Recently, we have shown that chronic exercise increases endothelium-derived relaxing factor (EDRF)/nitric oxide (NO)-mediated epicardial coronary artery dilation in response to brief occlusion and acetylcholine. This finding suggests that exercise can provide a stimulus for the enhanced production of EDRF/NO, thus possibly contributing to the beneficial effects of exercise on the cardiovascular system. Therefore, the purpose of the present study was to examine whether chronic exercise could influence the production of NO (measured as the stable degradation product, nitrite) and endothelial cell NO synthase (ECNOS) gene expression in vessels from dogs after chronic exercise. To this end, dogs were exercised by running on a treadmill (9.5 km/h for 1 hour, twice daily) for 10 days, and nitrite production in large coronary vessels and microvessels and ECNOS gene expression in aortic endothelial extracts were assessed. Acetylcholine (10`to mol/L) dose-dependently increased the release of nitrite (inhibited by nitro-L-arginine) from coronary arteries and microvessels in control and exercised dogs. Moreover, acetylcholine-stimulated nitrite production was markedly enhanced in large coronary arteries and microvessels prepared from hearts E a ndothelium-derived relaxing factor (EDRF), identified as nitric oxide (NO)12 or a closely related molecule,3 is released by the endothelium in response to local hormones, changes in blood flow velocity, or endothelial shear stress.4 However, the physiologically relevant stimulus for EDRF/NO release in vivo is not known and may reflect the summation of local hormones and blood flow. Evidence supporting the contribution of basally released EDRF/NO to the regulation of vascular tone is derived from experiments showing that specific inhibitors of NO synthase (NOS) elicit a prolonged pressor response and reduce regional blood flows in vivo and inhibit endothelium-dependent relaxations in vitro.5

Having identified renin in cardiac mast cells, we assessed whether its release leads to cardiac dysfunction. In Langendorff-perfused guinea pig hearts, mast cell degranulation with compound 48/80 released Ang I-forming activity. This activity was blocked by the selective renin inhibitor BILA2157, indicating that renin was responsible for Ang I formation. Local generation of cardiac Ang II from mast cell-derived renin also elicited norepinephrine release from isolated sympathetic nerve terminals. This action was mediated by Ang II-type 1 (AT 1 ) receptors. In 2 models of ischemia/reperfusion using Langendorff-perfused guinea pig and mouse hearts, a significant coronary spillover of renin and norepinephrine was observed. In both models, this was accompanied by ventricular fibrillation. Mast cell stabilization with cromolyn or lodoxamide markedly reduced active renin overflow and attenuated both norepinephrine release and arrhythmias. Similar cardioprotection was observed in guinea pig hearts treated with BILA2157 or the AT 1 receptor antagonist EXP3174. Renin overflow and arrhythmias in ischemia/reperfusion were much less prominent in hearts of mast cell-deficient mice than in control hearts. Thus, mast cell-derived renin is pivotal for activating a cardiac renin-angiotensin system leading to excessive norepinephrine release in ischemia/reperfusion. Mast cell-derived renin may be a useful therapeutic target for hyperadrenergic dysfunctions, such as arrhythmias, sudden cardiac death, myocardial ischemia, and congestive heart failure.

Our goal was to determine whether angiotensin II (Ang II) and its metabolic fragments release nitric oxide and the mechanisms by which this occurs in blood vessels from the canine heart. We incubated 20 mg of microvessels or large coronary arteries in phosphate-buffered saline for 20 minutes and measured nitrite release. Nitrite release increased from 27 +/- 2 up to 103 +/- 5, 145 +/- 17, 84 +/- 4, 107 +/- 16, and 54 +/- 4 pmol/mg (P < .05) in response to 10(-5) mol/L of Ang I, II, III, IV, and Ang-(1-7), respectively. The effects of all angiotensins were blocked by N omega-nitro-L-arginine methyl ester (100 mumol/L), indicating that nitrite was a product of nitric oxide metabolism, and by Hoe 140 (10 mumol/L), a specific bradykinin B2 receptor antagonist, indicating a potential role for local kinin formation. The protease inhibitors aprotinin (10 mumol/L) and soybean trypsin inhibitor, which block local kinin formation, inhibited nitrite release by all of the angiotensins. Angiotensin nonselective (saralasin), type 1-specific (losartan), and type 2-specific (PD 123319) receptor antagonists abolished the nitrite released in response to all the fragments. Angiotensin type 1 and type 2 and receptors mediate nitrite release after Ang I, II, III, and Ang-(1-7), whereas only type 2 receptors mediate nitrite release after Ang IV. Similar results were obtained in large coronary arteries. In summary, formation of nitrite from coronary microvessels and large arteries in the normal dog heart in response to angiotensin peptides is due to the activation of local kinin production in the coronary vessel wall.

These studies indicate that nitrate is a reliable measure of NO metabolism in vivo but that because of the long half-life, nitrate will accumulate in plasma once it is produced. Because of the large volume of distribution (21% of body weight versus the 4% of body weight usually attributed to plasma volume, the compartment in which nitrate is measured), simple measures of plasma nitrate underestimate by a factor of 4 to 6 the actual production of nitrate or NO by the body. In disease states, such as heart failure, in which renal function and extracellular volume are altered, caution should be exercised when increases in nitrate in plasma as an index of NO formation are evaluated.

The goal of this study was to determine whether coronary endothelial function was altered after pacing-induced heart failure in conscious dogs. Fourteen mongrel dogs were chronically instrumented for measurements of systemic hemodynamics, left circumflex coronary artery diameter (CD) and blood flow, and for left ventricular pacing for 4 wk. Heart failure developed during this pacing regimen and was characterized by a significant reduction in arterial pressure, an increase in left atrial pressure, a resting tachycardia, a depression of left ventricular dP/dt to isoproterenol injection, a significant reduction of the slope of the end-systolic pressure-diameter relationship, and all of the characteristic clinical signs. During heart failure, the dilation of CD after release of a brief coronary artery occlusion, acetylcholine, and arachidonic acid was attenuated, whereas prostaglandin (PG) I2- and nitroglycerin-induced dilations of CD were unchanged. The coronary blood flow responses to occlusion, acetylcholine, and nitroglycerin were depressed, but not to PG. Large coronary arteries and microvessels were isolated from normal and failing hearts. Both isolated large coronary arteries and microvessels from failing hearts produced significantly less nitrite, the immediate metabolite of nitric oxide in aqueous solution, than those of normal hearts. Thus endothelium-mediated control of the coronary circulation was depressed during heart failure. A decrease in the production of nitric oxide-endothelium-derived relaxing factor was most likely responsible for this depression.