Short- and long-term A3 adenosine receptor activation inhibits the Na+/H+ exchanger NHE3 activity and expression in opossum kidney cells

The renal function of the A(3) adenosine receptor (A3AR) is poorly characterized. In this study, we report that the A3AR-selective agonist, 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purine-9-yl]-1-deoxy-N-methyl-b-D-ribofuranuronamide (2-Cl-IBMECA) regulates the Na+/H+ exchanger-3 (NHE3) in a dose- and time-dependent fashion. In opossum kidney (OK) cells, 2-Cl-IBMECA at high (10(-6) M) and low (10(-8) M) dose inhibits NHE3 by a multiphasic time course with an acute phase of NHE3 inhibition from 15 min to 1 h, followed by a chronic phase of NHE3 inhibition from 24 to 48 h. Pre-incubation with either the selective A3AR-antagonist MRS1523 (10(-7) M) or the protein kinase C inhibitor, Calphostin C (10(-8) M) completely blocked 10(-6) M 2-Cl-IBMECA-induced acute (15 min) and chronic (24 h) phases of NHE3 inhibition. In contrast, the acute inhibitory phase (15 min) of 10(-8) M 2-Cl-IBMECA was completely prevented only when Calphostin C (10(-8) M) was added in conjunction with the protein kinase A inhibitor, H89 (10(-7) M). Acute (15 or 30 min depending on the A3AR-agonist concentration) A3AR-dependent inhibition of NHE3 activity was accompanied by decrease in cell surface NHE3 protein with no change in total NHE3 antigen. Chronic (24 h) A3AR-mediated down-regulation of NHE3 was associated with reduction of surface NHE3, decreased total NHE3 protein (70%) and a paradoxical rise of NHE3 RNA (40%). In summary, these results indicate that A3AR directly regulates NHE3 at multiple levels in a complex pattern. A3AR-dependent short- and long-term inhibition of NHE3 may be a fundamental mechanism of net sodium and fluid balance.

Acute regulation of Na/H exchanger NHE3 by adenosine A(1) receptors is mediated by calcineurin homologous protein

Adenosine is an autacoid that regulates renal Na(+) transport. Activation of adenosine A(1) receptor (A(1)R) by N(6)-cyclopentidyladenosine (CPA) inhibits the Na(+)/H(+) exchanger 3 (NHE3) via phospholipase C/Ca(2+)/protein kinase C (PKC) signaling pathway. Mutation of PKC phosphorylation sites on NHE3 does not affected regulation of NHE3 by CPA, but amino acid residues 462 and 552 are essential for A(1)R-dependent control of NHE3 activity. One binding partner of the NHE family is calcineurin homologous protein (CHP). We tested the role of NHE3-CHP interaction in mediating CPA-induced inhibition of NHE3 in opossum kidney (OK) and Xenopus laevis uroepithelial (A6) cells. Both native and transfected NHE3 and CHP are present in the same immuno-complex by co-immunoprecipitation. CPA (10(-6) M) increases CHP-NHE3 interaction by 30 - 60% (native and transfected proteins). Direct CHP-NHE3 interaction is evident by yeast two-hybrid assay (bait, NHE3(C terminus); prey, CHP); the minimal interacting region is localized to the juxtamembrane region of NHE3(C terminus) (amino acids 462-552 of opossum NHE3). The yeast data were confirmed in OK cells where truncated NHE3 (NHE3(delta552)) still shows CPA-stimulated CHP interaction. Overexpression of the polypeptide from the CHP binding region (NHE3(462-552)) interferes with the ability of CPA to inhibit NHE3 activity and to increase CHPNHE3(Full-length) interaction. Reduction of native CHP expression by small interference RNA abolishes the ability of CPA to inhibit NHE3 activity. We conclude that CHPNHE3 interaction is regulated by A(1)R activation and this interaction is a necessary and integral part of the signaling pathway between adenosine and NHE3.

Bimodal acute effects of A1 adenosine receptor activation on Na+/H+ exchanger 3 in opossum kidney cells

Regulation of renal apical Na+/H+ exchanger 3 (NHE3) activity by adenosine has been suggested to contribute to acute control of mammalian Na(+) homeostasis. The mechanism by which adenosine controls NHE3 activity in a renal cell line was examined. The adenosine analog, N(6)-cyclopentyladenosine (CPA) exerts a bimodal effect on NHE3: CPA concentrations >10(-8) M inactivate NHE3, whereas concentrations <10(-8) M stimulate NHE3 activity. Acute CPA-induced control of NHE3 was blocked by antagonists of A1 adenosine receptors and inhibition of phospholipase C, pretreatment with BAPTA-AM (chelator of cellular calcium), and exposure to pertussis toxin. Stimulatory and to some extent also inhibitory CPA concentrations attenuated 8-bromo-cAMP and dopamine-mediated inhibition of NHE3. BAPTA eliminated the ability of a stimulatory dose of CPA to attenuate 8-bromo-cAMP-induced suppression of NHE3 activity. Upon inhibition of protein kinase C, CPA at an inhibitory dose provoked activation of NHE3, which is partially reverted by 8-bromo-cAMP and suppressed by pre-incubation with BAPTA-AM. Cytochalasin B, an actin-modifying agent, selectively prevented downregulation but did not affect upregulation of NHE3 activity by CPA. In conclusion, these observations demonstrate that (1) CPA modulates NHE3 activity by elevation of cellular Ca(2+) exerting a negative control on adenylate cyclase activity, (2) protein kinase C is the determining factor leading to CPA-induced downregulation of NHE3 activity, and (3) alterations of surface NHE3 abundance may contribute to A1 adenosine receptor-dependent inhibition of NHE3 activity.

Molecular aspects of acute inhibition of Na(+)-H(+) exchanger NHE3 by A(2)-adenosine receptor agonists

Adenosine regulates Na(+) homeostasis by its acute effects on renal Na(+) transport. We have shown in heterologously transfected A6/C1 cells (renal cell line from Xenopus laevis) that adenosine-induced natriuresis may be effected partly via A(2) adenosine receptor-mediated inactivation of the renal brush border membrane Na(+)-H(+) exchanger NHE3. In this study we utilized A6/C1 cells stably expressing wild-type as well as mutated forms of NHE3 to assess the molecular mechanism underlying A(2)-dependent control of NHE3 function. Cell surface biotinylation combined with immunoprecipitation revealed that NHE3 is targeted exclusively to the apical domain and that the endogenous Xenopus NHE is located entirely on the basolateral side of A6/C1 transfectants. Stimulation of A(2)-adenosine receptors located on the basolateral side for 15 min with CPA (N6-cyclopentyladenosine) acutely decreased NHE3 activity (microspectrofluorimety). This effect was mimicked by 8-bromo-cAMP and entirely blocked by pharmacological inhibition of PKA (with H89) or singular substitution of two PKA target sites (serine 552 and serine 605) on NHE3. Downregulation of NHE3 activity by CPA was attributable to a reduction of NHE3 intrinsic transport activity without change in surface NHE3 protein at 15 min. At 30 min, the decrease in transport activity was associated with a decrease in apical membrane NHE3 antigen. In conclusion, two highly conserved target serine sites on NHE3 determine NHE3 modulation upon A(2)-receptor activation and NHE3 inactivation by adenosine proceeds via two phases with distinct mechanisms.

[The body muscle compartment and its relationship to food absorption and blood chemistry during an extreme endurance performance]

The purpose of the study was to examine the changes of the muscle's fat-free compartment and its relation to the corresponding biochemical and nutritional parameters of 42 men and 13 women, the participants of an ultra long-distance run of 1000 km (20 days of daily running 50 km). The muscle-fractions initially increased, decreased in the middle phase, and remained stable for the rest of the run. Significant changes of the fat-free weight were registered from the 11th day on, the LBM decreasing until the middle of the distance; then the lean body mass enlarged. All the muscle-circumferences were reduced with the exception of the thigh, which grew, paralleling the CK/CKMB-concentrations, this phenomenon being due to the high mechanical stress of the lower extremities. The biochemical parameters exhibit a strain-related reaction of adaptation within the initial 6 days, the hormones and protein-concentration increasing in the beginning and falling from the third day on, uric acid and CK/CKMB-activity decreasing from the 6th day on. The consecutive parallel reduction of both uric acid, urea, and muscle measurements might be seen as a special endurance-related clearance-mechanism of potential toxicants. The negative relationship between the changes of muscle measurements and the cumulative protein intake and the catabolic constellation of the clinical-chemical values might suggest that the absolute protein intake of 1.7 g/kg body mass should be increased in order to diminish the loss of musculature during an ultra-long distance run.