Confocal imaging and function of the juxtaglomerular apparatus

PURPOSE OF REVIEW: Monitoring interactions between dissimilar cells is a key to understanding the function of the juxtaglomerular apparatus. This review will summarize new findings on tubuloglomerular feedback-mediated reductions in the glomerular filtration rate and renin release signals to elucidate how current research can benefit from visual approaches. RECENT FINDINGS: Macula densa cells are capable of producing significant changes in cell volume depending on the rate of entry of salt through their apical membrane. Tubuloglomerular feedback activation by increased tubular salt content is associated with macula densa cell swelling and the simultaneous contraction of the terminal, intraglomerular, renin-positive segment of the afferent arteriole and the intraglomerular mesangium. Macula densa cells release adenosine triphosphate, which causes tubuloglomerular feedback-mediated vasoconstriction either directly through purinergic signaling or indirectly through its hydrolysis to adenosine. During conditions of low extracellular fluid volume, reduced tubular salt delivery causes macula densa cells to synthesize and release prostaglandin E2, which in turn stimulates the proliferation of juxtaglomerular cells, the synthesis and release of renin, and activation of the renin-angiotensin system. Renin release from juxtaglomerular cells represents a unique form of exocytosis: even large granules can release their content very rapidly without any significant movement relative to the juxtaglomerular cell membrane. SUMMARY: A trend in biomedical research is to re-visit earlier studies using state-of-the-art experimental techniques. Imaging methods including the newest innovations in confocal fluorescence microscopy provide direct, visual information on juxtaglomerular apparatus function, with exceptional time and spatial resolution at the level of individual cells and organelles.