The teleost gas gland is truly remarkable in its abilities to secrete gases into the swim bladder of physoclistous fish. The physiological and metabolic adaptations of this tissue have been elegantly summarized in a recent review article by Pelster and Scheid (1992). There are two key contributors to the function of the gland. First, a specialized metabolism of the swim bladder, involving copious and simultaneous production of lactate and CO2 from anaerobic glycolysis and the pentose phosphate pathway (also known as the hexose monophosphate shunt), respectively, contributes to gas exchange through pH and salting-out effects on the oxygen-carrying capacity of the blood. Second, a countercurrent multiplier system (i.e. a rete mirabile) enables gas tensions to be elevated further by back diffusion. Several features of metabolism and acid-base physiology remain unclear. First, despite the remarkable ability of this tissue to produce acid, it is not clear if or how intracellular pH (pHi) is regulated. Since ultimately the blood must be acidified, one would predict that the pHi of the tissue would be well regulated via high rates of membrane exchange of protons and/or high tissue buffering capacity. Second, although the functioning of the pentose phosphate pathway has been strongly inferred from measurements of enzyme activities (Bostrom et al. 1972; Pelster and Scheid, 1991), and from measurements of enhanced rates of CO2 excretion relative to the rates of oxygen uptake (Pelster et al. 1989), direct evidence for the existence of the shunt is lacking. Lastly, although the pentose phosphate pathway is expected to produce CO2, and thus contribute to the acidification of blood entering the gland, the pathway may have a different primary, or perhaps a dual, role, namely to maintain high tissue levels of NADPH for protection against oxygen radical damage to cells (Pelster and Scheid, 1992). The composition of the gas stored in the swim bladder can approach pure oxygen in some species, so it is not surprising that the teleost gas gland contains substantial levels of the enzymes catalase, superoxide dismutase and glutathione peroxidase, which scavenge deleterious radicals of oxygen and related harmful compounds (Morris and Albright, 1984). Noteworthy is glutathione peroxidase, which requires a constant supply of NADPH (presumably from the shunt) to maintain glutathione in a reduced state. Reduced glutathione is then used in a variety of oxygen radical detoxification mechanisms. If the pentose phosphate pathway has a role in oxygen detoxification, one would predict that flux rates through the pathway would increase with increased oxygen levels.
The Active Transport of Oxygen and Carbon Dioxide into the Swim-Bladder of Fish
