Juxtamedullary nephrons

Distinguish between a cortical nephron and a juxtamedullary nephron... 

The vasa recta are modified peritubular capillaries. As with the peritubular capillaries, the vasa recta arise from efferent arterioles. However, these vessels are associated only with the juxtamedullary nephrons and are found only in the medullary region of the kidney. The vasa recta pass straight through to the inner region of the medulla, form a hairpin loop, and return straight toward the cortex. This structure allows these vessels to lie parallel to the Loop of Henle and collecting ducts. 

Depending on the choice of control parameters, the amplitudes of the pressure oscillations in the nephron tree are found to be different at different positions in the tree. Due to model symmetry, two nephrons connected to the same node have the same oscillation amplitudes. Thus, we can refer to the number of the branching point to describe the amplitude properties. Branching points 2, 3, and 4 may correspond to deep nephrons and branching points 6 and 7 to superficial nephrons. Experimentally, only the pressure oscillations in nephrons near the surface of the kidney have been investigated. However, we suppose that deep (juxtamedullary) nephrons can exhibit oscillations in their pressures and flows as well. 

Secretion is not uniform along the proximal tubule, and may differ between superficial and juxtamedullary nephrons. This heterogeneity of secretion, varying between species and substrates, might reflect different... 

Table 8.Techniques for study of renal microcirculation. Juxtamedullary nephron preparation Elydronephrotic kidney...

Limitations of the in vitro perfused juxtamedullary nephron preparation include 1) the availability of only a select population of juxtamedullary glomeruli near the inner surface of the kidney 2) underestimation of the contribution of flow in large vessels to preglomeru-lar resistance and 3) lack of characterization of tubular transport. 

To date, the in vitro perfused juxtamedullary nephron preparation has not been used to examine the effects of nephrotoxic agents. However the preparation has been used to examine the pathophysiology of tubuloglomerular feedback. It has also been used to study the effect of mediators like adenosine, oxygen radicals and nitric oxide. Some recent studies are discussed below. 

The influence of neuronal nitric oxide synthase (nNOS) on renal arteriolar tone has been studied in the perfused juxtamedullary nephron preparation [126]. Superfusion with a specific nNOS inhibitor decreased afferent and efferent arteriolar diameters, and these decreases in arteriolar diameters were prevented by interruption of distal volume dehvery by papillectomy. When volume delivery to the macula densa segment was increased, afferent arteriolar vasoconstrictor responses to the nNOS inhibitor were enhanced, but this effect was again completely prevented after papillectomy. In contrast, the arteriolar diameter responses to a nonselective NOS inhibitor were only attenuated by papillectomy. Specific nNOS inhibition enhanced the efferent arteriolar vasoconstrictor response to ANG II but did not alter the afferent arteriolar vasoconstrictor responsiveness to ANG II. In contrast, non specific NOS inhibition enhanced both afferent and efferent arteriolar vasoconstrictor responses to ANG It. This study demonstrates that the modulating influence of nNOS on afferent arteriolar tone of juxtamedullary nephrons is dependent on distal tubular fluid flow and that nNOS exerts a differential modulatory action on... 

The role of the cyclooxygenase pathway interaction with nNOS [133, 134], tyrosine kinase [135]. intracellular calcium [136] and insulin-like growth factor 1 (IGF-1) [137] on afferent arteriolar function has been demonstrated in the perfused juxtamedullary nephron. 

The use of knockout mice in the perfused juxtamedullary nephron model has provided much insight into the physiology of the angiotensin system on renal microcirculation. Some recent studies of the perfused juxtamedullary nephron model are shown in Table 9. 

Casellas D, Navar LG /n vitro perfusion of juxtamedullary nephrons in rats. Am.J.Physiol 246 F349-F358,1984... 

Casellas D, Carmines PK, Navar LG Microvascular reactivity of in vitro blood perfused juxtamedullary nephrons from rats. Kidney Int 28 752-759,1985... 

Pittner J, Wolgast M, Casellas D, Persson AE Increased shear stress-released NO and decreased endothelial calcium in rat isolated perfused juxtamedullary nephrons. Kidney International 67 227-236,2005... 

Each kidney also contains over one million nephrons, which are the functional units of the kidney. Nephrons originate in the cortex where an afferent arteriole forms a specialized capillary bed known as the glomerulus (Figure 1). Some nephrons originate near the surface of the kidney and are called superficial nephrons, while other nephrons originate near the cortical-medullary region and are called juxtamedullary nephrons. 

The terminal portion of the proximal tubule leads into the descending thin limb of Henle in the region corresponding to the demarcation of outer and inner stripes of the outer zone of the medulla. The thin limb loops upward to form the ascending thin limb of Henle, which differentiates into the ascending thick limb of Henle at the intersection of the inner zone of medulla and the inner stripe of the outer medullary zone. This is characteristic of juxtamedullary nephrons that have long loops of Henle. In contrast, as noted earlier, cortical nephrons have short loops of... 

The method for the in vitro perfused juxtamedullary nephron technique in Sprague-Dawley rats is as follows A common dissection procedure is used to expose the arteries and related nephrons located at the inside surface of the renal cortex hning the pelvic cavity. Rats are systemically heparinized. The right kid-... 

Advantages of the in vitro perfused juxtamedullary nephron preparation are 1) preservation of the circulatory network 2) no microdissection trauma to vessels 3) the elimination of neural and hormonal influences 4) maintenance of tubulovascular relationship and 5) simultaneous evaluation of both vascular and tubular effects of toxic substances. 

The influence of neuronal nitric oxide synthase (nNOS) on renal arteriolar tone has been studied in the perfused juxtamedullary nephron preparation [241]. Superfusion with a specific nNOS inhibitor decreased afferent and efferent arteriolar diameters, and these decreases in arteriolar diameters were prevented by interruption of distal volume delivery by papillectomy. When volume delivery to the macula densa segment was increased, afferent arteriolar vasoconstrictor responses to the nNOS inhibitor were enhanced. 

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