Role of nitric oxide in ischemia/reperfusion injury of the heart

Fascinating research of the last decade in areas as diverse as human memory, penile erection and immune response has revealed an important transduction mechanism in which nitric oxide (NO) acts as the central factor. In the vascular system, NO acts on blood vessels, especially arteries, and keeps them patent against overwhelming odds in a number of diseases such as hardening of the vessel wall (atherosklerosis), diabetic angiopathy, and lack of oxygen (ischemic syndromes). In this organ system, NO exerts its powerful beneficial effects through stimulation of an enzyme, soluble guanylyl cyclase, that constantly turns out cyclic guanosine 3,-5,-monophosphate, a potent vasodilator and vasoprotectant. The current project aims at enhancing our understanding, still incomplete, of NO biosynthesis and its roles in heart function. It comprises a biochemical part in which the working of the enzyme forming the mediator (NO synthase) will be scrutinized, especially with respect to its interaction with the pteridine cofactor tetrahydrobiopterin. These studies will be performed with recombinant isoforms of NO synthase obtained in preparative scale from a baculovirus-insect cell expression system. The pharmacological aspects of the project will be concerned with the dual role of NO in cardiac injury, i. e. its basically beneficial role in countering oxygen deprivation (ischemia) due to narrowing or hardening of arteries (anti-ischemic effects of NO), but also with its potentially deleterious effects. In fact, restoration of oxygen to a previously ischemic region (reperfusion) results in the generation of NO-related reactive intermediates such as oxgen-derived free radicals which may exert powerfully toxic effects in the vasculature as well as in heart muscle cells. Because ischemia and reperfusion are constantly occurring, especially in the arteries of the heart and brain, eventually leading to myocardial infarction and ischemic stroke, the biochemical basis for the regulation of vascular function by products of NO synthase needs firmly to be established. By integrating biochemical, pharmacological and päthophysiological aspects of vascular function in the current project, the authors are confident to contribute to elucidating the mechanisms leading to, and its eventual overcoming of, human myocardial infarction.

Nitric oxide (NO) is an intracellular messenger molecule which is formed in virtually all cells of the human body and is involved in the regulation of various biological processes. In the vasculature system, NO causes dilatation of blood vessels and thus a decrease in blood pressure. NO is also formed in cardiomyocytes, but its physiological and/or pathophysiological role as a modulator of heart function is not very well understood. Since cardiovascular diseases are among the most frequent causes of death inn the western industrial countries, there is a high priority for the development of new drugs for the treatment of cardiovascular disorders. Besides biochemical studies in which the function of the NO biosynthesis cofactor tetrahydrobiopterin was clarified, fundamental work on the function of NO in heart was a key issue of the present project. In classical pharmacological studies, cardiac parameters (contractility, heart rate) were measured in isolated perfused hearts obtained from genetically engineered mice with cardiomyocyte-selective overexpression of endothelial NO synthase (eNOS). Comparison with hearts from wild-type control animals showed that physiologically low concentrations of NO cause an increase in myocardial contractility, i.e. increased cardiac output, whereas higher concentrations of NO, as occurring in various pathological situations such as cardiomyopathies or septic conditions, had the opposite effect, i.e. reduced contractility. These results show that a successful treatment of these diseases may be achieved by blockade of the pathological overproduction of NO with specific NO synthase inhibitors which do not affect the physiologically important basal NO formation. In addition, ischemia/reperfusion studies revealed that hearts from eNOS overexpressing mice were much more resistant against tissue injury caused by oxygen deprivation as compared to hearts obtained from wild-type animals. Therefore, cardiomyocyte-derived NO appears to counteract myocardial injury and loss of heart function in cardiac ischemia (coronary artery disease and myocardial infarction). These results provide a new explanation for the clinical benefit of the so-called nitrovasodilators (organic nitrates, molsidomine) drugs that have been used since more than a century for the successful treatment of coronary artery disease and myocardial infarction. Since publication of the results in December 2001, several colleagues from renowned universities have already expressed their interest in collaborative work using the transgenic eNOS overexpressing mice that were created in the course of the present project. Thus, this world-wide unique animal model may soon become an invaluable tool of experimental cardiovascular pharmacology.