CALCITONIN ACTIVATES AN NA-INDEPENDENT HCO3--DEPENDENT PATHWAY IN THERABBIT DISTAL CONVOLUTED TUBULE()

Calcitonin is known to stimulate Ca2+ reabsorption and natriuresis and to increase adenosine 3',5'-cyclic monophosphate levels in early dist al tubule, but its effects on acid-base transport mechanisms are not w ell characterized in this segment. We found that recovery of cell pH ( pH(i)) from an induced acid load (using NH4+) in single isolated segme nts of the initial portion (''bright'') of the rabbit distal convolute d tubule (DCTb) was due to an ethylisoprapylamiloride-sensitive Na+/H exchanger both in the absence and presence of HCO3-, but we found no evidence for participation of other mechanisms such as an H+ pump or a n HCO3--dependent mechanism. Introduction of calcitonin stimulated an Na+-independent, HCO3--dependent mechanism (0.17 +/- 0.04 pH units/min , n = 14) that reestablishes normal pH(i) after an induced acid load. This mechanism was observed only in the presence of CO2/HCO3- and was not inhibited by N-ethylmaleimide (1 mM), 4,4'-diisothiocyanostilbene- 2,2'-disulfonic acid (200 mu M), or Sch-28080 nor stimulated by glutam ine (2 mM) or ketoglutarate (0.5 mM), but it was dependent on chloride . We conclude that, in the DCTb, salmon calcitonin activates a latent Na+-independent, HCO3--dependent mechanism.