Calcium modulates basal phosphodiesterase activity in mouse rod photoreceptors

Abstract

Purpose : Absorption of photon to rhodopsin in rod photoreceptors leads to an increase in the catalytic activity of phosphodiesterase (PDE) and rapid cGMP hydrolysis. In addition to brief stimuli, rods have to adapt to varying lighting conditions. Fast light adaptation in rods is mainly accomplished by regulating light sensitivity and response kinetics through calcium sensor proteins: recoverin modulates the lifetime of active rhodopsin and GCAPs regulates the rate of cGMP synthesis. However, when these proteins are knocked out, some calcium-dependent regulation remains. Here we tested the hypothesis that the basal PDE activity (β_dark), one of the main factors setting the kinetics of photoresponse recovery, might be modulated by calcium ions.

Methods : PDE inhibitors decrease the catalytic activity of PDE. A decrease in the basal PDE activity can be compensated by introducing a corresponding increase in the PDE activity with light, effectively keeping the [cGMP]_in at its dark value (cGMP clamp). When certain phototransduction parameters and the PDE inhibitor efficiencies are known, the β_dark can be calculated from the amount of light needed for the compensation. Electroretinography (ERG) across the rod outer segment layer was used for recording of rod signal and a PID-control feedback from the ERG signal to stimulus light enabled the cGMP clamp. Recordings were conducted on isolated GCAPs^-/- recoverin^-/- mouse retinas in HEPES buffered solution containing either normal (1 mM) or low (≈20 nM) free calcium concentration ([Ca²⁺]). IBMX was used as the PDE inhibitor.

Results : Lowering [Ca²⁺] led to a 30% desensitization of rods without having significant effect on response activation (Fig. 1). In our cGMP clamp experiments, β_dark shifted from 4.3 ± 0.5 s^-1 in normal [Ca²⁺] (n=9) to 5.7 ± 0.5 s^-1 in low [Ca²⁺] (n=7) (Fig. 2). This effect was statistically significant.

Conclusions : Calcium modulates basal PDE activity in mouse rod photoreceptors. Lowering [Ca²⁺] caused an increase in β_dark that was accompanied by an acceleration of flash response shutdown and desensitization of rods.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Population averaged dim flash responses in normal and low [Ca²⁺] (n=10).

 

Determination of β_dark with cGMP clamp. The inhibition constant for IBMX, K_I,apparent, was determined separately in normal [Ca²⁺] and in low [Ca²⁺]. The error bars indicate SEMs.