Structure and functions of angiotensinogen.

Angiotensinogen (AGT) is the sole precursor of all angiotensin peptides. Although AGT is generally considered as a passive substrate of the renin-angiotensin system, there is accumulating evidence that the regulation and functions of AGT are intricate. Understanding the diversity of AGT properties has been enhanced by protein structural analysis and animal studies. In addition to whole-body genetic deletion, AGT can be regulated in vivo by cell-specific procedures, adeno-associated viral approaches and antisense oligonucleotides. Indeed, the availability of these multiple manipulations of AGT in vivo has provided new insights into the multifaceted roles of AGT. In this review, the combination of structural and functional studies is highlighted to focus on the increasing recognition that AGT exerts effects beyond being a sole provider of angiotensin peptides.

Renin-angiotensin system components in the interstitial fluid of the isolated perfused rat heart. Local production of angiotensin I.

We used a modification of the isolated perfused rat heart, in which coronary effluent and interstitial transudate were separately collected, to investigate the uptake and clearance of exogenous renin, angiotensinogen, and angiotensin I (Ang I) as well as the cardiac production of Ang I. The levels of these compounds in interstitial transudate were considered to be representative of the levels in the cardiac interstitial fluid. During perfusion with renin or angiotensinogen, the steady-state levels (mean +/- SD) in interstitial transudate were 64 +/- 34% (P < .05 for difference from the arterial level, n = 8) and 108 +/- 42% (n = 6) of the arterial level, respectively; the levels in coronary effluent were not significantly different from those in interstitial transudate. Ang I was not detectable in interstitial transudate during perfusion with Tyrode's buffer or angiotensinogen. It was very low in interstitial transudate during perfusion with renin and rose to much higher levels during combined renin and angiotensinogen perfusion. The total production rate of Ang I present in interstitial fluid could be largely explained by the renin-angiotensinogen reaction in the fluid phase of the interstitial compartment. In contrast, the total production rate of Ang I present in coronary effluent and the net ejection rate of Ang I via coronary effluent were, respectively, 4.6 +/- 2.2 and 2.8 +/- 1.3 (P < .01 and P < .05 for difference from 1.0, n = 6) times higher than could be explained by Ang I formation in the fluid phase of the intravascular compartment. Ang I from the interstitial fluid contributed little to the Ang I in the intravascular fluid and vice versa. These data reveal two tissue sites of Ang I production, ie, the interstitial fluid and a site closer to the blood compartment, possibly vascular surface-bound renin. There was no evidence that the release of locally produced Ang I into coronary effluent and interstitial transudate occurred independently of blood-derived renin or angiotensinogen.

Role of the kidney in the plasma clearance of angiotensinogen in the rat: plasma clearance and tissue distribution of 125I-angiotensinogen.

We studied the tissue distribution and plasma clearance of angiotensinogen (AGN) in rats following an i.v. injection of 125I-labeled AGN. The plasma clearance rate of [125I]AGN fits a two-compartment model with half-lives of 10.2 +/- 1.5 min and 4.1 +/- 0.5 h in non-treated rats, and the half-life of slower phase significantly increased to 10.2 +/- 1.1 h following bilateral nephrectomy. Radioactivity was predominantly distributed in the kidneys (4.9%), and to a lesser extent in the liver (1.8%), testis (1.2%), spleen (0.61%), heart (0.35%), lung (0.18%), thymus (0.03%) and brain (0.03%). The subcellular distribution of radioactivity in the kidney was 64% in the soluble fraction and 33% in the crude mitochondrial-lysosomal fraction. Sodium dodecylsulfate-polyacrylamide gel electrophoresis revealed that the radioactivity in the soluble fraction consisted of proteins corresponding to intact [125I]AGN, whereas the mitochondrial-lysosomal fraction contained additional radioactive proteins with molecular weights between 18,000 and 29,000. When isolated kidney cells were incubated with [125I]AGN at 0 degree C, the radioactive binding was saturable and specific with a Kd value of 4.8 x 10(-11)M, whereas incubation at 37 degrees C resulted in the appearance of degraded products of [125I]AGN in the medium. These results suggested that circulating AGN is cleared mainly by the kidneys via receptor-mediated endocytosis, which may play an important role in regulating plasma level of AGN.