Cardiovascular changes and physiological response during nitrite exposure in rainbow trout

Nitrite-exposed (1 mM) rainbow trout Oncorhynchus mykiss fell into two dist inct groups with regard to susceptibility and physiological response. Group 1 accumulated nitrite in plasma to a concentration of 2.9 mM within 24 h a nd died before 48 h. Group 2 survived for 96-144 h. and the accumulation of nitrite was slower, levelling off at a concentration c. 2.3 mh 1 at 72 h. Methaemoglobin (metHb) formation was faster in group 1 than in group 2. but both groups had a metHb fraction 1. 70% before dying. The extracellular el ectrolyte balance was perturbed significantly only in group 1. where plasma [Cl-] decreased and plasma [K+] increased. Heart rate increased rapidly, m ore in group 1 than in group 2. The tachycardia occurred before any signifi cant changes in metHb or [K+] had developed. suggesting that it was due to nitrite-induced vasodilation, possibly via nitric oxide generated from nitr ite, that was countered by an increased cardiac pumping to re-establish blo od pressure. Arterial blood pressure and pulse pressure were accordingly ke pt reasonably constant. Heart rare variability was significantly depressed in group I. The ventilation rate was significantly increased fter 9 h of ni trite exposure in group 1. while the ventilation in group 2 did not increas e significantly before 21 h. The data reveal that nitrite has substantial i nfluence on the cardio-respiratory function in fishes (C) 2001 The Fisherie s Society of the British Isles.