Diel changes in endolymph aragonite saturation rate and mRNA expression of otolith matrix proteins in the trout otolith organ

Abstract

In the teleost fish otolith, the alternate deposition of CaCO 3-rich and protein-rich layers results in the formation of daily increments. In order to clarify the mechanism of daily increment formation, a precise understanding of the relationship between ionic and organic controls of otolith growth is essential. In the present study, we studied diel variations in the aragonite saturation rate (Sa) of the endolymph and the mRNA expression of 2 major otolith matrix proteins, OMP-1 and otolin-1, in the saccular tissue. A new technique for simultaneously quantifying endolymph S a and mRNA expression in the saccular tissue from an individual rainbow trout was developed. Endolymph Ca2+ levels, CO2 partial pressure (PCO2) and pH were simultaneously measured using an automatic pH/gas/electrolyte analyzer, and the Sa was calculated. Total RNA was isolated from sacculi of individual fish after the endolymph was obtained. cDNAs were synthesized and used to quantify OMP-1 and otolin-1 mRNA expression levels by real-time PCR. Significant diel variations were observed in endolymph pH and PCO2 levels in an antiphasic manner. Endolymph Sa did not exhibit significant diel variations and was maintained at a value of more than 2.0, indicating that the endolymph was kept supersaturated with respect to aragonite during the day-night cycle. Expression of otolin-1 mRNA had apparent diel variations with high levels at night, whereas that of OMP-1 mRNA was almost constant. These data strongly suggest an organic control of daily increment formation in the otolith. The most probable candidate protein for daily increment formation is otolin-1. © Inter-Research 2005.