Introduction

A. Background

The kidney, heart, and circulatory system together play an indispensable role in the maintenance of life in higher organisms, not only from the perspective of maintaining the constancy of many components of the extracellular fluid, including filtering out of nitrogenous wastes and maintenance of normal blood pressure, but also as major endocrine organs.

The kidney as an endocrine gland is the site of production of renin and the following hormones: (1) erythropoietin, which is a peptide hormone essential for the process of erythropoiesis or red blood cell formation by the bone marrow; (2) la,25-dihydroxyvitamin D3, the hormonally active form of vitamin D, which is essential for calcium homeostasis (see Chapter 9); and (3) the kallikreins, a group of serine proteases that act on blood proteins to produce bradykinin, a potent vasodilator. Renin is an enzyme with proteolytic activity that acts on a plasma protein, a2-globulin, to produce the hormonal angiotensins, which in turn act at the adrenal cortex to stimulate the biosynthesis and secretion of the mineralocorticoid aldosterone. In addition, the kidney serves as an endocrine target organ for a number of hormones. Table 15-1 summarizes the endocrine aspects of the kidney both as an endocrine secretory gland and as an endocrine target organ.

An important physiological function of the kidney was observed by J. Peters in 1835, "The kidneys appear to serve as the ultimate guardians of the constitution of the internal environment." In this regard, it is clear that the kidney occupies a unique position within the

HORMONES, SECOND EDITION

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