefore activates RetGC1 soon after the high dark Ca²⁺ levels in rods start to decline at the beginning of the photoresponse. The early activation of RetGC1 restricts the amplitude of a photon response. GCAP2 has higher affinity for Ca²⁺ and converts to the Mg²⁺-liganded state only after the free Ca²⁺ levels further decline in mid-phase of the response; it then activates the ancillary RetGC2 (and possibly a fraction of RetGC1 unoccupied by GCAP1) [68, 69]