Gap junctional permeability is affected by cell volume changes and modulates volume regulation

Glucose-evoked alterations in connexin43-mediated cell-to-cell communication in human collecting duct: a possible role in diabetic nephropathy

Aberrant sodium absorption has been linked to the development of hypertension in both renal disease and diabetes. Efficient absorption depends on coordination of cellular activity across the entire epithelium via cell-to-cell coupling. In the current study we have utilized a model human collecting duct cell line (HCD) to assess the role of connexin43 (Cx43)-mediated gap junctions in the transfer of intracellular Ca(2+) transients within coupled cell clusters. HCD cells express Cx43 mRNA and protein, as well as that for the mechanosensitive transient receptor potential receptor (TRPV4). Mechanical stimulation of individual cells within a cluster evoked a transient rise in cytosolic Ca(2+) concentration ([Ca(2+)](i)) that propagated between cells via a heptanol-sensitive mechanism. The rise in [Ca(2+)](i) was dependent on both store release and Ca(2+)-influx pathways. Lucifer yellow dye transfer and Cx43 knockdown experiments confirmed direct cell-to-cell communication. Application of the Ca(2+) ionophore ionomycin, or an increase in glucose (5 to 25 mM), produced a time-dependent (48 h) increase in Cx43 protein expression. The transmission rate of touch-evoked Ca(2+) transients between coupled cells was accelerated after exposure to high glucose, providing a functional correlate to increased Cx43 expression. These data suggest a pivotal role for Cx43-mediated gap junctions in the synchronization of activity between HCD cells in response to stimuli that mimic osmotic and physical changes. Cx43 expression and cell-to-cell communication increased in response to high glucose and may protect the collecting duct from renal damage associated with more established diabetic nephropathy.

Regulation of ion fluxes, cell volume and gap junctional coupling by cGMP in GFSHR-17 granulosa cells

Gap junctional communication between granulosa cells seems to play a crucial role for follicular growth and atresia. Application of the double whole-cell patch-clamp- and ratiometric fura-2-techniques allowed a simultaneous measurement of gap junctional conductance ( G(j)) and cytoplasmic concentration of free Ca(2+) ([Ca(2+)](i)) in a rat granulosa cell line GFSHR-17. The voltage-dependent gating of G(j) varied for different cell pairs. One population exhibited a bell-shape dependence of G(j) on transjunctional voltage, which was strikingly similar to that of Cx43/Cx43 homotypic gap junction channels expressed in pairs of oocytes of Xenopus laevis. Within 15-20 min, gap junctional uncoupling occurred spontaneously, which was preceded by a sustained increase of [Ca(2+)](i) and accompanied by shrinkage of cellular volume. These responses to the whole-cell configuration were avoided by absence of extracellular Ca(2+), blockage of K(+) efflux, or addition of 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) to the pipette solution. Even in the absence of extracellular Ca(2+) or blockage of K(+) efflux, formation of whole-cell configuration generated a Ca(2+) spike that could be suppressed by the presence of 8-Br-cGMP. We propose that intracellular cGMP regulates Ca(2+) release from intracellular Ca(2+) stores, which activates sustained Ca(2+) influx, K(+) efflux and cellular shrinkage. We discuss whether gap junctional conductance is directly affected by cGMP or by cellular shrinkage and whether gap junctional coupling and/or cell shrinkage is involved in the regulation of apoptotic/necrotic processes in granulosa cells.

Effect of membrane tension on gap junctional conductance of supporting cells in Corti's organ

The effects of turgor pressure-induced membrane tension on junctional coupling of Hensen cell isolates from the inner ear were evaluated by input capacitance or transjunctional conductance measurement techniques. Turgor pressure was altered by changing either pipette pressure or the osmolarities of extracellular solutions. Both positive pipette pressure and extracellular applications of hypotonic solutions, which caused cell size to concomitantly increase, uncoupled the cells as indicated by reduced input capacitance and transjunctional conductance. These changes were, in many cases, reversible and repeatable. Intracellular application of 50 microM H-7, a broad-based protein kinase inhibitor, and 10 mM BAPTA did not block the uncoupling effect of positive turgor pressure on inner ear gap junctions. The transjunctional conductance at a holding potential of -80 mV was 53.6 +/- 5.8 nS (mean +/- SEM, n = 9) and decreased approximately 40% at a turgor pressure of 1.41 +/- 0.05 kPa. Considering the coincident kinetics of cell deformation and uncoupling, we speculate that mechanical forces work directly on gap junctions of the inner ear. These results suggest that pathologies that induce imbalances in cochlear osmotic pressure regulation may compromise normal cochlear homeostasis.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Role of phosphoinositide hydrolysis in astrocyte volume regulation

Astrocytes exposed to hypoosmotic stress swell and subsequently reduce their size to almost their original volume, a phenomenon called regulatory volume decrease (RVD). We found that during hypoosmotic swelling there was a twofold increase in phosphatidylinositol (PI) hydrolysis. This increase was inhibited by the phospholipase C inhibitor, U-73122 (10 microM). Inhibition of PI hydrolysis resulted in blockage of RVD. We also examined whether agents that stimulate PI hydrolysis would enhance RVD. These agents significantly accelerated RVD. The rank order of potency was endothelin (20 nM) > or = norepinephrine (100 microM) > endothelin-3 (7 nM) > thrombin (1 U/ml) > or = ATP (500 microM) > bradykinin (20 microM) > or = carbachol (500 microM), as indicated by RVD rate constants. The extent of PI hydrolysis induced by these agents at the beginning of RVD exhibited a logarithmic relationship with the magnitude of RVD enhancement. Also, there was a linear relationship between the rate of PI hydrolysis and RVD rate constants. Our results suggest that stimulated PI hydrolysis is involved in the regulation of cell volume in astrocytes.