Article 6

Computerized videomicroscopy; Photoreceptor rods orientation; Ouabaine; Dark current

It was shown earlier [E. Kovacs, T. Savopol, A. Dinu, The polar behavior of frog photoreceptors, Biochim. Biophys. Acta, 1273 (1995) 217-222] that intact photoreceptor rod outer segments (ROS) are permanent dipoles, their polarity being, very probably, related to the dark current which flows along the cell in dark-adapted, living specimens [R.D. Penn, W.A. Hagins, S. Yoshikami, Dark current and photocurrent in retinal rods, Biophys. J. 10 (1970) 380-412; J.L. Schnapf, Dependence of the single photon response on longitudinal position of absorbtion in toad rod outer segment, J. Physiol. 343 (1983) 147-158] The light induced change in the dark current (the ‘photocurrent’) is the electrophysiological response of the photoreceptor cell to the light stimulus, being large enough to account for the visual system’s ability to detect single photons in the rod outer segments [R.D. Penn, W.A. Hagins, Kinetics of the photocurrent of retinal rods, Biophys. J. 12 (1972) 1073-1094]. The dark current is maintained by cell metabolism and is present only in freshly isolated, dark-adapted, living cells. If the cell polarity is related to the charge accumulation due to the dark current, it must also depend on cell metabolism. In order to check if the observed cell polarity is dependent indeed on cell energy, the sensitivity of photoreceptor orientation ability to Na+/K+-ATP-ase inhibitor, ouabaine was tested. It was found that photoreceptor rods loose their electrical polarity in the presence of ouabaine. That means that cell polarity is dependent on functioning of the membrane ion pumps, which provide a continuous flow of the dark current along the cell. Computerized videomicroscopy catching of photoreceptor cells orientation patterns in static electric fields thus seems to be a possible test for structural and functional integrity of photoreceptor cells to be used before performing elaborated microelectrophysiological measurements. (C) 1998 Elsevier Science S.A. All rights reserved.