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The Journal of Experimental Biology 206, 2685-2692 (2003)
doi: 10.1242/jeb.00479

Daily variations of endolymph composition: relationship with the otolith calcification process in trout

G. Borelli¹, M. E. Guibbolini¹, N. Mayer-Gostan¹, F. Priouzeau¹, H. De Pontual², D. Allemand¹, S. Puverel³, E. Tambutte³ and P. Payan^1,*

¹ Laboratoire Réponse des Organismes aux Stress Environnementaux, UMR INRA-UNSA 1112, Université de Nice-Sophia Antipolis, Faculté des Sciences, BP 71, 06108 Nice Cedex 2, France
² IFREMER, DRV, RH, Laboratoire de Sclérochronologie des Animaux Aquatiques, BP 70, 29280 Plouzané, France
³ Centre Scientifique de Monaco, Avenue Saint-Martin, MC-98000 Monaco, Principality of Monaco

^* Author for correspondence (e-mail: payan{at}unice.fr)

Accepted 7 May 2003

Ionic and organic parameters of the otolith calcification process in the trout Oncorhynchus mykiss were analysed in plasma and endolymph over the day:night cycle. Plasma pH remained constant and total CO₂ concentration was significantly lower (by 21%) during the day than at night. Calcifying parameters (total CO₂, total calcium concentration) were measured in the proximal and distal endolymphs and were unchanged in the latter during the day:night cycle, but fluctuated in the former. Non-collagenous protein and collagen concentrations in endolymph were higher (1.5- and 10-fold, respectively) during the day than at night. As there was no change in total calcium concentration, we propose that Ca²⁺ increases during the dark period and was maximal by the end of the night when the total CO₂ concentration has also increased (by 14%). Measurements of endolymph pH in situ revealed significant differences between samples from proximal and distal endolymph (7.38 and 7.87, respectively), but no variation between values obtained during the day and at night. Thus, the saturation state of aragonite (Sa) in the proximal endolymph should fluctuate around unity during the day:night cycle, and CaCO₃ precipitation should occur when supersaturation is reached. The electrophoretic pattern of proximal endolymph showed variations in both major and minor components. Immunoblotting of endolymph, using a rabbit antiserum raised against the otolith soluble organic matrix revealed an increase in the expression of two proteins (65 kDa and 75 kDa) during the day period. We propose that organic matrix and calcium carbonate deposition on the otolith vary antiphasically: organic matrix deposition begins by the end of the day period, when the concentration of organic precursors is maximal in the endolymph, whereas CaCO₃ precipitation starts once the solubility of CaCO₃ is exceeded.

Key words: otolith, calcification, day–night cycle, endolymph