Inotropic agents in hypoxic cat myocardium: depression and potentiation

Identification and characterization of a functional mitochondrial angiotensin system

The renin-angiotensin (Ang) system regulates multiple physiological functions through Ang II type 1 and type 2 receptors. Prior studies suggest an intracellular pool of Ang II that may be released in an autocrine manner upon stretch to activate surface membrane Ang receptors. Alternatively, an intracellular renin-Ang system has been proposed, with a primary focus on nuclear Ang receptors. A mitochondrial Ang system has not been previously described. Here we report that functional Ang II type 2 receptors are present on mitochondrial inner membranes and are colocalized with endogenous Ang. We demonstrate that activation of the mitochondrial Ang system is coupled to mitochondrial nitric oxide production and can modulate respiration. In addition, we present evidence of age-related changes in mitochondrial Ang receptor expression, i.e., increased mitochondrial Ang II type 1 receptor and decreased type 2 receptor density that is reversed by chronic treatment with the Ang II type 1 receptor blocker losartan. The presence of a functional Ang system in human mitochondria provides a foundation for understanding the interaction between mitochondria and chronic disease states and reveals potential therapeutic targets for optimizing mitochondrial function and decreasing chronic disease burden with aging.

Effects of angiotensin-converting enzyme inhibitors on tissue renin-angiotensin systems

The renin-angiotensin system (RAS) plays a major role in the control of blood pressure and cardiovascular homeostasis and is involved in the pathogenesis of a number of cardiovascular disorders. The efficacy of angiotensin-converting enzyme (ACE) inhibitors in the treatment of hypertension and congestive heart failure has led to the widespread clinical use of ACE inhibitors in primary or secondary prevention of heart disease. The demonstration of the expression of the components of the RAS in several extrarenal tissues, as well as local generation of angiotensin II, has confirmed the existence of a tissue RAS that may serve organ-specific functions and act independently from the plasma RAS. The concept of paracrine/autocrine functions of the local RAS has changed our understanding of the functions of the RAS and suggests that tissue ACE inhibition may be of greater importance than inhibition of circulating ACE in the treatment of congestive heart failure and other cardiovascular disorders. Whereas the circulating endocrine RAS appears to be responsible for mediation of acute effects, the tissue RAS seems to be involved in more chronic situations, such as secondary structural changes of the cardiovascular system, and therefore could contribute to the pathogenesis of hypertension as well as other cardiovascular disorders, such as cardiac hypertrophy, coronary artery disease, and atherosclerosis. Several experimental and clinical findings suggest that reversal of cardiovascular structural changes secondary to cardiovascular disease and enhancement of renal sodium excretion by ACE inhibitors are important long-term antihypertensive actions possibly mediated by inhibition of the tissue RAS.

Regulation by angiotensin II of phosphate transport in cardiac myocytes

The purpose of this study was to test the hypothesis that angiotensin II (ang II) affects the transport of inorganic phosphate in adult ventricular myocytes. Ventricular myocytes were isolated from Dahl rats and allowed to take up 32P inorganic phosphate (32P-Pi). The intracellular 32P concentration increased rapidly and reached a peak at 5 min. Ang II, 10(-10) to 10(-5) M, produced a significant (P less than 0.05) and concentration-dependent reduction in 32P-Pi uptake that plateaued at 0.1 to 1.0 microM. Ang II at 1 microM produced a 30% reduction in Vmax and a slightly greater reduction in the Km of 32P-Pi uptake, compared to myocytes that were not exposed to ang II. The ang II receptor antagonist saralasin (Sar1-Val5-Ala8-angiotensin II), significantly (P less than 0.05) antagonized the action of ang II on 32P-Pi uptake. TPA (12-O-tetradecanoylphorbol-13-acetate) also produced a significant (P less than 0.05) reduction of 32P-Pi uptake, suggesting that protein kinase C is involved in the transduction of ang II effects on intracellular Pi. 32P Efflux from myocytes, pulsed with 32P-Pi and chased with Pi-free medium, was accentuated markedly by ang II; this effect was blunted by saralasin. These data suggest that ang II is capable of regulating total intracellular Pi in the heart via two actions: (i) by inhibiting uptake of Pi into the myocyte, and (ii) by increasing the efflux of phosphates out of the cell.

Characterization of the mechanical behavior of isolated papillary muscle preparations of the ferret

The characteristics of isometric and isotonic contraction were studied in isolated papillary muscle preparations from the right ventricle of the ferret and compared with those previously reported for comparable preparations from cat and rat. The effects of stimulation rate, temperature, extracellular calcium concentration, caffeine, verapamil, and ouabain were measured. Afterloaded contractions and load clamps were used to study the mechanics of relaxation. The contractile characteristics of ferret papillary muscle preparation appear to be intermediate in many respects between those reported for cat and rat preparations.

Potentiation of the contractile effects of norepinephrine by hypoxia

Hypoxia has been found to depress the concentration response curve of norepinephrine (NE) in isoalted cat papillary muscles. To investigate the effects of hypoxia in intact hearts, a heart-lung preparation was developed and maximum left ventricular dp/dt (max dp/dt) was measured at constant heart rate, preload, and after load. Left main coronary arterial flow (Q(e)) was measured with an electromagnetic flow probe. As arterial P(O2) decreased from 90 mm Hg (96% saturation) to 20-25 mm Hg (40% saturation) at constant P(CO2) and pH, no change in max dp/dt occurred and Q(e) increased 298%. In contrast to cat papillary muscles, the contractile responses to NE were augmented in hypoxia. The NE dose-response curves shifted to the left. No deterioration of contractility occurred after exposure to NE. In contrast, the chronotropic response was unaltered in hypoxia. Dose-response curves to isoproterenol also were shifted to the left in hypoxia, but responses to paired pacing were unchanged. The responses to NE under oxygenated conditions were unaltered by mechanically increased coronary flow or by increased coronary flow with nitroglycerin. Although the mechanisms responsible for these effects are unknown, the results suggest that hypoxia may open previously nonfunctioning vascular channels and thereby allow more extensive exposure of beta adrenergic receptors to circulating catecholamines.