Onset rates of three feedback reactions that are considered to underlie light adaptation in frog rods
Purpose. Light adaptation in rod photoreceptors is thought to be mediated by a number of feedback reactions which cause the photoresponse to become faster and less sensitive. We evaluated how fast the changes in two intracellular second messengers, Ca and cGMP, lead to onsets of three of these feedback mechanisms. Ca* regulation of guanylate cyclase, Ca regulation of rhodopsin phosphorylation and regulation of transducin GTPase by cGMP. Methods. The kinetics of these reactions were measured in suspensions of rod outer segments after rapid changes hi either Ca*+ or cGMP. Results. Guanylate cyclase and rhodopsin kinase changed their activities in response to Ca-4* reduction on subsecond thne scales. To the contrary, the onset of transducin GTPase acceleration did not directly follow the decline of cGMP. It was limited by cGMP dissociation from the noncatalytic sites on rod cGMP phosphodiesterase that occurs on the time scale of tens of seconds. Each of the two sites on the phosphodiesterase contributed equally to this regulation. Conclusion. There is a temporal hierarchy among reactions that underlie rod light adaptation. Two Ca dependent reactions studied here quickly follow the light dependent decline in intracellular Ca. The cGMP dependent reaction requires a substantial reduction in intracellular cGMP over prolonged periods of illumination. We suggest that the former mechanisms follow the time course of light dependent current suppression while the latter mechanism requires prolonged illumination.