Effects of prolonged copper exposure in the marine gulf toadfish (Opsanus beta) II: Copper accumulation, drinking rate and Na⁺/K ⁺-ATPase activity in osmoregulatory tissues

Gulf toadfish were exposed to sublethal levels of copper (12.8 or 55.2 μM) for 30 days. Drinking in control fish averaged 1 ml kg^-1 h^-1 but exposure to 55.2 μM copper resulted in a complex biophasic pattern with initial (3 h and 1 day) inhibition of drinking rate, followed by an elevation of drinking rate from day 3 onwards. Drinking led to copper accumulation in the intestinal fluids at levels three to five times higher than the ambient copper concentrations, which in turn resulted in intestinal copper accumulation. The gill exhibited more rapid accumulation of copper than the intestine and contributed to early copper uptake leading to accumulation in internal organs. Muscle, spleen and plasma exhibited little if any disturbance of copper homeostasis while renal copper accumulation was evident at both ambient copper concentrations. The liver exhibited the highest copper concentrations and the greatest copper accumulation of all examined internal organs during exposure to 55.2 μM. Elevated biliary copper excretion was evident from measurements of gall bladder bile copper concentrations and appeared to protect partially against internal accumulation in fish exposed to 12.8 μM copper. No inhibition of Na⁺/K⁺-ATPase activity in either gills or intestine was seen despite copper accumulation in these organs. Calculations of inorganic copper speciation suggest that Cu(CO ₃)₂^2- complexes which dominate in seawater and intestinal fluids are of limited availability for uptake while the low levels of ionic Cu²⁺, CuOH⁺ and CuCO₃ may be the forms taken up by the gill and the intestinal epithelium.