CARDIORESPIRATORY FUNCTION IN CARP EXPOSED TO ENVIRONMENTAL NITRITE

Continuous monitoring of heart rate, breathing episodes and blood pres sure showed that the cardio-respiratory response of carp exposed to ni trite (water concentration, 1 mmol l(-1)) changes with length of expos ure. The animals developed a severe methaemoglobinaemia over the first 24 h of nitrite exposure. The minor changes in plasma HCO3- and lacta te concentration, suggest that the observed hyperventilatory response was sufficient to maintain aerobic metabolism throughout most of the b ody during this time. During the second 24-h period, the rate of breat hing increased further and short periods of bradycardia and hypotensio n were seen. Over this latter period, the animals increased their use of anaerobic metabolism as illustrated by the mean 48 h blood lactate concentration of 4.8 mmol l(-1), a greater than 10-fold increase over pre-exposure values. The increase in blood lactate was accompanied by the predicted metabolic acidosis, however, an alkalosis of respiratory origin and buffering combined to keep the plasma pH absolutely stable throughout the study. This study shows that as the blood oxygen suppl y is reduced through the development of methaemoglobinaemia, cardio-re spiratory compensation by the carp is probably adequate to maintain ti ssue oxygenation for short periods of nitrite exposure. However, as ni trite exposure proceeds past 24 h, the animals progress into a positiv e feedback cycle where the high cost of additional ventilation rapidly accelerates their oxygen deficit which cannot be repaid, because <25% of their haemoglobin is available for oxygen binding. Additionally, o ur data demonstrate a circadian rhythm of physiological response to ni trite and contradict the hypothesis that catecholamine release promote s CO2 retention in water breathing animals. (C) 1997 The Fisheries Soc iety of the British Isles