The acute direct action of Ang-(1-7) on net bicarbonate reabsorption (JHCO3-) was evaluated by stationary microperfusion of in vivo middle proximal tubules of rat kidney, using H ion-sensitive microelectrodes. The mean control value of JHCO3- is 2.82 ± 0.078 nmol. cm-2. s-1 (50) and Ang-(1-7) (10-12 or 10-9 M) into luminally perfused tubules causes a significant decrease [to 1.80 ± 0.21 nmol. cm-2. s-1 (52) and 1.11 ± 0.119 nmol. cm-2. s-1 (78), respectively] but Ang-(1-7) (10-6 M) increases it to 5.09 ± 0.46 nmol. cm-2. s-1 (70). A779 [10-6 M; an Ang-(1-7) receptor Mas antagonist] alone increases the JHCO3- to 3.68 ± 0.394 nmol. cm-2. s-1 (64); these data indicate that in the basal situation intratubular Ang-(1-7) inhibits JHCO3-. However, A779 prevents the inhibitory and the stimulatory effects of Ang-(1-7) in JHCO3-. S3226 (10-6 M; an inhibitor of NHE3 isoform of Na+/H+) alone decreases the JHCO3- to 1.56 ± 0.168 nmol. cm-2. s-1 (53), does not affect the inhibitory effect of Ang-(1-7) and changes its stimulatory effect on an inhibitory effect [1.98 ± 0.358 nmol. cm-2. s-1 (47)]. So, our results indicate that the biphasic dose-dependent effect of Ang-(1-7) on JHCO3- is mediated by the Mas receptor and via NHE3. The mean control value of cytosolic free calcium ([Ca2+]i), monitored by FURA-2-AM, is 101 ± 2 nM [6 (n° of tubules; each tubule is the average of 10 areas)] and Ang-(1-7) (10-12, 10-9 or 10-6 M) causes a transient (3 min) increase of it [to 252 ± 8 nM (5), 203 ± 3 nM (6) or 153 ± 8 nM (5), respectively]. A779 alone increases the [Ca2+]i to 126 ± 2 nM (5) but impairs the stimulatory effect of Ang-(1-7) all doses. The results are compatible with stimulation of NHE3 by a moderate increase in [Ca2+]i with Ang-(1-7) (10-6 M) and its inhibition by large increase in [Ca2+]i with Ang-(1-7) (10-12 or 10-9 M). Our findings are the opposite of those found for the biphasic dose-dependent effect of Ang II on the Na+/H+, since several studies indicate the stimulation of the Na+/H+ with low doses of Ang II by a small increase in [Ca2+]i and inhibition of the Na+/H+ with high doses of Ang II by a large increase in [Ca2+]i. It is therefore reasonable to assume that, in the intact animal, the interaction of the opposite dose-dependent effects of Ang-(1-7) and Ang II on the Na+/H+ and [Ca2+]i may represent an important physiological regulation in terms of intra- and extracellular volume and/or pH changes.