HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) All Versions of this Article: 567/3/923    most recent jphysiol.2005.090662v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Nymark, S Articles by Koskelainen, A Search for Related Content PubMed PubMed Citation Articles by Nymark, S Articles by Koskelainen, A

¹ Laboratory of Biomedical Engineering, Helsinki University of Technology, FI-02015 HUT, Finland
² Department of Biological and Environmental Sciences, University of Helsinki, FI-00014, Finland

Rod responses to brief pulses of light were recorded as electroretinogram (ERG) mass potentials across isolated, aspartate-superfused rat retinas at different temperatures and intensities of steady background light. The objective was to clarify to what extent differences in sensitivity, response kinetics and light adaptation between mammalian and amphibian rods can be explained by temperature and outer-segment size without assuming functional differences in the phototransduction molecules. Corresponding information for amphibian rods from the literature was supplemented by new recordings from toad retina. All light intensities were expressed as photoisomerizations per rod (Rh*). In the rat retina, an estimated 34% of incident photons at the wavelength of peak sensitivity caused isomerizations in rods, as the (hexagonally packed) outer segments measured 1.7 µm x 22 µm and had specific absorbance of 0.016 µm^–1 on average. Fractional sensitivity (S) in darkness increased with cooling in a similar manner in rat and toad rods, but the rat function as a whole was displaced to a ca 0.7 log unit higher sensitivity level. This difference can be fully explained by the smaller dimensions of rat rod outer segments, since the same rate of phosphodiesterase (PDE) activation by activated rhodopsin will produce a faster drop in cGMP concentration, hence a larger response in rat than in toad. In the range 15–25°C, the waveform and absolute time scale of dark-adapted dim-flash photoresponses at any given temperature were similar in rat and toad, although the overall temperature dependence of the time to peak (t_p) was somewhat steeper in rat (Q₁₀ 4 versus 2–3). Light adaptation was similar in rat and amphibian rods when measured at the same temperature. The mean background intensity that depressed S by 1 log unit at 12°C was in the range 20–50 Rh* s^–1 in both, compared with ca 4500 Rh* s^–1 in rat rods at 36°C. We conclude that it is not necessary to assume major differences in the functional properties of the phototransduction molecules to account for the differences in response properties of mammalian and amphibian rods.

(Received 16 May 2005; accepted after revision 14 July 2005; first published online 21 July 2005)
Corresponding author S. Nymark: Laboratory of Biomedical Engineering, Helsinki University of Technology, PO Box 2200, FI-02015 HUT, Finland. Email: soile.nymark{at}hut.fi

This article has been cited by other articles:

				T. D. Lamb and E. N. Pugh Jr Phototransduction, Dark Adaptation, and Rhodopsin Regeneration The Proctor Lecture Invest. Ophthalmol. Vis. Sci., December 1, 2006; 47(12): 5138 - 5152. [Full Text] [PDF]

				S. Nymark, C. Haldin, H. Tenhu, and A. Koskelainen A new method for measuring free drug concentration: retinal tissue as a biosensor. Invest. Ophthalmol. Vis. Sci., June 1, 2006; 47(6): 2583 - 2588. [Abstract] [Full Text] [PDF]