Interactions of external and internal K+ with K+-HCO3- cotransporter of rat medullary thick ascending limb

We studied [K+](i) and [K+](o), where subscripts i and o refer to intracellular and extracellular, respectively, concentration dependency of the kinetic properties of the electroneutral K+-HCO3-, cotransport, using suspensions of rat medullary thick ascending limb (mTAL). With the use of nigericin and monensin, [K+](i) was clamped at various values, While maintaining [Na+](i) = [Na+](o) = 37 mM, [HCO3-](i) = [HCO3-](o) = 23 mM, and pH(i) = pH(o) = 7.4. As indicated by 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein HCO3--dependent rates of change in pH(i), at constant [K+](i), increasing the magnitude of the outward K+ gradient by varying [K+], saturated HCO3- efflux with a Michaelis-Menten curve (apparent Michaelis constant for [K+](o) = 2 mM, Hill coefficient = 1). On the other hand, increasing [K+](i) from 30 to 140 mM, while either [K+](o) or the magnitude of the K+ concentration gradient was fixed, saturated HCO3- efflux with a sigmoidal curve and yielded a Hill coefficient of 3.4 and 50% of maximum velocity at 70 mM [K+](i). These results indicate that [K+](i), independent of its role as a transportable substrate for the cotransport with HCO3-, has a role as an allosteric activator of the K+-HCO3- cotransporter. Such an allosteric modulation may contribute to the maintenance of net HCO3- absorption despite large in vivo physiological variations of K+ concentration in the medullary interstitium.