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First published online October 21, 2005
Journal of Experimental Biology 208, 4035-4047 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01874

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Oxygen delivery to the fish eye: Root effect as crucial factor for elevated retinal P_O2

W. Waser^* and N. Heisler

Department of Animal Physiology, Humboldt-Universität zu Berlin, 10115 Berlin, Germany

^* Author for correspondence (e-mail: wolfgang.waser{at}rz.hu-berlin.de)

Accepted 6 September 2005

Although the retina has one of the highest metabolic rates among tissues, certain teleost fishes lack any vascular supply to this organ which, in combination with the overall thickness of the organ, results in extremely long diffusion distances. As the only way to compensate for these obstacles, oxygen partial pressure (P_O2) in the eyes of such fish is elevated far above atmospheric values. Although not supported by any direct evidence, the enhancement of P_O2 is considered to be related to the Root effect, the release upon acidification of Hb-bound O₂ into physical dissolution, possibly supported by counter-current multiplication similar to the loop of Henle.

The present study evaluates the magnitude of intraocular P_O2 enhancement under tightly controlled physiological conditions, to directly confirm the involvement of the Root effect on intraocular P_O2 in the retina of rainbow trout Oncorhynchus mykiss. Intraocular P_O2 was determined with special polarographic microelectrodes inserted into the eye. P_O2 profiles established in vivo by driving electrodes through the entire retina yielded average P_O2 values between 10 mmHg (1.3 kPa) at the inner retinal surface and 382 mmHg (50.9 kPa) close to the outer retinal limit (Bruch's membrane). According to estimates on the basis of the diffusion distances determined from sections of the retina (436 µm at the site of P_O2 measurement) and literature data on specific oxygen consumption, the in vivo determined values would be sufficient to cover the oxygen demand of the retina with some safety margin.

For a clear and direct in-tissue-test as to the involvement of the Root effect, an isolated in vitro eye preparation was established in order to avoid the problem of indirect blood supply to the eye from the dorsal aorta only via the pseudobranch, a hemibranch thought to modulate blood composition before entry of the eye. Any humoral effects (e.g. catecholamines) were eliminated by perfusing isolated eyes successively with standardized red blood cell (RBC) suspensions in Ringer, using trout (with Root) and human (lacking any Root effect) RBC suspension. To optimize perfusate conditions for maximal Root effect, the Root effect of trout RBCs was determined in vitro via graded acidification of individual samples equilibrated with standardized gas mixtures. During perfusion with trout RBC, P_O2 at the outer retinal limit was 99 mmHg (13.2 kPa), but fell by a factor of 3.3 upon perfusion with human RBC in spite of higher total oxygen content (T_O2 2.8 for trout vs 3.9 mmol l^-1 for human RBC). Upon reperfusion with trout RBC, P_O2 was restored immediately to the original value. This regularly observed pattern indicated a highly significant difference (P=0.003) between perfusion with trout (with Root effect; high retinal P_O2) and perfusion with human (no Root effect; low retinal P_O2) RBC suspension, thus clearly demonstrating that the Root effect is directly involved and a crucial prerequisite for the enhancement of P_O2 in the retina of the teleost eye.

Key words: rainbow trout, Oncorhynchus mykiss, ocular oxygen partial pressure, avascular retina

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