WHAT IS RESPONSIBLE FOR THE DIURNAL-VARIATION IN POTASSIUM EXCRETION

Potassium excretion exhibits a diurnal pattern, with most excretion oc curring close to noon in humans. Each component of the K+ excretion ra te {urinary K+ concentration ([K+]) and flow rate} was measured and ba ck-calculated to reflect events in the cortical collecting duct (CCD). Our purpose was to determine to what extent each component contribute d 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+ excret ion rose after meals; the rise in the [K+](CCD) (3.3-fold) predominate d at earlier times, and the rise in flow rate occurred later and was p rimarily 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 bi carbonaturia; the transtubular potassium concentration gradient was no w close to 15 in the morning and evening.