AN INVITRO APPROACH TO THE STUDY OF MACULA DENSA-MEDIATED GLOMERULAR HEMODYNAMICS

Tubuloglomerular feedback (TGF), which operates between the tubule and the parent glomerulus, is important to renal autoregulation and homeostasis of body fluid and electrolytes [1,2]. Since the juxtaglomerular apparatus (JGA) displays an intimate anatomical relationship between the specialized tubular epithelial cells (macula densa) and the vasculature (afferent and efferent arterioles), it has long been suggested as the anatomical site of TGF [3]. However, attempts to obtain direct evidence to support this have been hindered by the anatomical complexity of the JGA. Since the JGA is located beneath the tubular layer at some distance from the surface of the kidney, the macula densa is not accessible to direct micropuncture in vivo, nor is direct observation of the vascular pole possible. To circumvent these limitations of the in vivo approach, we have developed an in vitro preparation in which both the afferent arteriole and the macula densa of a microdissected rabbit JGA are microperfused simultaneously. This allows us to study the JGA directly without being influenced by systemic hemodynamic and hormonal factors; moreover, real-time images of the afferent arteriole can be continuously monitored and recorded. Unlike other preparations, there are only a few structures to interfere with direct observation of the vascular pole, permitting accurate measurement of the luminal diameter of the arterioles even in segments which are overlapped by the glomerulus. Using this preparation, we have observed that increasing the NaCl concentration of the macula densa perfusate constricts the afferent arteriole in the segment close to the glomerulus, and that this constriction is abolished by furosemide, a loop diuretic known to inhibit TGF.