Acid-base transport in Henle's loop: the effects of reduced renal mass and diabetes.

The loop of Henle (LOH) is an important site of renal acidification. Using the in vivo microperfusion technique of LOH combined with quantitative polymerase chain reaction (PCR) performed on isolated thick ascending limbs (TAL), we demonstrated that the Na + -H + exchanger is the main transport mechanism involved, although a small, but significant contribution from the H+-ATPase also occurs. Among the various Na+-H+ exchanger isoforms we have evidenced that NHE3 is expressed and functionally active along the TAL. Since the LOH is exposed to osmotic stress, bicarbonate transport was also measured under medullary hypotonicity conditions, which led to the stimulation of bicarbonate reabsorption. We demonstrated that the LOH can participate in the tubular adaptation to an increased filtered bicarbonate load by increasing net LOH bicarbonate transport. In this setting, at the molecular level, mRNA and protein abundance of NHE3 were also stimulated, and coincided with an increase in NHE3 activity. Finally, NHE3 expression and abundance was highly stimulated in the early phase of diabetes, which is characterized by increased glomerular filtration rate (GFR).

[Glomerulo-tubular balance in diabetes mellitus: molecular evidence and clinical consequences]. / Bilancio glomerulo-tubulare nel diabete mellito: evidenze molecolari e conseguenze cliniche.

Diabetes mellitus is fast becoming a world epidemic. About one-third of individuals with diabetes, after 10 yrs, develop diabetic nephropathy, the first cause of end-stage kidney disease. The evolution of diabetic nephropathy can be considered in three stages: glomerular hyperfiltration, microalbuminuria (30-300 mg/24 hr) and proteinuria (>300 mg/24 hr). This study was designed to investigate the tubular basis of glomerular hyperfiltration in early diabetes mellitus. Diabetes was inducted in rats with i.p. streptozotocin (65 mg/kg bw) for 6 days. At the end of the treatment, the glomerular filtration rate (GFR), measured by inulin clearance, had substantially increased in diabetic rats compared with controls. Quantitative polymerase chain reaction (PCR) and Western blot analysis reveal that in diabetic rats compared with controls, mRNA and protein abundance was higher for type 3 sodium/hydrogen exchanger (NHE3) in proximal tubule and ascending limbs of Henle's loop, and higher for bumetanide-sensitive sodium-potassium-2 chloride cotransporter (NKCC2) in ascending limbs of Henle's loop. Western blot analysis confirmed the PCR results. Finally, the abundance of á -ENaC protein was unchanged in diabetic rats compared to controls. These results show that the primary sodium reabsorption increase in proximal tubule reduces salt concentrations at the macula densa. This elicits a tubuloglomerular feedback-dependent increase in single nephron GFR.