Development of enzyme immunoassays for 3,5,3'-triiodo-L-thyronine and L-thyroxine: Time-course studies on the effect of food deprivation on plasma thyroid hormones in two marine teleosts, sea bass (Dicentrarchus labrax L) and sea bream (Sparus aurata L)

The effects of short-term food deprivation and photoperiod on plasma thyroid hormone levels of sea bass and sea bream were studied. Animals were acclimated under constant photoperiod regime (15L/9D) and feeding times (2 hr after light onset and 2 hr before light offset). Time-course studies involved monitoring plasma hormone levels every 4 hr throughout 1.5 24-hr cycles. Plasma 3,5,3'-Triiodo-L-thyronine (T-3) and L-thyroxine (T-4) were assayed using a newly developed competitive enzyme immunoassay, utilizing acetylcholinesterase as a label of enzymatic tracers. Enzyme immunoassays had sensitivities of 1.25-0.02 and 62.5-0.2 ng/ml for T-3 and T-4, respectively, and reproducibilities of 3.7 and 5.6% intraassay variation for T-3 and T-4, respectively; interassay variations for T-3 and T-4 assays respectively were 1.6%, 11% and 6.6%, 8% for sea bass and sea bream plasma similar to RIA. In sea bass, 3 days of food deprivation resulted in depressed plasma T-3 and T-4, overriding significant diel variations seen during the second day of starvation. Sea bream displayed a slight decrease of T-4 plasma levels while T-3 levels remained constant for the whole sampling period. Both thyroidal systems responded to photoperiod with a significant increase in plasma T-4 level at the time of light onset. In addition, sea bass also displayed increased T-3 levels and decreases in both hormone levels coinciding with ''light off.'' Data show different responses of the sea bass and sea bream thyroidal systems to both nutritional state and photoperiod in that the latter state is influenced by the former. Data suggest plasma thyroid levels can be used asa rapid indicator of nutritional status. (C) 1996 Academic Press, Inc.