Nycthemeral relationship between renal function and sodium homeostasis.

Nycthemeral changes in renal function have been known for a long time, but our knowledge about the underlying mechanisms is still far from complete. From a theoretical point of view, many physiological systems could be involved in turning the kidney from a sodium-excretory organ during daytime into a sodium-conserving organ during nighttime. Although blood pressure itself may regulate sodium output directly through pressure natriuresis, this mechanism can be unmasked only when other influences on the kidney can be eliminated. Hence, there is not enough evidence that pressure natriuresis governs cyclic variations in sodium output in the intact human being. Although the glomerular filtration rate follows a nocturnal variation pattern that is more or less in phase with the pattern of sodium excretion, the amplitudes of both differ so much that it is not likely that filtration alone can account for the changes in electrolyte output. The available data are compatible with tubular mechanisms, notably in the distal parts of the nephron. Recent molecular studies have identified several circadian gene products that may entrain the sodium excretory rhythm.