[K.sup.+] secretion in the rat kidney: [Na.sup.+] channel-dependent and -independent mechanisms

Renal [Na.sup.+] and [K.sup.+] excretion was measured in rats with varying dietary [K.sup.+] intake. The requirement for channel-mediated distal nephron [Na.sup.+] reabsorption was assessed by infusing the animals with the [K.sup.+]-sparing diuretic amiloride via osmotic minipumps. At infusion rates of 2 nmol/min, the concentration of amiloride in the urine was 38 [micro]M, corresponding to concentrations of 9-23 [micro]M in the distal tubular fluid, sufficient to block >98% of [Na.sup.+] transport through apical [Na.sup.+] channels (ENaC). With a control [K.sup.+] intake (0.6% KC1), amiloride reduced [K.sup.+] excretion rates ([U.sub.k]V) from 0.85 [+ or -] 0.15 to 0.05 [+ or -] 0.01 [micro]mol/min during the first 2 h of infusion, suggesting that distal nephron [K.sup.+] secretion was completely dependent on the activity of [Na.sup.+] channels. When [K.sup.+] intake was increased by feeding overnight with a diet containing 10% KC1, amiloride reduced [U.sub.K]V from 7.5 [+ or -] 0.7 to 1.3 [+ or -] 0.1 [micro]mol/min despite an increased plasma [K.sup.+] of 9 mM, again suggesting a major but not exclusive role for the [Na.sup.+] channel-dependent pathway of [K.sup.+] secretion. The maximal measured rates of amiloride-sensitive [K.sup.+] excretion correspond well with estimates based on apical [K.sup.+] channel activity in distal nephron segments. However, when the animals were adapted to the high-[K.sup.+] diet for 7-9 days, the diuretic decreased [U.sub.K]V less, from 6.1 [+ or -] 0.6 to 3.0 [+ or -] 0.8 [micro]mol/min, indicating an increasing fraction of [K.sup.+] excretion that was independent of [Na.sup.+] channels. This indicates the upregulation of a [Na.sup.+] channel-independent mechanism for secreting [K.sup.+]. amiloride; [K.sup.+] adaptation; [K.sup.+] channels