TUBULAR FLUID CONCENTRATIONS AND KIDNEY CONTENTS OF ANGIOTENSIN-I ANDANGIOTENSIN-II IN ANESTHETIZED RATS

Previous micropuncture studies have reported nanomolar concentrations of angiotensin II in proximal tubular fluid and have indicated that an giotensin II or a precursor may be secreted into the tubular lumen. Fu rther experiments were performed to determine if proximal tubular flui d angiotensin I concentrations are also greater than plasma and kidney levels and to estimate the degree of intrarenal compartmentalization of the angiotensin peptides. Free-flow proximal tubular fluid samples were collected in micropipets and were pooled for each animal. At the end of each experiment, a blood sample was collected and the micropunc tured left kidney was harvested and homogenized in methanal. The angio tensin I concentration in proximal tubular fluid samples averaged 6.1 +/- 1.2 pmol/mL, whereas the angiotensin II concentration averaged 8.1 +/- 1.6 pmol/mL (N = 13). HPLC analysis of a separate sample pooled f rom collections in five rats indicated that the immunoreactive angiote nsin I and angiotensin II primarily represented authentic angiotensin I and II, Plasma concentrations of angiotensin I and angiotensin II av eraged 0.39 +/- 0.09 and 0.15 +/- 0.03 pmol/mL, respectively. The kidn ey contents of angiotensin I and angiotensin II were 1.28 +/- 0.24 and 0.97 +/- 0.17 pmol/g of kidney, respectively. These findings indicate that proximal tubular fluid contains nanomolar concentrations of angi otensin I as well as angiotensin II, These high tubular fluid concentr ations, which greatly exceed the plasma and kidney levels, likely refl ect net secretion of the angiotensin peptides by proximal tubule cells . Comparison of the tissue levels of angiotensin I and angiotensin II, measured in the same kidneys subjected to micropuncture procedures, w ith the proximal tubular fluid samples suggests substantive intrarenal compartmentalization of approximately 10 to 20%.