Temperature Effects on Trimethylamine Oxide Accumulation and the Relationship Between Plasma Concentration and Tissue Levels in Smelt (Osmerus mordax) JASON R. TREBERG 1n , JASON S. BYSTRIANSKY, 2 AND WILLIAM R. DRIEDZIC 1 1 Ocean Sciences Centre, Memorial University of Newfoundland St. John’s, Newfoundland, Canada A1C 5S7 2 Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1 ABSTRACT Rainbow smelt (Osmerus mordax) were maintained in a long term acclimation study to elucidate temperature effects on the accumulation of trimethylamine oxide (TMAO) and to determine if the activity of trimethylamine oxidase (TMAoxi) plays a role in modulating the seasonally variable levels of TMAO. In the same experiment, the TMAO content was determined for several tissues at varying plasma TMAO concentrations. TMAO accumulation begins at 5–71C, well above that which might be normally associated with an antifreeze response. The plasma concentration reached a plateau of 20 mM as temperatures reached 01C. Plasma TMAO concentration drops to pre-accumulation levels, less than 5 mM, when fish are held at elevated temperature (8–111C) and increases when fish are chilled below ambient seawater temperatures. However, despite temperatures near or below 01C, plasma TMAO decreases after the winter season. Changes in TMAoxi activity do not correlate with TMAO levels, suggesting that the activity of this enzyme does not play a key role in regulating TMAO concentrations in smelt. For the first time in any teleost fish, tissue TMAO contents in liver, kidney, brain, and intestine were found to strongly correlate with plasma TMAO concentrations. For these tissues, the intracellular and extracellular concentration of TMAO appears to be approximately equal. Conversely, the heart and white muscle accumulate TMAO, and in the case of white muscle, intracellular concentration is maintained at a constant level of approximately 35 mmol/kg, despite fluctuating plasma concentrations over a range from 0 to over 25 mM. J. Exp. Zool. 303A:283–293, 2005. r 2005 Wiley-Liss, Inc. INTRODUCTION Trimethylamine oxide (TMAO) is a small nitrogenous solute found in most, if not all, marine teleost fishes with muscle contents fre- quently in the range of 20–60 mmol/g (see Hebard et al., ’82 for review). Despite the near ubiquitous occurrence of TMAO in marine teleosts, little is known of its metabolism or the regulation of tissue levels. Several species of teleosts found at tempe- rate and polar latitudes have unusually high levels of TMAO in serum and liver during periods of near and subzero temperatures (Hebard et al., ’82; Raymond, ’94; Raymond and DeVries, ’98). These species may be exploited to provide insights into the regulation of this compound. Rainbow smelt (Osmerus mordax) elevate plas- ma TMAO during periods of low temperature associated with winter (Raymond, ’94; Treberg et al., 2002). A previous study partially articulated a seasonal oscillation and the magnitude of this TMAO increase in smelt (Treberg et al., 2002). Plasma TMAO concentration was elevated by mid- December when ambient temperature was ap- proximately 51C, and increased at lower winter temperatures, but maintaining the temperature at approximately 51C resulted in a gradual decrease in concentration. This past study was deficient in that the very early changes in TMAO accumu- lation that presumably occur in the autumn period were not documented. As such, a primary Grant support: Natural Sciences and Engineering Research Council of Canada; Grant support: AquaNet, a Network of Centres of Excellence n Correspondence to: Jason R. Treberg, Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, NL. Canada– A1C 5S7. E-mail: jtreberg@mun.ca Received 27 February 2004; Accepted 22 October 2004 Published online in Wiley Interscience (www.interscience.wiley. com). DOI: 10.1002/jez.a.140 r 2005 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 303A:283–293 (2005)