In an attempt to clarify the relationship of the circulating and myocardial renin-angiotensin systems, active renin concentration, its constituent major glycoforms (active renin glycoforms I through V), and angiotensinogen were measured in plasma and left ventricular homogenates from sodium-depleted rats under control conditions or 2 minutes, 3 hours, 6 hours, and 48 hours after bilateral nephrectomy (BNX). Control myocardial renin concentration was 1.4+/-0.1 ng angiotensin I (Ang I) per gram myocardium per hour and plasma renin concentration was 6.7+/-1.1 ng Ang I per milliliter plasma per hour. Control myocardial angiotensinogen was 0.042+/-0.004 micromol/kg myocardium and plasma angiotensinogen was 1.5 micromol/L plasma. Two minutes after BNX and corresponding stimulation of renin secretion by anesthesia and surgery, plasma renin concentration was increased disproportionately compared with myocardial renin. Three, 6, and 48 hours after BNX, renin decay occurred significantly faster from the plasma than from the myocardium. Forty-eight hours after BNX, myocardial renin concentrations had fallen to 15% of control values, while myocardial angiotensinogen concentrations had increased 12-fold and plasma angiotensinogen concentrations had increased by only 3.5-fold. Myocardial renin glycoform proportions were identical in myocardial homogenates and plasma in control animals. At 6 hours BNX, the proportions of plasma active renin glycoforms I+II fell, while those in the myocardium significantly increased. We conclude that in control rats, active renin and active renin glycoforms are distributed as if in diffusion equilibrium between plasma and the myocardial interstitial space. After BNX, myocardial renin concentration falls dramatically, suggesting that most cardiac renin is derived from plasma renin of renal origin. After BNX, renin glycoforms I+II are preferentially cleared from the plasma but preferentially retained by the myocardium. Control myocardial angiotensinogen concentrations are too low to result from simple diffusion equilibrium between plasma and the myocardial interstitium.

A local renin-angiotensin system is present within the myocardium and can play a role in the initiation and maintenance of cardiac hypertrophy. The source of myocardial renin maybe direct cardiac renin gene expression, or plasma renin of renal origin. A primary indication that myocardial renin is derived from plasma renin of renal origin was from work showing that cardiac renin activity was no longer detected 30 hours after bilateral nephrectomy (BNX). However, more recent studies have been able to detect myocardial renin after BNX. We measured normal rat cardiac renin before and after 48-hour BNX using a myocardial renin assay with improved sensitivity. The myocardial renin assay was also used to assess normal rat cardiac myocyte renin levels. Since cardiac tissue contains cathepsin D, a lysosomal enzyme capable of renin-like activity, a rat cathepsin D assay was also developed to assess cathepsin D contribution to renin-like activity. Several artifacts were shown to contribute to myocardial renin-like enzymatic activity levels after BNX, including initial plasma renin stimulation during BNX surgery, assay pH, and cardiac cathepsin D activity. Myocardial renin concentration after 48-BNX was found to be only approximately 1% of normal control levels, and renin concentration in normal cardiac myocytes was only 2-fold greater than assay blanks. Both results were probably overestimated due to cathepsin D contamination. In conclusion, no evidence was found for myocardial renin synthesis in the normal adult rat heart, and myocardial renin decays to near zero levels after 48-hour BNX.

We tested the hypothesis that the myocardial renin-angiotensin system (RAS) is both necessary and sufficient to initiate and maintain all classes of ventricular hypertrophy. Myocardial and plasma renin and angiotensinogen were measured in rats during initiation and maintenance of ventricular hypertrophy associated with DOCA implants and 1% NaCl drinking water, with and without the AT(1) ANG II receptor blocker losartan. Additional groups of rats were given a low-sodium diet (0.04%) for 3 wk. Ventricular hypertrophy was initiated within 7 days and maintained for 35 days in DOCA-treated rats despite significantly low myocardial and plasma renin, normal or low myocardial and plasma angiotensinogen, or the presence of losartan. Furthermore, there was no ventricular hypertrophy in low-salt diet-fed animals despite increased myocardial and plasma renin levels and normal angiotensinogen levels. Therefore, the myocardial RAS is not necessary to initiate or maintain cardiac hypertrophy in DOCA-treated rats and is not sufficient to initiate cardiac hypertrophy in low-salt diet-fed rats. Additionally, myocardial renin and angiotensinogen were significantly correlated with corresponding plasma levels.

Active renin and five major active renin glycoforms were measured in plasma and the carotid wall of anesthetized rabbits before and after 1.5- and 24-h bilateral nephrectomy (BNX). Before BNX, there was no difference in renin glycoform proportions between plasma and the carotid wall. Plasma renin concentration (PRC) fell by 67% after 1.5-h BNX due to preferential clearance of renin glycoforms I+II, but no significant change in renin concentration was seen in the carotid artery (or aorta). Twenty-four hours after BNX, PRC and carotid wall renin concentrations were reduced by 99.7 and 97.7%, respectively, while the proportion of renin glycoforms I+II in the carotid wall was significantly elevated. These data are consistent with the view that vascular renin is derived from plasma renin of renal origin. After BNX, renin disappearance from the carotid (and aortic wall) is slower than renin decay from plasma, and the less negatively charged active renin glycoforms I+II exit the carotid wall much more slowly than the more negatively charged glycoforms. After 24-h BNX, renin glycoforms I+II were still effluxing from the vascular wall and represented the only glycoforms present in the carotid wall.