Background: Transient Receptor Potential (TRP) channels are expressed in many epithelial cells and respond to mechanical or osmotic stress by initiating a number of calcium-dependent mechanisms. The human serum and glucocorticoid regulated kinase (hSGK) is one such calcium-dependent protein found in the kidney. Stimulated by cell shrinkage, this volume-regulated kinase stimulates sodium transport. Aberrant absorption of sodium has been implicated in the development of hypertension associated with renal disease and diabetic nephropathy. Osmotic-induced changes in cell volume can have dramatic repercussions for connexin-mediated cell-to-cell contact and may further exacerbate sodium-induced hypertension.
Objectives: In the present study we have utilised a novel cell line of the human collecting duct (HCD), as a model system to examine expression and localisation of these key signal recognition response elements and have investigated the extent of direct cell-to-cell communication.
Results: Mechanical stimulation of fura-2-loaded HCD cells evoked a transient increase in cytosolic calcium (5 separate experiments). The heptanol-sensitive calcium signal propagated rapidly (30plus/minus9.4microns/sec) across adjacent cells within a defined cell cluster (10-15 cells/cluster). Microinjected Lucifer yellow, permeated within cell clusters and confirmed a high degree of direct cell-coupling between HCD cells. Expression of TRPV4, connexin-43 (Cx-43) and hSGK mRNA was confirmed by RT-PCR. Western blotting of HCD cells identified Cx-43 and hSGK protein expression and immunocytochemistry localised hSGK mainly to the nucleus and Cx-43 to the cytosol and plasma membrane.
Conclusions: These data suggest that the TRPV4 mechanoreceptors mediate touch-evoked changes in intracellular calcium in HCD-cells. The evoked calcium-signal propagates across HCD cell clusters via gap-junctions. These touch-evoked calcium-signals, that mimic osmotically-driven changes in cytosolic calcium, may have important implications for hSGK-mediated sodium re-absorption and the development of hypertension.
This study was supported by the Diabetes, Endocrine and Immersion Research Trust