Interactions of external and internal K+ with K (+) -HCO3- cotransporter of rat medullary thick ascending limb
Blanchard, A.; leviel, F.; Bichara, M.; Podevin, R.A.; Paillard, M.
American Journal of Physiology 271(1 Pt 1): C218-C225
1996
ISSN/ISBN: 0002-9513 PMID: 8760049 Document Number: 456292
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+HCO-3- 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, (HCO-3-)-i = (HCO-3-)-o = 23 mM, and pH-i = pH-o = 7.4. As indicated by 2',7'-bis(carboxyethyl)-5(6)carboxyfluorescein HCO-3-dependent rates of change in pH-i, at constant (K+)-i, increasing the magnitude of the outward K+ gradient by varying (K+)-o saturated HCO-3- 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 HCO-3- 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 HCO-3-, has a role as an allosteric activator of the K+-HCO-3- cotransporter. Such an allosteric modulation may contribute to the maintenance of net HCO-3- absorption despite large in vivo physiological variations of K+ concentration in the medullary interstitium.