Intracellular ANG II induces biological effects in nonrenal cells, but it is not known whether it plays a physiological role in renal proximal tubule cells (PTCs). PTCs express angiotensinogen, renin, and angiotensin-converting enzyme mRNAs, suggesting the presence of high levels of intracellular ANG II. We determined if microinjection of ANG II directly in single PTCs increases intracellular calcium concentration ([Ca2+]i) and, if so, elucidated the cellular mechanisms involved. Changes in [Ca2+]i responses were studied by fluorescence imaging using the Ca2+ indicator fluo 3. ANG II (1 nM) was microinjected directly in the cells, whereas cell-surface angiotensin type 1 (AT1) receptors were blocked by losartan (10 microM). When ANG II (1 nM) was added to the perfusate, there was a marked increase in [Ca2+]i that was blocked by extracellular losartan. With losartan in the perfusate, intracellular microinjection of ANG II elicited a robust increase in cytoplasmic [Ca2+]i that peaked at 30 s (basal: 2.2 +/- 0.3 vs. ANG II: 14.9 +/- 0.4 relative fluorescence units; P < 0.01). Chelation of extracellular Ca2+ with EGTA (2 mM) did not alter microinjected ANG II-induced [Ca2+]i responses (Ca2+ free + ANG II: 12.3 +/- 2.6 relative fluorescence units, not significant vs. ANG II); however, pretreatment with thapsigargin to deplete intracellular Ca2+ stores or with U-73122 to inhibit phospholipase C (1 microM each) markedly attenuated microinjected ANG II-induced [Ca2+]i responses. Combined microinjection of ANG II and losartan abolished [Ca2+]i responses, whereas a combination of ANG II and PD-123319 had no effect. These data demonstrate for the first time that direct microinjection of ANG II in single PTCs increases [Ca2+]i by stimulating intracellular AT1 receptors and releases Ca2+ from intracellular stores, suggesting that intracellular ANG II may play a physiological role in PTC function.