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 pHi = pHo = 7.4. As indicated by 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein HCO3(-)-dependent rates of change in pHi, at constant [K+]i, increasing the magnitude of the outward K+ gradient by varying [K+]o 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.