Renal endosomes contain angiotensin peptides, converting enzyme, and AT(1A) receptors

Kidney cortex and proximal tubular angiotensin IT (ANG II) levels are great er than can be explained on the basis of circulating ANG II, suggesting int rarenal compartmentalization of these peptides. One possible site of intrac ellular accumulation is the endosomes. In the present study, we tested for endosomal ANG I, ANG II, angiotensin type 1A receptor (AT(1A)), and angiote nsin converting enzyme (ACE) activity and determined whether these levels a re regulated by salt intake. Male Sprague-Dawley rats were fed chow contain ing either high or low dietary sodium for 10-14 days. Blood and kidneys wer e harvested and processed for measurement of plasma, kidney, and renal inte rmicrovillar cleft and endosomal angiotensin levels. Kidney ANG I averaged 179 +/- 20 fmol/g and ANG II averaged 258 +/- 36 fmol/g in rats fed a high- sodium diet and were significantly higher, averaging 347 +/- 58 fmol/g and 386 +/- 55 fmol/g, respectively, in rats fed a low-salt diet. Renal intermi crovillar clefts and endosomes contained ANG I and ANG II. Intermicrovillar cleft ANG I and ANG II levels averaged 8.4 +/- 2.6 and 74 +/- 26 fmol/mg, respectively, in rats fed a high-salt diet and 7.6 +/- 1.7 and 70 +/- 25 fm ol/mg in rats fed a low-salt diet. Endosomal ANG I and ANG II levels averag ed 12.3 +/- 4.4 and 43 +/- 19 fmol/mg, respectively, in rats fed a high-sal t diet, and these levels were similar to those observed in rats fed a low-s alt diet. Renal endosomes from rats fed a low-salt diet demonstrated signif icantly more AT(1A) receptor binding compared with rats fed a high-salt die t. ACE activity was detectable in renal intermicrovillar clefts and was 2.5 -fold higher than the levels observed in renal endosomes. Acute enalaprilat treatment decreased ACE activity in renal intermicrovillar clefts by 90% a nd in renal endosomes by 84%. Likewise, intermicrovillar cleft and endosoma l ANG II levels decreased by 61% and 52%, respectively, in enalaprilat-trea ted animals. These data demonstrate the presence of intact angiotensin pept ides and ACE activity in renal intermicrovillar clefts and endosomes, indic ating that intact angiotensin peptides are formed and/or trafficked through intracellular endosomal compartments and are dependent on ACE activity.