AJP - Regulatory, Integrative and Comparative Physiology, Vol 267, Issue 2 554-R560, Copyright © 1994 by American Physiological Society
What is responsible for the diurnal variation in potassium excretion?
A. Steele, H. deVeber, S. E. Quaggin, A. Scheich, J. Ethier and M. L. Halperin
Department of Medicine, St. Michael's Hospital, Toronto, Ontario, Canada.
Potassium excretion exhibits a diurnal pattern, with most excretion
occurring close to noon in humans. Each component of the K+ excretion rate
[urinary K+ concentration ([K+]) and flow rate] was measured and
back-calculated to reflect events in the cortical collecting duct (CCD).
Our purpose was to determine to what extent each component contributed to
this diurnal variation in each 2-h portion of the day. In humans, K+
excretion rose threefold from nadir (0600 h) to peak (1200-1400 h), 18 h
after the principal intake of K+. The variation in K+ excretion was due
almost exclusively to changes in [K+] in the terminal CCD ([K+]CCD) rather
than via changes in flow rate. In rats, the bulk of K+ excretion occurred
shortly after eating. Both components of K+ excretion rose after meals; the
rise in the [K+]CCD (3.3-fold) predominated at earlier times, and the rise
in flow rate occurred later and was primarily a result of a higher rate of
excretion of urea. The rise in [K+]CCD did not correlate with aldosterone
levels or administration. A very large rise in the [K+]CCD only occurred in
the presence of bicarbonaturia; the transtubular potassium concentration
gradient was now close to 15 in the morning and evening.
