Morphology and function of the distal part of the afferent arteriole.

The morphological features of the juxtaglomerular part of the afferent arteriole presented here are in favor of a bulk fluid flow through the fenestrated endothelium and the urinary space of the glomerulus towards the renin-granulated epithelioid cells and the extraglomerular mesangium. In addition there should be some fluid movement from the glomerular mesangium draining via the glomerular stalk. These flows can be important codeterminants of the interstitial fluid composition around the granular cells and act themselves as a signal to granular or extraglomerular mesangial cells to trigger renin secretion and/or a short-loop (vasculo-vascular) feedback mechanism in governing the glomerular filtration. These studies also provide evidence that the terminal part of the afferent arteriole, consisting of granular cells, lacks the immunoreactive myosin. The role of this vessel segment not only as a putative effector of the tubuloglomerular feedback mechanism, but also as an intrarenal vascular receptor, should therefore be reconsidered. The electrophysiological data presented herein suggest an intrarenal extravascular conversion of Ang I to Ang II in the vicinity of the granular cell. This de novo locally-formed angiotensin II may directly influence the renin secretion, mesangial cell function, and thus tubuloglomerular feedback mechanism and glomerular hemodynamics.