Effects of nitrite exposure on blood respiratory properties, acid-base and electrolyte regulation in the carp (Cyprinus carpio)

Adult carp were subjected to 1 mM environmental nitrite for 48 h and nitrite uptake and changes in blood respiratory properties, extracellular electrolyte composition and acid-base status were examined. A constant influx of nitrite caused an accumulation of NO ₂ ⁻ in plasma to 5.4 mM in 48 h. The fraction of methaemoglobin rose with plasma [NO ₂ ⁻ ] to 83%, and the arterial oxygen content decreased to extremely low values. Arterial \(P_{{\text{O}}_2 }\) increased as a compensation to this O₂-shortage, whereas the O₂ saturation of the functional (unoxidized) haemoglobin decreased, revealing a reduction in its O₂ affinity. Blood haematocrit decreased as a result of red cell shrinkage, which caused very high red cell haemoglobin (Hb) concentrations. The erythrocytic nucleoside triphosphate (NTP) concentration showed a parallel increase whereby NTP/Hb, as well as the relative contributions of ATP and GTP to NTP, remained unchanged. Plasma [Cl⁻] declined by 15 mM in 48 h, off-setting the plasma [NO ₂ ⁻ ] increase, minor changes in plasma [HCO ₃ ⁻ ] and a considerable increase in plasma [lactate]. Arterial pH and [HCO ₃ ⁻ ] rose slightly during the first 24 h of nitrite exposure, but returned to control values at 48 h. The rise in plasma [lactate] was not reflected in an extracellular metabolic acidosis. Plasma [K⁺] increased by 94% in 48 h, revealing an uncompensated extracellular hyperkalemia, whereas plasma [Na⁺] decreased, and plasma [Ca⁺⁺] was unchanged. Plasma osmolality remained essentially constant. The NO ₂ ⁻ accumulation could be reversed by transfer of the fish to NO ₂ ⁻ -free water, but nitrite ‘off-loading’ was slower than the preceding NO ₂ ⁻ loading.