What is responsible for the diurnal variation in potassium excretion?

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.