Serum osmolality in the coelacanth, Latimeria chalumnae: urea retention and ion regulation.

Samples of blood (hemolyzed) were obtained from the renal vein, the hepatic portal vein, and the heart of a freshly thawed specimen of Latimeria chalumnae. The coelacanth uses high concentrations of urea to maintain its serum osmolality at approximately that of sea water. The mean value for the total osmolality was 1181 milliosmoles per liter. The mean values (milliequivalents per liter) were: for sodium, 181; for potassium, 51.3; for calcium, 6.9; for magnesium, 28.7; for chloride, 199; and for bicarbonate, 4.7. The mean urea concentration was 355 millimoles per liter, and the mean nonprotein nitrogen was 1343 milligrams percent. Heart blood showed significantly lower values for osmolality (921 milliosmoles per liter) and nonprotein nitrogen (1030 mg percent) and was probably less severely contaminated with products of protein breakdown. Fluid from the anterior chamber of the eye showed values of 952 milliosmole/liter; the urea value for this fluid was 303 mmole/liter, and the magnesium was 7.3 meq/liter. The magnesium value for the aqueous humor was used to correct the abnormally high concentrations in the hemolyzed serum. The high level of serum potassium also was attributed to hemolysis.