INTERACTIONS OF EXTERNAL AND INTERNAL K-HCO3- COTRANSPORTER OF RAT MEDULLARY THICK ASCENDING LIMB( WITH K+)

We studied [K+](i) and [K+](o), where subscripts i and o refer to intr acellular and extracellular, respectively, concentration dependency of the kinetic properties of the electroneutral K+-HCO3-, cotransport, u sing suspensions of rat medullary thick ascending limb (mTAL). With th e 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(carboxy ethyl)-5(6)-carboxyfluorescein HCO3--dependent rates of change in pH(i ), at constant [K+](i), increasing the magnitude of the outward K+ gra dient by varying [K+], saturated HCO3- efflux with a Michaelis-Menten curve (apparent Michaelis constant for [K+](o) = 2 mM, Hill coefficien t = 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 Hil l coefficient of 3.4 and 50% of maximum velocity at 70 mM [K+](i). The se results indicate that [K+](i), independent of its role as a transpo rtable substrate for the cotransport with HCO3-, has a role as an allo steric activator of the K+-HCO3- cotransporter. Such an allosteric mod ulation may contribute to the maintenance of net HCO3- absorption desp ite large in vivo physiological variations of K+ concentration in the medullary interstitium.