Kinetics of phototransduction in retinal rods of the newt Triturus cristatus.

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Abstract
The kinetics of photoresponses to flashes and steps of light of rods, from the retina of the newt Triturus cristatus, were analysed by recording the membrane current with a suction electrode. In dark-adapted conditions the relation between the normalized amplitude of the photoresponse at a fixed time 1 s after the onset of light and the light intensity could be fitted by an exponential or a polynomial relation. In the presence of a steady bright light the same relation could be fitted by a Michaelis-Menten relation. The kinetics of photoresponses to light stimuli were reconstructed using a model in which: (i) three molecules of guanosine 3'',5''-cyclic monophosphate (cyclic GMP) open a light-sensitive channel; (ii) light activates the enzyme phosphodiesterase, which hydrolyses cyclic GMP, thus closing light-sensitive channels; (iii) Ca2+ ions permeate through light-sensitive channels; and (iv) intracellular Ca2+ inhibits, in a co-operative way, the enzyme cyclase, which synthesizes cyclic GMP. The model reproduces the shortening of the time to peak of brief flash photoresponses from about 1080 ms to about 690 ms with brighter lights. The model also explains the shortening of the time to peak to 350 ms observed in the presence of a steady light and the lack of a further acceleration with brighter flashes of lights. The presence in the model of an intracellular calcium buffer accounts for the partial reactivation of the photocurrent following a step of light, lasting several seconds. The time course of this reactivation is not accelerated by a steady bright light both experimentally and in the model. After the extinction to a long step of light the photocurrent showed a rapid partial reactivation, which was followed by a slow component of the photoresponse which extinguished with a rate constant of about 0.05 s-1. The model exlains the origin of this slow component by assuming that the inactivation of excited rhodopsin is partially reversible. The model is also able to explain the particular chnges of kinetics when different amounts of exogenous calcium buffers are incorporated into rods (Torre, Matthews and Lamb, 1986).