Stimulates two retinal guanylyl cyclase (GCs) GUCY2E and GUCY2F when free calcium ions concentration is low, and inhibits GUCY2E and GUCY2F when free calcium ions concentration is elevated (By similarity). This Ca(2+)-sensitive regulation of GCs is a key event in recovery of the dark state of rod photoreceptors following light exposure (By similarity). May be involved in cone photoreceptor response and recovery of response in bright light (PubMed:25673692). Subcellular location is not affected by light or dark conditions. Expressed in rod photoreceptors in the retina (at protein level). Expressed in cone photoreceptor cells (PubMed:9620085). The N-terminus is blocked. Guca1a and Guca1b double knockout mice show an increase in response to light in dark-adapted cone photoreceptors (PubMed:25673692). Dark-adapted cone photoreceptors show a delayed response time and a delayed recovery time in response to light (PubMed:25673692). Guca1a, Guca1b and Rcvrn triple knockout mice show rod photoreceptors have a reduced current decay during light response (PubMed:29435986). Binds three calcium ions.