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

Abstract

Abstract: : Purpose: The shutoff of the transduction cascade in rods is dominated by a time constant of around 2 s (Pepperberg et al., (1992) Vis. Neurosci. 8, 9) which is unaffected by [Ca²⁺]_i (Lyubarsky et al., (1996) J. Gen. Physiol. 107, 19) and which may represent GTP hydrolysis by transducin (Sagoo & Lagnado, (1997) Nature 389, 392). Instead, in rods, [Ca²⁺]_i acts on a step early in transduction with a time constant of ∼0.5 s (Matthews, (1997) J. Gen. Physiol. 109, 141) which appears to represent rhodopsin quenching (Matthews et al., (2001) J. Gen. Physiol. 118, 377). In contrast, response recovery in cones is poorly understood. We have therefore investigated the Ca²⁺–dependence of the dominant time constant in salamander cones. Methods: Recordings were made from isolated red–sensitive salamander cones drawn inner segment first into a suction pipette. The dynamic fall in [Ca²⁺]_i which normally accompanies the flash response was delayed by rapidly superfusing the outer segment with a 0Ca²⁺/0Na⁺ solution designed to minimise surface membrane Ca²⁺ fluxes. Results: If the cone outer segment 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 (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. Conclusions: These results resemble those obtained from bleached rods regenerated with 11–cis–9–demethylretinal, which prolongs Ca²⁺–dependent photopigment quenching so that it dominates response recovery (Matthews et al., (2001) J. Gen. Physiol. 118, 377). We therefore conclude that in cones the time constant that normally dominates 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 speeding response recovery.

Keywords: photoreceptors • calcium