RELATIONSHIPS BETWEEN BRANCHIAL CHLORIDE CELLS AND GAS TRANSFER IN FRESH-WATER FISH

The gill lamellar epithelium is composed of two predominant cell types , pavement cells and mitochondria-rich chloride cells. The chloride ce lls play a vital role in ionic regulation because they are the sites o f Ca2+ and Cl- uptake from water. Consequently, lamellar chloride cell proliferation occurs in response to ionoregulatory challenges so as t o increase the ion-transporting capacity of the gill. It has been argu ed that such chloride cell proliferation might increase the thickness of the blood-to-water diffusion barrier and thereby impede gas diffusi on. This review focuses on the potential negative consequences of chlo ride cell proliferation on gas transfer and possible compensatory mech anisms that might minimise the extent of respiratory impairment. Two a pproaches were used to evoke chloride cell proliferation in rainbow tr out, hormone treatment (growth hormone/cortisol) and exposure to soft water. In all cases, chloride cell proliferation was associated with a pronounced thickening of the lamellar diffusion barrier. The thickeni ng of the diffusion barrier was associated with a significant impairme nt of gas transfer. Subsequent studies revealed that several compensat ory physiological responses occurred concurrently with the chloride ce ll proliferation to alleviate or reduce the detrimental consequences o f the thickened diffusion barrier. These included hyperventilation, an increased affinity of haemoglobin-oxygen binding and earlier onset of catecholamine release during acute hypoxia.