Background: Full-length SPAK is thought to be necessary and sufficient to activate NCC in the distal convoluted tubule (DCT). Results: SPAK knock-out disrupts a signaling network, involving OSR1, in the DCT but not the TAL, preventing NCC activation. Conclusion: SPAK and OSR1 function interdependently in the DCT to positively regulate NCC. Significance: This study provides insights into the mechanisms whereby SPAK/OSR1 regulates renal salt transport.

Objective: Update of the Hohenheim consensus on monosodium glutamate from 1997: Summary and evaluation of recent knowledge with respect to physiology and safety of monosodium glutamate. Design: Experts from a range of relevant disciplines received and considered a series of questions related to aspects of the topic. Setting: University of Hohenheim, Stuttgart, Germany. Method: The experts met and discussed the questions and arrived at a consensus. Conclusion: Total intake of glutamate from food in European countries is generally stable and ranged from 5 to 12 g/day (free: ca. 1 g, protein-bound: ca. 10 g, added as flavor: ca. 0.4 g). L-Glutamate (GLU) from all sources is mainly used as energy fuel in enterocytes. A maximum intake of 16.000 mg/kg body weight is regarded as safe. The general use of glutamate salts (monosodium-L-glutamate and others) as food additive can, thus, be regarded as harmless for the whole population. Even in unphysiologically high doses GLU will not trespass into fetal circulation. Further research work should, however, be done concerning the effects of high doses of a bolus supply at presence of an impaired blood brain barrier function. In situations with decreased appetite (e.g., elderly persons) palatability can be improved by low dose use of monosodium-L-glutamate.

Aberrant activation of with no lysine (WNK) kinases causes familial hyperkalemic hypertension (FHHt). Thiazide diuretics treat the disease, fostering the view that hyperactivation of the thiazide-sensitive sodium-chloride cotransporter (NCC) in the distal convoluted tubule (DCT) is solely responsible. However, aberrant signaling in the aldosterone-sensitive distal nephron (ASDN) and inhibition of the potassium-excretory renal outer medullary potassium (ROMK) channel have also been implicated. To test these ideas, we introduced kinase-activating mutations after Lox-P sites in the mouse gene, which encodes the terminal kinase in the WNK signaling pathway, Ste20-related proline-alanine-rich kinase (SPAK). Renal expression of the constitutively active (CA)-SPAK mutant was specifically targeted to the early DCT using a DCT-driven Cre recombinase. CA-SPAK mice displayed thiazide-treatable hypertension and hyperkalemia, concurrent with NCC hyperphosphorylation. However, thiazide-mediated inhibition of NCC and consequent restoration of sodium excretion did not immediately restore urinary potassium excretion in CA-SPAK mice. Notably, CA-SPAK mice exhibited ASDN remodeling, involving a reduction in connecting tubule mass and attenuation of epithelial sodium channel (ENaC) and ROMK expression and apical localization. Blocking hyperactive NCC in the DCT gradually restored ASDN structure and ENaC and ROMK expression, concurrent with the restoration of urinary potassium excretion. These findings verify that NCC hyperactivity underlies FHHt but also reveal that NCC-dependent changes in the driving force for potassium secretion are not sufficient to explain hyperkalemia. Instead, a DCT-ASDN coupling process controls potassium balance in health and becomes aberrantly activated in FHHt.

Large, Ca(2+)-activated K(+) channels (BK), comprised of alpha- and beta-subunits, mediate K(+) secretion during high flow rates in distal nephron segments. Because the BK-beta1 subunit enhances Ca(2+) sensitivity of BK in a variety of cells, we determined its role in flow-induced K(+) secretion and its localization in the mammalian nephron. To determine the role of BK-beta1 in the kaliuretic response to volume expansion, the rate of K(+) excretion (U(K)V) vs. varied urinary flow rates were determined in wild-type and BK-beta1 knockout mice (BK-beta1(-/-)). When flow rate was varied by volume expansion (2 ml.h(-1).25 g body wt(-1)) for 30 to 60 min in wild-type mice, we found that the U(K)V increased significantly with increasing urine flow rates (r(2) = 0.50, P < 0.00001, n = 31), as demonstrated previously in distal nephron of rats and rabbits. However, in BK-beta1(-/-) mice, U(K)V did not vary with changing flow rates (r(2) = 0.15, P = 0.08, n = 20). Using immunohistochemical techniques, we found that BK-beta1 was strongly expressed in the apical membrane of the murine distal nephron and that 98% of BK-beta1 protein detected by histochemistry colocalized with NCX, a marker of connecting tubules (CNT). Both BK-beta1 and NCX colocalized with BK-alpha in separate experiments. Furthermore, we confirmed BK-beta1 protein expression in the apical membrane of connecting tubules in rabbits. BK-beta1 RNA from rabbit CNT was sequenced and was identical to previously published rabbit muscle sequences. These data show that the BK-beta1 accessory subunit is present in the CNT segment of the mammalian distal nephron and has a significant role in the kaliuretic response to increased urinary flow induced by volume expansion.

Mice lacking the ␤1-subunit (gene, Kcnmb1; protein, BK-␤1) of the large Ca-activated K channel (BK) are hypertensive. This phenotype is thought to result from diminished BK currents in vascular smooth muscle where BK-␤1 is an ancillary subunit. However, the ␤1-subunit is also expressed in the renal connecting tubule (CNT), a segment of the aldosterone-sensitive distal nephron, where it associates with BK and facilitates K secretion. Because of the correlation between certain forms of hypertension and renal defects, particularly in the distal nephron, it was determined whether the hypertension of Kcnmb1 ؊/؊ has a renal origin. We found that Kcnmb1 ؊/؊ are hypertensive, volume expanded, and have reduced urinary K and Na clearances. These conditions are exacerbated when the animals are fed a high K diet (5% K; HK). Supplementing HK-fed Kcnmb1 ؊/؊ with eplerenone (mineralocorticoid receptor antagonist) corrected the fluid imbalance and more than 70% of the hypertension. Finally, plasma [aldo] was elevated in Kcnmb1 ؊/؊ under basal conditions (control diet, 0.6% K) and increased significantly more than wild type when fed the HK diet. We conclude that the majority of the hypertension of Kcnmb1 ؊/؊ is due to aldosteronism, resulting from renal potassium retention and hyperkalemia.adrenal medulla ͉ BK ͉ eplerenone ͉ mineralcorticoid ͉ volume expansion