Proceedings of The Physiological Society

University of Cambridge (2004) J Physiol 555P, C161


Control by calcium of the rate-limiting step of phototransduction in red-sensitive salamander cones

Hugh R. Matthews * and Alapakkam P. Sampath †

* Physiological Laboratory, University of Cambridge, Downing Street, Cambridge CB2 3EG and †Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA

The shutoff of the phototransduction cascade in rod photoreceptors is dominated by a time constant of around 2 s (Pepperberg et al. 1992) which is unaffected by Ca2+i (Lyubarsky et al. 1996) and which may represent the hydrolysis by transducin of GTP (Sagoo & Lagnado, 1997). Instead, in rods, Ca2+i acts on a step early in phototransduction which takes place with a time constant of ~0.5 s (Matthews, 1997) which appears to represent rhodopsin quenching (Matthews et al. 2001). In contrast, response recovery in cones is poorly understood. We have therefore investigated the Ca2+-dependence of the dominant time constant in salamander red-sensitive cones by rapidly superfusing the outer segment with a 0Ca2+/0Na+ solution designed to minimise surface membrane Ca2+ fluxes, in order to delay the dynamic fall in Ca2+i which normally accompanies the flash response.

Aquatic tiger salamanders, Ambystoma tigrinum, were killed by stunning followed by decapitation and rostral and caudal pithing. Recordings were made from isolated red-sensitive cones drawn inner segment first into a suction pipette.

If the outer segment of a red-sensitive cone was stepped to 0Ca2+/0Na+ solution just before a bright flash and returned to Ringer shortly before recovery then response saturation was prolonged in comparison to the response in Ringer, increasing linearly by 0.50 ± 0.07 (least squares regression, n = 9 cells) of the time spent in this solution. Furthermore, if the cone was pre-exposed to steady subsaturating light, thereby reducing Ca2+i, the prolongation of the bright flash response evoked by 0Ca2+/0Na+ solution decreased progressively with increasing background intensity. These results resemble those obtained from rods which have been bleached and regenerated with 11-cis-9-demethylretinal, which prolongs Ca2+-dependent photopigment quenching so that it dominates response recovery (Matthews et al. 2001). We therefore conclude that in cones the time constant that normally dominates response recovery is controlled by Ca2+i in a graded manner. During adaptation to steady light the accompanying fall in Ca2+i will act to accelerate the dominant time constant in cones, thereby contributing to the speeding of response recovery.

This work was supported by the Wellcome Trust.

Where applicable, experiments conform with Society ethical requirements