Changes in Nitrogen excretion pattern in Channa gachua fishes kept in a water-scarce condition

Abstract

Freshwater fishes, typically ammonotelic, excrete highly toxic ammonia directly into the water, but many species shift to urea excretion under water-restricted conditions to avoid internal toxicity. This study investigated how Channa gachua adapts its nitrogen metabolism—shifting from ammonotelism to ureogenesis—to survive water scarcity or environmental stress. A total of 100 (one hundred) Channa gachua (an air-breathing teleost) were used to study the effects of water restriction. Over a 10-day period, the metabolic response to water restriction was tracked. Both blood and excreted urea and ammonia levels were measured to track the shift in nitrogen excretion. Hepatic glutamate dehydrogenase (GLDH or GDH) activity was measured to evaluate how the liver converts amino acids and ammonia into urea during stress. Environmental conditions, including alkalinity, pH, dissolved oxygen, and free carbon dioxide, were also monitored to simulate habitat degradation. During the experimental period, Channa gachua showed a decreasing trend of ammonia excretion (r = 0.9626, in correlation with duration).However, excretion of urea, in Channa gachua showed significant correlation with increasing duration (r = 0.936). Absence of not so prominent correlation of GLDH with duration in the fishes (r = 0.501) was also observed. Only a moderate degree of correlation was observed between the activities of glutamate dehydrogenase and excretory ammonia and urea. Though a very high correlation was found between free carbon dioxide and excretory ammonia and urea, but no correlation was found between excretory ammonia and urea with alkalinity of the surrounding medium. The study found that Channa gachua triggers ureogenesis as a protective mechanism under water-restricted, stressful conditions to prevent the accumulation of toxic ammonia. The results of the present study may suggest possible impact of stress factors on physiological changes regarding nitrogen excretion in these experimental fishes and the role played by the enzyme glutamate dehydrogenase in regulating the entire mechanism.