The renal medulla is considered to play a critical role in the regulation of extracellular fluid volume and electrolytes and long-term control of blood pressure. In response to an increase in renal perfusion pressure, the renal medulla is proposed to release a neutral antihypertensive lipid, medullipin, into the circulation. This substance is proposed to have three distinct properties: natriuretic and diuretic action in the kidney, vasodepressor, and inhibitor of central sympathetic outflow, actions which counteract those of the renal renin- angiotensin-aldosterone system. The identity of this substance has so far not been uncovered. In this proposal, we propose that medullipin corresponds to anandamide or one of its associated metabolites, in particular, the ethanolamide of prostaglandin E2 also called prostamide E2. Anandamide is one of a group of endogenous neutral unsaturated fatty acid amides collectively known as endocannabinoids due to their ability to stimulate endogenous cannabinoid receptors. The possibility that prostamide metabolites represent renal medullary medullipin is supported by several lines of preliminary evidence. Anandamide is present in the kidneys at high concentration relative to most tissues and was shown in our preliminary studies to be enriched the renal medullary region. The kidney medulla is also unique in expressing high constitutive levels of cyclooxygenase 2 (Cox-2), an enzyme that catalyzes the rate-limiting step in synthesis of prostamides from anandamide. Prostamide E2 formation is higher in the medulla than cortex. These findings contrast with the renal distribution of fatty acid amide hydrolase (FAAH), the enzyme that hydrolyzes anandamide to arachidonic acid and ethanolamide. The renal medulla expresses lower Faah protein and enzyme activity than cortex, thereby facilitating the conversion of anandamide to prostamide. Infusion of anandamide into the renal medulla produces diuresis and natriuresis, effects which are blocked by a Cox-2 inhibitor. Intravenous prostamide E2 lowers blood pressure and increases renal blood flow, and its effects directly counteract angiotensin II-induced effects on arterial pressure and renal blood flow. This proposal aims to investigate the relationship between prostamide formation in the renal medulla and changes in renal perfusion pressure. The formation of prostamides will be manipulated genetically by using Faah knockout mutant mice, which exhibit increased levels of prostamides in renal medulla, plasma, and urine. Faah activity and prostamide synthesis will also be modulated by genetic and chemical inhibition. A better understanding of mechanisms in the renal medulla that are protective in the long-term control of blood pressure will result in improved prevention and treatment of hypertension.
Hypertension is a critical public health issue and in the United States is a leading cause of heart disease, kidney failure and stroke. An understanding of mechanisms underlying protective responses of the kidney to increased blood pressure will contribute to prevention and treatment of hypertension. This project will investigate the role of renal medullary cyclooxygenase-2 metabolites of anandamide, the prostamides, in protection against increased kidney perfusion pressure and long-term control of blood pressure.
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