The signalling pathways allowing hormonal regulation of Na+ transport in murine collecting duct cells
The collecting duct of the distal nephron marks the final location where adjustments to Na+ excretion can be made, therefore determining the final concentration of Na+ conserved in the extracellular fluid which plays a role in governing overall blood volume and pressure. This transport of Na+ is subject to hormonal regulation but the signalling pathways underpinning this regulation however, are not fully understood. In this thesis the signalling pathways allowing both basal and insulin-stimulated Na+ absorption were explored in the murine collecting duct cell line, mpkCCDcl4. The effects of two insulin-sensitizing drugs, TZDs, on ENaC-mediated Na+ transport were investigated and the signalling pathways underlying two other hormonal regulators of ENaC, dexamethasone and vasopressin, were also examined. Unstimulated monolayers of mpkCCDcl4 cells generated spontaneous Na+ absorption which was quantified by measuring equivalent short circuit current (Ieq). Selective inhibition of PI3-kinase, mTORC2 and SGK1 left ~80 % of the current intact, indicating these signalling molecules are not required for basal Na+ transport. Acute addition of insulin stimulated Ieq and this occurred with a concomitant increase in mTORC2, SGK1 and Akt activity. Inhibition of PI3-kinase abolished the insulin-stimulated response as well as phosphorylation of downstream substrates, indicating a crucial role of PI3-kinase. Inhibition of mTORC1 with rapamycin did not alter basal or insulin-stimulated Na+ transport. The mTOR inhibitors TORIN1 and PP242 could therefore be used to evaluate the role of mTORC2. These inhibitors greatly reduced insulin-stimulated ENaC-mediated Na+ transport and also abolished SGK1 and mTORC2 activity, indicating a novel role of mTORC2. An inhibitor of SGK1, GSK650394A abolished insulin-stimulated Na+ transport and specifically inhibited SGK1 acitivty demonstrating the importance of SGK1 in insulin signalling. The inhibitor Akti-1/2 also abolished insulin-mediated Na+ transport but this compound inhibited both Akt and SGK1 activity. The TZDs pioglitazone and rosiglitazone did not alter basal or insulin-stimulated Na+ transport and had no effect on SGK1 activity indicating these drugs do not alter Na+ absorption in this cell line. Dexamethasone stimulated ENaC-mediated Na+ transport in a similar manner to insulin and this could be blocked with rapamycin. This drug did not alter phosphorylation of NDRG1 indicating that dexamethasone stimulates Na+ transport in an mTORC1-dependent manner but without altering SGK1 activity. Arginine vasopressin also stimulated Ieq but did so by reducing Rt with an associated depolarisation of Vt. Ieq could be blocked with amiloride and vasopressin-stimulated Ieq was insensitive to TORIN1 and PP242. Vasopressin suppressed SGK1 phosphorylation of NDRG1 but did stimulate protein kinase A (PKA) activity. Therefore vasopressin stimulates Ieq via a PKA-dependent but mTOR- and SGK1-independent pathway.
Insulin, Kinase inhibitors, Epithelial Na+ Channel, Cortical Collecting Duct, SGK1
Networked Digital Library of Theses and Dissertations