These studies examine the effect of dietary K content on the rate efflux coefficient (KK) assessed both isotopically (42K) and by net K flux in segments dissected from the inner stripe of the outer medullary collecting tubule (MCT). Female New Zealand White rabbits were given either a K-replete diet (KCl diet) or one of two K-restricted diets with similar (142 meq K/kg diet) K content [basic (Na-deplete) diet or NaCl (Na-replete) diet]. After 7 days, renal K excretion (mmol/24 h) varied with K intake: KCl diet, 13.8 +/- 3.0; basic diet, 4.36 +/- 0.12; NaCl diet, 4.13 +/- 0.48. To determine whether alterations in KK of MCT could contribute to these changes in urinary K excretion, the MCT from the three groups of rabbits was perfused in vitro and volume reabsorption and KK were measured. The KK of tubules from the K-replete control group (KCl diet) was 82 +/- 14 nm/s but was significantly greater in tubules from either K-restricted group: 224 +/- 49 nm/s (basic diet group) or 193 +/- 34 nm/s (NaCl diet group). Net K flux in the presence of a 15 mM lumen-to-bath K gradient confirmed a significantly greater K reabsorptive flux and net rate coefficient in either K-restricted group (KNetK = 179 +/- 77 nm/s for the NaCl diet, KNetK = 170 +/- 27 nm/s for the basic diet) than the K-replete control group (KNetK = 12.0 +/- 18 nm/s for the KCl diet).(ABSTRACT TRUNCATED AT 250 WORDS)