Salt and Water Balance — Extrarenal Mechanisms

Abstract

Current theories of vertebrate evolution suggest that primitive fish-like vertebrates originally evolved in a freshwater habitat and that the appearance of marine forms resulted from a subsequent reinvasion of the sea. It appears that such a reinvasion followed two basic patterns — whilst the bony fish retained a relatively low osmotic concentration of their body fluids and developed in the sea as hypo-osmotic regulators, the early cartilagenous fish elevated the osmotic concentration of their body fluids so as to become isosmotic, or slightly hyperosmotic, to their marine environment. The elevation of body fluid osmotic concentration in the latter group was mainly achieved, however, not by large increases in inorganic ion concentrations, but by the accumulation and retention of certain organic nitrogenous compounds, such as urea and trimethylamine oxide (TMAO). The net result of this early divergence in osmoregulatory strategy means that the two major groups of marine fish, the teleosts and the elasmobranchs, are fundamentally different in their osmotic status, although they do both face broadly similar problems of ionic balance. For elasmobranchs, the basic problems of hydromineral regulation therefore centre on the retention of urea and related compounds, and the elimination of excess ions. This latter process involves an unusual organ, the salt-secreting rectal gland, which has attracted considerable interest, originally as an extremely rich source of the enzyme Na-K-ATPase (Hokin et al. 1973), and more recently as a model system for a particularly widespread, and medically highly important, transport process — the secondary active transport of chloride.