Nitrite and Red Cell Function in Carp: Control Factors for Nitrite Entry, Membrane Potassium Ion Permeation, Oxygen Affinity and Methaemoglobin Formation

Abstract

ABSTRACT Red cell function was studied in carp by a combination of in vivo and in vitro experiments with nitrite as the perturbing agent. In vivo accumulation of nitrite caused a marked increase in the red cell methaemoglobin content, and reduced the mean cellular volume. The oxygen affinity of unoxidized haemoglobin was strongly decreased, partly as result of the elevated concentration of cellular nucleoside triphosphates and haemoglobin associated with red cell shrinkage. Red cell pH was unchanged compared to controls, but reduced when referred to constant extracellular pH and O2 saturation. The mean cellular K+ content decreased, reflecting a K+ loss from the red cells during their shrinkage. This K+ loss contributed significantly to the large plasma hyperkalaemia during nitrite exposure. In vitro experiments revealed that nitrite influx into deoxygenated red cells was much larger than into oxygenated red cells. Nitrite permeation of the red cell membrane was not inhibited by DIDS and did not change extracellular pH. Methaemoglobin (MetHb) formation was more pronounced in deoxygenated blood than in oxygenated blood, but quasi-steady states were reached, reflecting a balance between nitrite-induced MetHb formation and the action of MetHb reductase. Red cells incubated in the oxygenated state released K+, whereas a net K+ uptake occurred in deoxygenated cells. Nitrite did not change the K+ loss from oxygenated cells, but shifted the K+ uptake in deoxygenated cells to a pronounced K+ release by the time high MetHb levels were reached. Both types of red cell K+ release were inhibited by DIDS and appeared to occur via a route involving Band 3. The data are consistent with the hypothesis that a significant DIDS-sensitive K+ efflux from the red cells occurs whenever a large fraction of the haemoglobin molecules assumes an R-like quaternary structure.