cAMP AFFECTS THE LEVEL OF THE BASAL (DARK) ACTIVITY OF PHOSPHODIESTERASE IN VERTEBRATE PHOTORECEPTORS.
Astakhova L.A., Govardovskii V.I., Firsov M.L.
Sechenov Institute for Evolutionary Physiology and Biochemistry RAS, St.- Petersburg, Russia; firstname.lastname@example.org
Earlier we have shown that a change in the level of cAMP have a regulating effect on the cascade of phototransduction. Partially the effect of cAMP on the phototransduction cascade is implemented through changes in intracellular calcium concentration. The increase in [Ca2+]in induced by increase in [cAMP]in affects the rhodopsinkinase, guanylate cyclase and cGMP-dependent channels of plasma membrane. In addition, the increase in [cAMP]in decreases basal (dark) activity of cGMP-specific phosphodiesterase type 6 (PDE). The level of basal activity of PDE depends on three parameters - the rate of spontaneous activation and inactivation of the PDE, and the maximum catalytic activity of a single molecule of PDE. According to modern concepts, none of these three parameters is calcium-dependent, so the most likely cAMP affects PDE function via one of the known mechanisms: cAMP-dependent protein kinase-A (PKA) or protein EPAC.
Among a few possible ways the study of PDE in the intact cell, one is an analysis of the power spectrum of the photoreceptor current noise (current variations around an average level) in the dark. It is known that below 1 Hz noise of the dark current is mainly determined by the PDE activity (Rieke & Baylor, 1996). Comparative analysis of the photoreceptor dark current noise has revealed that elevation of [cAMP]in increases the rate constant of spontaneous activation of PDE, while the rate of inactivation of PDE does not change. The observed effect of cAMP is likely mediated by PKA but not EPAC, as EPAC-selective cAMP analog 8-pCPT-2'-O-Me-cAMP do not reproduce this effect. Mechanism(s) of the effect of cAMP on the kinetic parameters of the PDE is currently unknown and requires further study.
This work was supported by RFBR grant 11-04-00892 for MLF.