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In vivo cGMP levels in frog photoreceptor cells as a function of light exposure

1986; Elsevier BV; Volume: 43; Issue: 5 Linguagem: Inglês

10.1016/s0014-4835(86)80004-5

1096-0007

Elizabeth Barbehenn, Kathleen L. Klotz, Dierdre M. Noelker, Ralph Nelson, Gerald J. Chader, Janet V. Passonneau,

Drug Transport and Resistance Mechanisms

By employing a combination of highly sensitive radioimmunoassays and histochemical techniques, an in vivo time course of cGMP levels has been determined in the outer segment, photoreceptor cell and outer plexiform layers of frog retina. Frogs (Rana pipiens) were darkadapted overnight and either frozen rapidly (approximately 3 sec) in liquid nitrogen or exposed to periods of light varying between 0·1 sec and 2 hr before freezing. Frozen retinal sections were cut, freeze-dried, and samples of individual layers dissected out and analysed for cGMP. In the outer plexiform layer, there was a 42% drop in cGMP concentration after 2 sec of light (250 ft candles) followed by a 34% rise after 2 min; a steep concentration gradient formed around the layer after the 2 min exposure. In both the outer-segment layer and photoreceptor-cell layer (which includes outer segments, inner segments and outer nuclear layers), cGMP levels declined from a dark value of 56 μmol kg−1 (dry) to 9 μmol kg−1 (dry) as a result of increasing exposure to several types of light source: levels appear to be primarily a function of total ft candle min. Cyclic GMP concentrations at the longest exposures (2 min with a fiber optie light source or 2 hr with fluorescent room light) reached identical minimum levels. In the outer segments, a 15% decrease in cGMP was observed after 0·1 see of light exposure. Although the freezing time is too long to be able to say whether the 15% decrease in cGMP at the 0·1 see exposure is involved in transduction, the low identical levels reached gradually after longer exposures appear to indicate that a light-induced biochemical adjustment in cGMP metabolism occurs over a relatively long time period separate from the msec time course of the transduction process.