Osmotic and ionic regulation in a freshwater teleost, Ictalurus nebulosus (Ie sueur) Public Deposited

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  • Aspects of hydromineral balance in the stenohaline fresh-water catfish, Ictalurus nebulosus (Le Sueur) were studied at 10° and 20°C. Temperature did not affect water content significantly. Water content was 78 ml/100 g body weight, 32% of which is in muscle and 49% in the skeleton and associated connective tissue. The extracellular space (inulin space) was 8.7 ml/100 g at 10° and 11.3 ml/100 g at 20°C. A 100 g animal contains 4680 microequivalents of Na, 3200 microequivalents of Cl, and 7180 microequivalents of K. About 77% of the Na and 82% of the Cl exchange with ²²Na and ³⁶CI in 36 hours at 10°C. Plasma concentrations of ions in mM/l at 10° are: Na - 104; Cl - 86; and K - 4. At the lower temperature there was a shift in these ions from the extracellular to the intracellular compartments. The endogenous Cl space was increased by 15% and the sodium space by 29% relative to 20° animals. Skeletal muscle comprises 40% of the wet weight of the fish. The extracellular space (inulin) was 5 ml/100 g at 10°C. About 12% of the total Na and Cl in the animal is in skeletal muscle. Both ions exchange completely within 40 hours. Intracellular concentration of Na was 11 mM/l and Cl was 5 mM/l at 10°C. The rate of osmotic uptake of water at 10°C was 4.27 ml/kg-hr or 0.68 μl/cm²-hr. At 20° the respective values were 10 ml/kg-hr and 1.93 μl/cm²-hr. Water uptake is related to body weight by the relation: uptake = 1.3 weight⁰[superscript .]⁶² . Uptake across the trunk region was separated from uptake across the head region by measuring uptake when one region was bathed in is osmotic sucrose solution. From a knowledge of the skin area and the gill area a permeability constant was derived for each epithelial region. The value was 0.85 μ/sec for skin and 0.5 μ/sec for gills at 10°C. The rate of urine flow equalled the rate of uptake of water at both temperatures. Glomerular filtration rates were 14 and 23 ml/kg-hr at 10° and 20° respectively. Over 98% of the Na and Cl filtered is reabsorbed. At 10°C the free water clearance is 3.64 ml/kg-hr. The sodium efflux from animals in tap water at 10°C was about 1.1 μM/10 g-hr of which about 27% is renal loss, 63% is gill loss, and 10% is skin loss. For Cl the efflux was 0.8 μM/10 g-hr, with 23% being renal loss, 65% being gill loss, and 12% being skin loss. The influxes of Na and Cl were the same as the respective effluxes. Virtually all of the influx occurred across the gills. The uptake of both Na and Cl is thermodynamically active. The body fluids are positive to the bath (tap water or pond water) by 2 to 3 mV. This potential difference was not affected by sodium concentration in the bath up to 100 mM/l. The potential was slightly reduced with an elevation of bath Cl to 100 mM/l.
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