Feed the Future Innovation Lab for Collaborative Research on Aquaculture & Fisheries

Glucocorticoids (GCs) regulate an array of physiological responses in vertebrates. Genomic GC actions mediated by nuclear steroid receptors require a lag time on the order of hours to days to generate an appreciable physiological response. Experimental evidence has accumulated that GCs, can also act rapidly through a non-genomic mechanism to modulate cellular physiology in vertebrates. Causal evidence in the Mozambique tilapia (Oreochromis mossambicus) suggests that the GC cortisol exerts rapid, non-genomic actions in the gills, liver, and pituitary of this euryhaline teleost, but the membrane receptor mediating these actions has not been characterized. Radio-receptor binding assays were conducted to identify a putative GC membrane receptor site in O. mossambicus. The tissue distribution, binding kinetics, and pharmacological signature of the GC membrane-binding activity were characterized. High affinity (Kd=9.527 , 0.001nM), low-capacity (Bmax = 1.008 , 0.116fmol/mg protein) [3H] cortisol binding was identified on plasma membranes prepared from the livers and a lower affinity (Kd=30.08 , 2.373nM), low capacity (Bmax = 4.690 , 2.373 fmol/mg protein) binding was found in kidney membrane preparations. Competitors with high binding affinity for nuclear GC receptors, mifepristone (RU486), dexamethasone, and 11-deoxycorticosterone, displayed no affinity for the membrane GC receptor. The association and dissociation kinetics of [3H] cortisol binding to membranes were orders of magnitude faster (t1/2=1.7-2.6min) than those for the intracellular (nuclear) GC receptor (t1/2=10.2h). Specific [3H] cortisol membrane binding was also detected in the gill and pituitary but not in brain tissue. This study represents the first characterization of a membrane GC receptor in fishes and one of only a few characterized in vertebrates.

NOP 13-321 (English)