Regulatory function of a primitive vertebrate kidney

1. 1. Determinations were mae of the major low molecular weight constituents of serum and urine of Pacific hagfish ( Eptatretus stouti) and of the sea water in which they were kept. 2. 2. The concentrations of sodium and chloride, and probably the osmotic pressure, of serum and urine were the same. 3. 3. Concentrations of potassium, magnesium, sulfate and phosphate were greater in urine than serum, indicating that these ions are secreted into the glomerular filtrate by the mesonephric duct cells. 4. 4. Concentrations of calcium and glucose were lower in urine than in serum. This is interpreted as a reabsorption of glucose by the mesonephric duct cells. Some of the plasma calcium is bound to proteins and is not filtered through the glomeruli, accounting for the low level of urine calcium. 5. 5. It is suggested that hydrogen-ion activity of the plasma is regulated in the kidney. 6. 6. Experiments with C 14-labeled inulin showed that water is not reabsorbed from the glomerular filtrate. This indicates that there is no renal mechanism for the active reabsorption of sodium ions, a lack that is unique among vertebrates. 7. 7. The lack of renal sodium reabsorption, which was requisite for the invasion of fresh water by vertebrates, is regarded as a primitive omission rather than a secondary loss. This strongly suggests that myxinoid ancestors have always been marine and, therefore, that vertebrates must have originated in the sea. 8. 8. The cationic concentrations in hagfish slime are compared with the concentrations in serum and cells. Slime may function in the extra-renal secretion of calcium, magnesium and potassium.