Thiazide-sensitive NaCl-cotransporter in the intestine: possible role of hydrochlorothiazide in the intestinal Ca2+ uptake.

Thiazides, such as hydrochlorothiazide (HCTZ), are used to control blood pressure and to reduce renal calcium excretion. These effects are a result of interactions with the NaCl-cotransporter (NCC). This is demonstrated by the fact that mutations within the NCC protein lead to salt-resistant hypotension and hypocalciuria, paralleled by an increase in bone mineral density. These symptoms are also known as Gitelman syndrome. It has become increasingly evident that the effect of HCTZ on blood pressure and calcium homeostasis cannot be attributed exclusively to kidney functions, where the primary action of HCTZ on NCC is postulated to occur. We demonstrated the presence of the NCC transporter in the rat small intestine (ileum and jejunum) and human HT-29 cells, by using reverse transcription-PCR, Northern blot, Western blot, and immunofluorescence. Furthermore, we show that HCTZ modulates Ca(2+) uptake by intestinal cells, while affecting the electrical parameters of the cellular membrane, thus suggesting a functional interaction between NCC and the epithelial voltage-dependent calcium channel. The experiments presented here support the hypothesis of a direct involvement of the intestinal cells in the interaction between HCTZ and NaCl, as well as calcium homeostasis.

Membrane thickness changes ion-selectivity of channel-proteins.

The plasma membrane is a highly dynamic cell-barrier if the nature and distribution of its constituents are considered. Ion channels are embedded in these double lipid bilayers, which modulate their 3D-structures. The structure modulations by the lipid bilayer can assume such a degree that channel activation depends on them, as was shown for the KcsA potassium channel. Here we show that the cation-over-anion selectivity of reconstituted ICln channels can be varied by the thickness of a bilayer build of phosphatidylcholines. The shorter the acyl-chains and therefore the thinner the bilayers of the membrane are, the more potassium selective the channels are. In contrast, the longer the acyl-chains and therefore the thicker the membranes are, the more chloride selective the channels become.

Mechanisms sensing and modulating signals arising from cell swelling.

Cell volume alterations are involved in numerous cellular events like epithelial transport, metabolic processes, hormone secretion, cell migration, proliferation and apoptosis. Above all it is a need for every cell to counteract osmotic cell swelling in order to avoid cell damage. The defence against excess cell swelling is accomplished by a reduction of the intracellular osmolarity by release of organic- or inorganic osmolytes from the cell or by synthesis of osmotically less active macromolecules from their specific subunits. De-spite the large amount of experimental data that has accumulated, the intracellular mechanisms underlying the sensing of cell volume perturbations and the activation of volume compensatory processes, commonly summarized as regulatory volume decrease (RVD), are still only partly revealed. Moving into this field opens a complex scenario of molecular rearrangements and interactions involving intracellular messengers such as calcium, phosphoinositides and inositolphosphates as well as phosphoryla-tion/dephosphorylation processes and cytoskeletal reorganization with marked cell type- and tissue specific variations. Even in one and the same cell type significant differences regarding the activated pathways during RVD may be evident. This makes it virtually im-possible to unambigously define common sensing- and sinaling pathways used by differ-ent cells to readjust their celll volume, even if all these pathways converge to the activa-tion of comparatively few sets of effectors serving for osmolyte extrusion, including ion channels and transporters. This review is aimed at providing an insight into the manifold cellular mechanisms and alterations occuring during cell swelling and RVD.