Modulation of Dim Flash Responses by Lowered Extracellular Calcium in GCAP-/- Mouse Rods

Abstract

Purpose: : Reducing extracellular [Ca²⁺] leads to a large transient increase of the photoreceptor outer segment current together with persistent deceleration and desensitization of dim flash responses (Yau et al., Nature 292:502-5; Bastian & Fain, J. Physiol. 330:307-329). These effects are probably caused either directly or indirectly by highly increased [cGMP] during low Ca²⁺ treatment in darkness (Matthews, J. Physiol. 484(2): 267-286), thus limiting the use of low [Ca²⁺]_out in investigating Ca²⁺ dependent light adaptation mechanisms. Here GCAP-/- mouse rods are used to investigate how lowered [Ca²⁺]_out modulates dim flash responses when the Ca²⁺-dependent acceleration of cGMP synthesis is absent.

Methods: : Rod photoreceptor flash (2 - 20 ms) responses were recorded with transretinal ERG from WT (C57Bl/6) and GCAP-/- mouse retinas in 1 mM [Ca²⁺]_out and in EGTA buffered ~30 nM [Ca²⁺]_out. HEPES buffered Ringer solution with 2 mM aspartate was used to remove b-wave and higher-order neuron components. The glial component (slow PIII) was removed with BaCl₂. Stability of the responses, especially under low [Ca²⁺]_out, was facilitated by conducting the experiments at 25^oC.

Results: : The saturated photoresponse amplitude of WT rods was 135 ± 20 µV in 1 mM [Ca²⁺]_out and it transiently exceeded 1 mV, reaching a steady state value of 340 ± 40 µV when the retinas were exposed to low [Ca²⁺]_out (n = 10). The fractional dim flash response sensitivity decreased from 3.1 ± 0.6 %/R* to 1.0 ± 0.2 %/R* and the time for the dim flash response to reach the peak amplitude (t_p) increased from 310 ± 20 to 560 ± 70 ms as the retinas were switched from 1 mM Ca²⁺ to low 30 nM Ca²⁺ perfusion. In GCAP-/- rods the low calcium treatment did not increase the saturated photoresponse amplitude significantly. Moreover, on the contrary to the WT retinas, the t_p of the GCAP-/- rod responses decreased from 670 ± 40 to 470 ± 10 ms when switched from 1 mM to 30 nM [Ca²⁺]_out (n = 2). The accelerated dim flash response kinetics in GCAP-/- rods was accompanied by ~4 -fold decrease in fractional sensitivity. In preliminary simulations, these changes in GCAP-/- rods under low [Ca²⁺]_out could be accounted for with ~4-5-fold reduction in R* lifetime.

Conclusions: : The GCAP-/- rods can be used to study how [Ca²⁺]_out modulates phototransduction in mammalian rods. Our results indicate that dim flash responses are significantly shaped by lowered extracellular [Ca²⁺] in the absence of GCAPs, most probably through shortening of R* lifetime.

Keywords: photoreceptors • electrophysiology: non-clinical • signal transduction: pharmacology/physiology