Lnteractions of external and internal k+ with the k+-hco3-
cotransporter of rat medullary thick ascending limb.
Blanchard, Anne, Fran[cedilla]coise Leviel, Maurice Bichara,
Ren[acute]e-Alexandre Podevin, and Michel Paillard.
D[acute]epartement de Physiologie, Universit[acute]e Pierre et
Marie Curie, Institut National de la Sant[acute]e et de la Recherche
M[acute]edicale Unit[acute]e 356, and H[circumflex]opital Broussais,
Paris, France
APStracts 3:0055C, 1996.
We studied intracellular ([K+]i) and extracellular ([K+]e)
concentration-dependency of the kinetic properties of the
electroneutral K+-HCO3- cotransport, using suspensions of rat
medullary thick ascending limb (MTAL). By using 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 BCECF HCO3--dependent rates of change in pHi, at
constant [K+]i, increasing the magnitude of the outward K+ gradient
by varying [K+]e saturated HCO3- efflux with a Michaelis-Menten curve
(apparent Km for [K+]e = 2 mM, Hill coefficient = 1). On the other
hand, increasing [K]i from 30 to 140 mM, while either [K]e 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 v = 0.5 Vmax 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.
Received 22 August 1995; accepted in final form 30 January 1996.
APS Manuscript Number C516-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 14 February 96