It appears difficult to build a coherent picture of the concentrating system of the mammalian renal medulla. This may be due to the diversity of the factors involved and to considerable interspecies differences. Several morphological adaptations that may be critical in the improvement of water conservation are described. They include variations in the length of the papilla, number of nephrons, percentage of long-looped nephrons, nephron heterogeneity, development of pelvic fornices, confluence of collecting ducts, vascular bundles in the inner stripe of the outer medulla, thin descending limb epithelium, and relative development of the three medullary zones. The organization of the medullary circulation is described; the medulla includes several functionally different compartments favoring preferential exchanges by the juxtaposition of certain tubules and vessels. This may improve the efficiency of certain recycling routes and hence the insulation of the different compartments. As discussed in section III, a better inner medullary insulation may be key, not (or not only) in achieving a high urine concentration but mainly in reducing the time required to reach this high concentration. This overview of the multiple interspecies variations in medullary organization underlines the importance, among other factors, of the inner stripe architecture and of the internephron differences in the process of urine concentration.

Our understanding of renal Mg handling has been expanded in recent years with the use of electron probe, ultramicroanalysis, and fluorescent dye techniques to determine total Mg and free Mg2+ in individual tubule segments and cells, respectively. Recent studies have shown that [Mg2+]i is a highly mobile cation that may be altered by a number of influences including hormones. It is likely that the hormonal changes in [Mg2+]i, reported here and elsewhere, are involved in intracellular metabolism and regulation rather than transepithelial transport. The role of intracellular Mg2+ in control of cell function is poorly understood. However, it is evident that [Mg2+]i may be rapidly charged through a number of different influences that may have important effects on cell function. These kinds of data have enlarged our understanding of Mg conservation by the renal tubule but have posed many questions for further study. Magnesium is handled in different ways along the nephron. About 80% of the total plasma Mg (1.5-2.0 mM) is ultrafilterable across the glomerular membrane. Of the ultrafilterable Mg (1.2-1.6 mM), only 20-25% is reabsorbed by the proximal tubule, including the convoluted and straight portions. This is in contrast to Na and Ca reabsorption, which amounts to approximately 70 and 60%, respectively, in the proximal nephron. Accordingly, the fractional delivery of Mg to the thick ascending limb of the loop of Henle is much greater than that of Na or Ca. It is now evident from micropuncture studies that proportionally greater amounts of Mg (50-60%) are reabsorbed in the loop compared with Na (20-25%) or Ca (30-35%). Because the terminal nephron segments, including the DCT and collecting tubule, reabsorb only a small portion of the filtered Mg (approximately 5%), the loop of Henle plays a major role in the determination of Mg reabsorption, and it is in this segment that the major regulatory factors act to maintain Mg balance. Magnesium reabsorption in the thick ascending limb takes place in the cortical segments, at least in the mouse and rat. Evidence summarized here suggests that Mg is passively reabsorbed via the paracellular pathway in the cTAL of the loop of Henle. Several factors affect Mg reabsorption in the loop of Henle. Hypermagnesemia and hypercalcemia inhibit reabsorption leading to increased urinary excretion of Mg and Ca. These effects have been reviewed in detail elsewhere (113, 149). Magnesium depletion, for instance through dietary Mg deprivation, enhances Mg reabsorption in the loop of Henle before the fall in plasma Mg concentration and filtered Mg load.(ABSTRACT TRUNCATED AT 400 WORDS)

Abstract,81-and fl2-adrenergic receptor (,B-ARs) expression in the thick ascending limb of rat kidney was studied at the level of mRNA and receptor coupling to adenylyl cyclase. Absolute quantitation off,l-and .82-AR mRNAs in microdissected nephron segments was performed with an assay based on reverse transcription and polymerase chain reaction, using in vitro transcribed mutant RNAs as internal standards. In the cortical thick ascending limb (CITAL), the number of mRNA molecules/mm of tubular length was 2,806±328 (n = 12) for,l1-AR and 159±26for,#2-AR (P < 0.01). Lower levels were obtained in the medullary thick ascending, fl1-AR mRNA still being predominant.The pharmacological properties of f-ARS was also studied in the CTAL. Cyclic AMP accumulation was stimulated by ,B-agonist with a rank order of potency of isoproterenol >norepineph-rine >epinephrine. This observation, and the higher efficiency of a 6i1 than of a f62 antagonist to inhibit isoproterenol-induced cAMP accumulation, establish the typical fl1-AR sensitivity of the CTAL. No detectable contribution of atypical or ,63-ARs to adenylyl cyclase stimulation could be found. In conclusion, this study, which shows markedly different levels of l1-and ,62-AR mRNAS in the CTAL, provides a molecular basis for the predominant expression of the ,l1 receptor subtype in this nephron segment. (J. Clin. Invest. 1993.91:264-272.)

The effect of parathyroid hormone (PTH) on transepithelial Na+, Cl-, K+, Ca2+ and Mg2+ transport was investigated in isolated perfused cortical thick ascending limbs (cTAL) and that of human calcitonin (hCT) was tested in both cortical and medullary thick ascending limbs (mTAL) of the mouse nephron. The transepithelial ion net fluxes (Jx) were determined by electron probe analysis of the perfused and collected fluids. Simultaneously, the transepithelial voltage (PDte) and resistance (Rte) were recorded. In cTAL segments, PTH and hCT significantly stimulated the reabsorption of Na+, Cl-, Ca2+ and Mg2+, hCT generated a net K+ secretion towards the lumen and PTH tended to exert the same effect. Neither PDte nor Rte were significantly altered by either PTH or hCT. However, in the post-experimental period a significant decrease in PDte was noted. Time control experiments carried out under similar conditions revealed a significant decrease in PDte with time, which could have masked the hormonal response. In mTAL segments, Mg2+ and Ca2+ transport was close to zero, hCT did not exert any detectable effect on either PDte or Jcl-, JNa+, JK+, JMg2+ and JCa2+ in these segments. In conclusion, our data demonstrate that PTH and hCT stimulate NaCl reabsorption as well as Mg2+ and Ca2+ reabsorption in the cTAL segment of the mouse. These data are in agreement with and extend data obtained in vivo in the rat.

Insulin (Ins) decreases Na+ delivery in the final urine. To determine whether the loop of Henle participates in this reduction, the effects of Ins were tested on cortical (CTAL) and medullary thick ascending limbs (MTAL) of the mouse nephron, microperfused in vitro. In the MTAL, Ins increased the transepithelial potential difference (Vt) and the Na+ and Cl- net reabsorption fluxes (JNa and JCl, respectively) in a dose-dependent manner, the threshold being below 10(-9) M. At 10(-7) M, Ins reversibly increased JNa and JCl, leaving Mg2+ and Ca2+ fluxes (JMg and JCa, respectively) close to zero. In the CTAL, 10(-7) M Ins reversibly increased Vt, JNa, JCl, JMg, and JCa. In CTAL segments perfused under asymmetrical conditions, with a bath-to-lumen-directed NaCl gradient (lumen 50 mM NaCl, bath 150 mM NaCl), addition of 10(-7) M Ins to the bath resulted in a large increase in JMg and JCa. Thus the responses of CTAL and MTAL to Ins are in all ways similar to those already reported for the adenosine 3',5'-cyclic monophosphate (cAMP)-generating hormones acting on these nephron segments. When 10(-10) M arginine vasopressin (AVP) and 10(-7) M Ins were used in combination, previous addition of one hormone to the bath potentiated the response to the second hormone. In cAMP accumulation experiments, performed in the presence of a phosphodiesterase inhibitor, the amounts of cAMP formed with 10(-7) M Ins and 10(-10) M AVP (which elicit maximal physiological responses in these segments) were in the same range.(ABSTRACT TRUNCATED AT 250 WORDS)