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Free amino play an important role in regulating cell volume in fishes. Four tissues/cells (skeletal muscle, RBC, brain, and myocardium) of the little skate, Raja erinacea, were selected for detailed study because of their special importance or unique advantage as experimental models. Three particular amino acids, beta-alanine, taurine, and sarcosine play a predominant role in all four tissues. As in higher vertebrates, amino acid uptake in skate brain, heart, and RBC is mediated via a Na+-dependent process. Amino acids leave the skate brain rapidly in response to a sudden decrease in plasma osmolality and/or to a simultaneous drop in extracellular Na+ concentration. However, although amino acids are important for volume regulation in normal brain cells, they do not appear to be likely candidates for the unidentified "idiogenic" osmolytes in mammalian brain cells. The high concentration of taurine in skate myocardium is of special interest because of the special role of this amino acid in myocardial contractility. Thus, unlike beta-alanine and sarcosine, taurine may play a dual role in regulating both cell volume and contractility of myocardial cells. The isolated skate atrium is well suited for in vitro studies of these two processes.

Concentrations of individual free amino acids were determined in various tissues of the skate (Raja erinacea) and the stingray (Dasyatis sabina), and the relationship of cellular free amino acid concentrations to intracellular osmoregulation was investigated by adapting these elasmobranchs to half-strength seawater. Each tissue characteristically had high levels of certain specific amino acids. Skate sing muscle contained high concentrations of sarcosine and beta-alanine, skate heart had high concentrations of taurine, and skate erythrocytes had high levels of taurine and beta-alanine. Amino acid levels in skate plasma were very low. High concentrations of aturine and glutamate were found in stingray brain. Adaptation of skates and stingrays to half-strength seawater was accompanied by significant decreases in concentrations of the major free amino acids in skate wing muscle and erythrocytes and in stingray brain, but not in skate heart. The data suggest that in these elasmobranchs certain specific free amino acids are selectively involved in intracellular osmoregulatory mechanisms.

Net renal tubular excretion of taurine was demonstrated by clearance techniques in the dogfish, Squalus acanthias; the little skate, Raja erinacea; and in the winter flounder, Pseudopleuronectes americanus. Dogfish acclimated to 70% seawater had increased renal excretion of taurine resulting from higher glomerular filtration rates of taurine as well as from increased rats of tubular secretion. Tubular secretion of taurine prevailed in the dogfish and flounders measured, whereas it occurred in only 30% of the population of skates where tubular reabsorption predominated. Taurine uptake into incubated dogfish thin kidney slices was investigated. Uptake occurs mainly across the peritubular membrane, is carrier mediated, dependent on metabolic energy, and completely inhibited by ouabain and low Na in the medium. The active transport of taurine is also inhibited by beta-alanine but not by alpha-aminoisobutyric acid, glycine, gamma-aminobutyric acid, PAH, or probenecid.

The level of activity of the ornithine-urea cycle is low in the liver of the permanently aquatic Australian lungfish. The rate of incorporation of (14)C-bicarbonate into urea by liver slices was only 100th of that previously observed in the estivating African lungfish Protopterus dolloi. The activities of enzymes of the ornithine-urea cycle were similarly reduced. The low activity of this cycle in Neoceratodus is consistent with its exclusively aquatic nature.

Creatine and trimethylamine oxide (TMAO) are the chief nitrogenous constituents of normal Lophius urine, and both of these organic bases characteristically have high urine/plasma concentration ratios. Competition studies involving various organic bases indicate that creatine and TMAO are excreted independently by separate transport mechanisms. TMAO excretion is inhibited competitively by tetraethylammonium ion (TEA) and by cyanine dye No. 863-compounds previously shown to be transferred actively by an organic base-secreting mechanism of general occurrence among vertebrates. TEA does not inhibit competitively the active tubular reabsorption of TMAO in Squalus with doses which markedly depress its tubular excretion in Lophius. Glycine, which inhibits creatine reabsorption in the dog, does not interfere competitively with its secretion in Lophius.

The relative importance of the purine pathway (uricolysis) and the ornithine cycle as routes for urea synthesis was assessed in isolated liver preparations from the African lungfish Protopterus dolloi. Incorporation of C(14)-labeled precursors into urea was used for comparison. Both pathways are present in the lungfish, but the ornithine cycle is quantitatively more important.

The maximal tubular transfer rates (Tm) of both p-aminohippurate (PAH) and diodrast (3,5-diiodo-4-pyridone-N-acetic acid or iodopyracet) were found to be fixed and reproducible when measured separately in Lophius (goosefish) under standard laboratory conditions. Expressed on a molar basis Tm(PAH) was four times Tm(D). However, when these transport competitors were presented simultaneously in equimolar concentrations with the plasma levels of each sufficiently high enough to saturate the carrier system, the relative rates of excretion were reversed with the diodrast transfer rate then four times that of PAH. The combined rate of excretion was far below Tm(PAH) alone, and roughly equal to Tm(D). Interaction with a common carrier was indicated by the gradations in degree of inhibition which resulted when plasma concentration ratios of diodrast to PAH were extended from 0.1 to 3.2, and PAH transfer rates expressed as percentage of Tm(PAH) were correspondingly depressed from 17 to 1.0 per cent respectively. These observations again point up the inverse relationship between transfer rate and competitive effectiveness which exists for members of a series of substances actively transported by a common mechanism. It appears that carrier affinity and dissociation characteristics may be quite different for various compounds in a series, and also that these parameters may vary significantly from species to species.

Quantitative evaluations have been made of the chief anions and cations in plasma, urine, and pericardial fluid taken both from freshly captured goosefish and from those undergoing "laboratory diuresis." Measurements included: Na, K, Ca, Mg, Cl, SO(4), PO(4), protein, HCO(3), NH(3), pH, titratable acidity, freezing point depression, creatine, trimethylamine oxide, and plasma volume. The total patterns of electrolyte distribution in these body fluids are presented. The morphologically undifferentiated aglomerular tubule acts as a barrier to the free diffusion of monovalent electrolytes, while transporting actively the divalent ions, especially Mg. Urine taken from freshly captured fish is hypotonic to plasma, low in electrolyte, and as much as 50 per cent of its total osmolarity is accounted for by nitrogenous components. Of these creatine is transported most actively by the renal tubule cells. With the onset of diuresis immediately after capture, plasma osmolarity slowly rises and urine suddenly becomes isotonic with plasma as chloride floods into the urine. The active movement of Mg continues during diuresis and urine/plasma concentration ratios of 100 or more are sustained for days while the animals are kept in the laboratory. Na follows chloride and never reaches 50 per cent of plasma values, and K never appears in urine in more than mere traces. Electrolytes in this system are viewed as not being in true equilibrium but rather as constituting a biological steady state with the distribution across renal cells being maintained against passive diffusion by the expenditure of cellular energy.