Flow-dependent potassium secretion by rabbit cortical collecting tubule in vitro

Cortical collecting tubules (CCT) dissected from rabbits fed a diet designed to stimulate potassium transport secreted potassium in direct proportion to the flow rate in the range of 0.4-3 nl/min (r = 0.79). This relationship was also evident in tubules from rabbits maintained on standard laboratory chow (r = 0.80). The slope of the line relating the two parameters was almost six times greater in tubules from animals fed the special diet. When the range of flow rates was expanded, potassium secretion in nine CCTs appeared to peak at 5-6 nl/min and then failed to increase despite further elevation of flow to nearly 15 nl/min. We investigated the effects of the electrical and chemical gradients on flow-dependent potassium secretion. Because transepithelial voltage was unaffected by changes in axial flow, we conclude that the flow-dependent fraction of potassium secretion is not explained by the electrical gradient. To evaluate the role of luminal potassium concentration on flow-dependent potassium secretion, 11 CCTs were perfused with both 5 and 50 mM potassium solutions at two flow rates (approximately 1.5 and 4.0 nl/min). Increases in both potassium secretion (15.6 +/- 3.9 peq.mm-1.min-1) and sodium reabsorption (11.9 +/- 5.2 peq.mm-1.min-1) were evident in the tubules perfused with 5 mM potassium. Potassium secretion was not reduced by 50 mM luminal potassium at the low flow rate when the largest chemical gradients opposing net secretion were generated. When 50 mM potassium was present in the lumen, increasing flow did not stimulate potassium secretion.(ABSTRACT TRUNCATED AT 250 WORDS)