Cobalt Can Replace Calcium in Mouse Rod Phototransduction and Light Adaptation

Abstract

Purpose:: To investigate the properties of Ca²⁺ -sensitive mechanisms in mouse rod phototransduction and light adaptation by comparing the actions of Ca²⁺ and Co²⁺ on rod photoresponses.

Methods:: Photoresponses were recorded with ERG technique across isolated mouse (Mus musculus) retinas at 25 ^oC. The retinas were perfused at the photoreceptor side. The b-wave and higher-order neuron components were blocked with 2 mM aspartate. The glial component (slow PIII) was removed by adding barium (10 mM BaCl₂) into the electrode space in contact with the proximal side of the retina. Photoresponses were recorded 1) in normal Ringer solution (1 mM Ca²⁺) 2) in Ringer with 1 mM Co²⁺ substituted for 1 mM Ca²⁺ (cobalt solution), and 3) in Ringer with very low (~ 100 nM) free Ca²⁺ (low Ca²⁺ solution). The calcium-dependent mechanisms underlying photoresponse termination and light adaptation were probed by 1) flashes and steps of light, 2) steps of light followed by an immediate saturating 20 ms flash of fixed intensity (the "step/flash paradigm", Fain et. al. 1989), and 3) determining the dominant time constant of recovery T_C (the "Pepperberg plot", Pepperberg et. al. 1992).

Results:: Saturated photoresponse amplitudes and fractional sensitivity were larger while small-stimulus photoresponse kinetics was slightly slower in cobalt solution compared to responses in normal Ringer. Rod responses to steps of light showed a prominent relaxation after the initial peak both in normal Ringer and in cobalt solution. This relaxation was not present in low Ca²⁺ solution. In "step/flash" experiments, the recovery phase of flash responses was accelerated with increasing step intensity similarly in normal Ringer and in cobalt solution, but not in low Ca²⁺. The dominant time constant T_C was approximately the same (0,35 - 0,4 s at 25 ^oC) in normal Ringer and in cobalt solution, while in low Ca²⁺ it was increased to ca. 0,8 s.

Conclusions:: Our results show that calcium can be replaced by cobalt in most if not all the calcium-dependent mechanisms of response termination and light-adaptation. These include the modulation of guanylate cyclase (GC) by GCAP and the recoverin-dependent modulation of light-activated rhodopsin lifetime. Our data also indicate that cobalt can permeate the cGMP-gated channels and that cobalt can be extruded by the Na⁺/Ca²⁺,K⁺ exchanger.

Keywords: calcium • photoreceptors • electrophysiology: non-clinical