Plasma Ca2+ concentration limits melatonin night production in two fish species

Abstract

In freshwater (FW) rainbow trout Oncorhynchus mykiss of spontaneously low plasma calcium concentrations ([Ca2+]pl), plasma melatonin at night was significantly lower than that measured in FW fish with the highest [Ca2+]pl. In brackish water adapted rainbow trout with originally high [Ca2+]pl, plasma melatonin concentration at night was elevated. In cannulated flounder Platichthys flesus, night plasma melatonin increases (ΔMel) corresponded to [Ca2+]pl. It is postulated that in physiological steady-state conditions, melatonin synthesis capacity is coupled to free calcium concentration in plasma of O. mykiss and P. flesus. © 2003 The Fisheries Society of the British Isles.