Renal potassium transport: contributions of individual nephron segments and populations

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Abstract
The extent to which distinguishable nephron populations, such as superficial and deep, may differ in their contributions to overall K excretion are not known. The nature of transport processes across the successive segments of the nephrons (including not only the underlying cellular mechanisms, but even the direction of transport) is not known for all segments in any 1 nephron population. Excreted K is derived both from filtered K that escapes reabsoprtion and from secreted K. The filtered portion is large in amphibians and may be larger than generally recognized in mammals. The remainder is secreted primarily by distal nephron segments (distal tubule and cortical collecting duct). K is also secreted into descending limbs of Henle loops; this fraction is recycled from collecting ducts, and so does not represent an additional quantity of K transferred from blood to tubule fluid. Systemic factors that affect K excretion (K intake, NaCl intake, mineralocorticoid hormone levels, acid-base balance, and diuretic treatments) do so by modifying the net uptake of K from blood to cell and by altering the rate of fluid flow through the distal nephron. Under most circumstances, the distal nephron in the cortex appears to secrete K and the medullary collecting duct reabsorbs K. Although successive nephron segments transport K in different ways, may not be handled differently by superficial nephrons than nephrons whose glomeruli lie in the deeper levels of the cortex.