Increased Na/H antiporter and Na/3HCO3 symporter activities in chronic hyperfiltration. A model of cell hypertrophy.
The effect of chronic hyperfiltration, a model of cell hypertrophy, on H/HCO3 transporters was examined in the in vivo microperfused rat proximal tubule. Hyperfiltration was induced by uninephrectomy with subsequent increased dietary protein. After 2 wk the hyperfiltration group had a higher glomerular filtration rate (2.21 +/- 0.13 vs. 1.48 +/- 0.12 ml/min), associated with increased kidney weight (1.71 +/- 0.05 vs. 1.23 +/- 0.04 g). HCO3 absorptive rate measured in tubules perfused with an ultrafiltrate-like solution (25 mM HCO3) was higher in the hyperfiltration group (183 +/- 17 vs. 109 +/- 16 pmol/mm per min). The activities of the apical membrane Na/H antiporter and basolateral membrane Na/3HCO3 symporter were assayed using the measurement of cell pH [(2'7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein] in the doubly microperfused tubule in the absence of contact with native fluids. After 2 wk of hyperfiltration Na/H antiporter activity, assayed as the effect of luminal Na removal on cell pH, was increased 114%. Basolateral membrane Na/3HCO3 symporter activity, assayed as the effect of a decrease in peritubular [HCO3] (25 to 5 mM) or in peritubular [Na] (147 to 25 mM) in the absence of luminal and peritubular chloride, was increased 77 and 113%, respectively, in the hyperfiltration group. Steady-state cell pH, measured with physiologic, ultrafiltrate-like luminal and peritubular perfusates, was significantly higher in the hyperfiltration group (7.27 +/- 0.02 vs. 7.14 +/- 0.03). In similar studies, performed 24 h after uninephrectomy and protein feeding, kidney weight was increased 10%, Na/H antiporter activity 39%, and Na/3HCO3 symporter activity 46%. At this time cell pH was not different between the two groups. The results demonstrate that chronic hyperfiltration is associated with parallel increases in Na/H antiporter and Na/3HCO3 symporter activities. If a decrease in cell pH is the signal that triggers these adaptations, it occurs early, and the adaptations can be maintained in the absence of sustained cell acidification.
Preisig
P A
PA
Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas 75235-8856.
Alpern
R J
RJ
D000222:Adaptation, Physiological; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006984:Hypertrophy; D007687:Kidney Tubules, Proximal / Q000378:metabolism* / Q000473:pathology; D007700:Kinetics; D008297:Male; D008954:Models, Biological; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism; D027982:Sodium-Bicarbonate Symporters; D017923:Sodium-Hydrogen Exchangers
D001639:Bicarbonates; D002352:Carrier Proteins; D027982:Sodium-Bicarbonate Symporters; D017923:Sodium-Hydrogen Exchangers; D012964:Sodium