Role of ureogenesis in the mud-dwelled Singhi catfish (Heteropneustes fossilis) under condition of water shortage

Abstract

The air-breathing Singhi catfish Heteropneustes fossilis was kept inside moist peat for 1 month mimicking their normal habitat in summer and the role of ureogenesis for their survival in a water-restricted condition was studied. The ammonia excretion rate by the mud-dwelled fish increased transiently between 6 and 12 h of re-immersion in water to approximately between eight and 10-fold, followed by a sharp decrease almost to the normal level at the later part of re-immersion. The urea-N excretion by the mud-dwelled fish increased to approximately 11-fold within 0–3 h of re-immersion, followed by a gradual decrease from 9 h onwards. The rate of urea-N excretion by the mud-dwelled fish, however, remained significantly higher (approx. threefold more) than the control fish even after 36–48 h of re-immersion. Although there was a significant increase of both ammonia and urea levels in the plasma and other tissues (except ammonia in the brain), the level of accumulation of urea was higher than ammonia in the mud-dwelled fish as indicated by the decrease in the ratio of ammonia: urea level in different tissues including the plasma. The activities (units/g tissue and /mg protein) of glutamine synthetase and three enzymes of the urea cycle, carbamyl phosphate synthetase, argininosuccinate synthetase and argininosuccinate lyase increased significantly in most of the tissues (except the brain) of the mud-dwelled fish as compared to the control fish. Higher accumulation of ammonia in vivo in the mud-dwelled Singhi catfish is suggested to be one of the major factors contributing to stimulation of ureogenesis. Due to this physiological adaptive strategy of ureogenesis, possibly along with other physiological adaptation(s), this air-breathing amphibious Singhi catfish is able to survive inside the moist peat for months in a water-restricted condition.

Introduction

The freshwater air-breathing catfishes, Heteropneustes fossilis and Clarias batrachus, found predominantly in the Indian subcontinent, are clearly amphibious since they spend a substantial part of their lives on mudflats in response to habitat drying and also they are observed to migrate terrestrially during wetter periods (Liem, 1987). These facultative air-breathers usually inhabit stagnant, slow-flowing swampy bodies of water or wet lands, which are often covered with macrovegetation like water hyacinth and are characterised by low dissolved oxygen, higher free carbon dioxide gas and bicarbonate, and high ammonia levels (for review, see Saha and Ratha, 1998). During summer, these swamps dry up due to prolonged drought and the air-breathing catfishes burrow inside the mudflats, possibly to avoid total dehydration.

Adaptation of nitrogen metabolism is one of the important prerequisites for any vertebrate species while migrating from aquatic to the amphibious or to the terrestrial habitat (Campbell, 1991). The aquatic amphibian, Xenopus laevis, which is primarily ammoniotelic, excreting ammonia as the major nitrogen excretory product, turns toward ureotelism during water shortage (Janssens and Cohen, 1968). Balinsky (1970) also reported increased activity of the urea cycle enzymes in aestivating X. laevis. In African lungfishes, Protopterus annectens and P. aethiopicus, a greater part of waste nitrogen is converted to urea via the urea cycle when they undergo aestivation (Janssens, 1964). In most freshwater teleost fishes, however, the involvement of the urea cycle, as a similar strategy for surviving semidry conditions has been ruled out mainly because of the absence of a functional urea cycle in most teleost fishes (for reviews, see Campbell, 1991, Anderson, 1995, Saha and Ratha, 1998). However, the presence of a functional urea cycle has recently been reported in several teleost fishes, e.g. the alkaline lake-adapted tilapia (Oreochromis alcalicus grahami) (Randall et al., 1989), the marine toadfishes Opsanus tau and O. beta (Mommsen and Walsh, 1989), and in some Indian air-breathing teleosts (Saha and Ratha, 1987, Saha and Ratha, 1989). These species also excrete a significant amount of urea in response to adverse environmental conditions such as confinement (stress), severely alkaline water, ammonia loading, and exposure to air (Randall et al., 1989, Walsh et al., 1990, Walsh et al., 1994, Saha and Ratha, 1998). It has been clearly shown, at least in two Indian air-breathing catfishes (H. fossilis and C. batrachus), that ureogenesis is stimulated under hyper-ammonia stress (Saha and Ratha, 1990, Saha and Ratha, 1994, Ratha et al., 1995, Saha et al., 1995, Saha and Das, 1999). These air-breathing catfishes are also reported to tolerate a very high ambient ammonia (50–75 mM NH₄Cl) (Saha and Ratha, 1990, Saha and Ratha, 1994, Saha and Ratha, 1998).

The role of ureogenesis in these air-breathing catfishes in their survival while living inside the mud for months in a semidry condition due to prolonged drought in the summer was investigated in the present study. The results demonstrate stimulation of ureogenesis in one of these air-breathing catfishes, Heteropneustes fossilis, by keeping the fish inside the moist peat for 30 days outside the laboratory, mimicking their normal habitat which they face during summer.