THE EFFECTS OF NITRITE ON CHLORIDE REGULATION IN THE CRAYFISH PACIFASTACUS-LENIUSCULUS DANA (CRUSTACEA, DECAPODA)

Nitrite in the external freshwater medium was found to be toxic to Pacifastacus leniusculus Dana (48 h LC50 approximately 0.7 mM NO-2). It produced significant changes in haemolymph ionic concentration and acid-base status. Exposure to 1.0 mM NO2- resulted in a rapid, active accumulation of nitrite in the haemolymph (to 25 mM NO2- after 24 h) and caused the partial inhibition of Cl- uptake. Some reduction in Cl- efflux rate was seen. In 1.0 mM NO2- a rapid depletion of haemolymph [Cl-] was observed (approximately 50 mM decrease in 27 h). Nitrite competitively inhibited active Cl- uptake (K(m) increased from 0.42 to 1.22 mM; K(i) = 0.45 mM). To achieve Cl- balance in this medium, depleted crayfish would require a two-fold increase in external [Cl-]. A lesser decrease in haemolymph [Na+] was found while osmotic pressure was relatively unaffected. Haemolymph [HCO3-] showed a significant increase and was accompanied, unexpectedly, by an acidosis. Possible sources of the excess HCO3-, perhaps by inhibition of normal Cl-/HCO3- branchial exchange or release from CaCO3 stores, are discussed. Haemolymph clearance of NO2- was slower than uptake as was the restoration of [Cl-] on recovery in nitrite-free medium.