Flow dependence of K+ secretion in cortical distal tubules of the rat

Superficial distal tubules were pump perfused (range 0-35 nl/min), generally with solutions similar in composition to early distal tubule fluid, in control, K+–depleted, acutely K+–loaded, and K+–adapted rats with the use of double-barreled resin-reference microelectrodes to measure K+ and Na+ activities and transepithelial potential differences (PD). When perfusion rate decreased from 35 to 5 nl/min in control animals, K+ increased from 2 mM to between 10 and 20 mM, remaining at these levels as perfusion rate was decreased further. In low-K+ rats, the change in K+ activity with flow was greatly attenuated. In K+–loaded and K+–adapted rats, K+ was higher than in controls at all flow rates. Na+ concentrations and lumen-negative PD increased with high flow rates in control rats. Addition of 10(-3) M amiloride blocked the increase in luminal K+ with low flow rates. In the physiological range of late distal flow rates, luminal K+ remains constant and similar to the concentration attained in the steady state. At higher flow rates, K+ declines, and K+ balance, PD, and Na+ reabsorption modulate the relationship between K+ secretion and flow rate.