抄録
The outer segment of vertebrate photoreceptors is a sophisticated molecular device with remarkably high sensitivity and flexibility in photic signal reception and transduction. This device can detect single photons when dark-adapted and is also able to adapt to light intensities over a range of more than 6 log units. This device, is only a few picoliters in volume, contains a stack of discs on which 'the enzymatic machine', i.e. the cGMP-cascade translates the photic signal into cGMP breakdown. This eventually elicits closure of ion channels in the plasma membrane, consequent hyperpolarization of the membrane potential and also in the depletion of intracellular Ca2+. This Ca2+ depletion facilitates the recovery of cGMP not only by activating cGMP synthesis but also by desensitizing the cGMP-cascade. This article is a short review of recent findings on the mechanisms of the cGMP cascade and of the Ca2+ mediated regulaion of cGMP metabolism in vertebrate rod photoreceptors.
