Effects of Amiloride on Distal Renal Tubule Sodium and Calcium Absorption: Dependence on Luminal pH

: The goal of our study was to examine the effects of amiloride on distal renal tubule calcium and sodium absorption at low and high luminal pH. In vivo microperfusion of distal convoluted tubules were performed on rats. Total Ca and Na concentrations in perfusion and collected fluids were measured by atomic absorption spectrometry. When tubules were perfused with low pH perfusate (pH = 5.6), net calcium absorption (JCa) averaged 5.4 pmol/min. With high pH perfusate (pH = 7.1), JCa was significantly higher averaging 8.9 pmol/min. (P < 0.01). In contrast, net sodium absorption (JNa) was not affected by luminal pH (231 pmol/min. versus 237 pmol/min.). At low luminal pH, 10–4 M amiloride analogue (5-N-methyl-N-isobutyl amiloride) (MIA) inhibited sodium absorption but stimulated calcium absorption. At high luminal pH MIA still inhibited Na absorption but it had no effect on calcium absorption. Thus, the natriuretic effect of amiloride appears to be independent of luminal pH, while the amiloride-stimulated distal calcium absorption is observed at low but not at high luminal pH.