Protein kinase C modulates cell swelling-induced Ca2+ influx and prolactin secretion in GH4C1 cells

Osmotic membrane stretch increases cytosolic Ca(2+) and inhibits bone resorption activity in rat osteoclasts

Although the importance of mechanical stress on bone metabolism is well known, the intracellular mechanisms involved are not well understood. To evaluate the role of mechanical stress on osteoclastic function, we investigated the effects of membrane stretch induced by osmotic cell swelling on cytosolic Ca(2+) and bone resorption activity in freshly isolated rat osteoclasts. The intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured by fura-2 microspectrofluorimetry. Exposure to hypotonic solution (211-151 mOsm) caused cell swelling and reversibly increased [Ca(2+)](i) in the osteoclasts. This [Ca(2+)](i) increase was abolished by the omission of extracellular Ca(2+), but was not affected by the depletion of intracellular Ca(2+) stores. Gd(3+) and La(3+) inhibited the swelling-induced [Ca(2+)](i) increase, while nifedipine and Bay K 8644 did not. Neither protein kinase A inhibitors (Rp-cAMP, H-89) nor protein kinase C inhibitors (staurosporine, chelerythrine) affected the [Ca(2+)](i) increase. Membrane depolarization was not essential for the [Ca(2+)](i) increase either. To assess the effects of membrane stretch on the bone resorption activity of osteoclasts, we investigated actin ring formation, the intracellular structure responsible for bone resorption in osteoclasts. Hypotonic stimulation acutely disrupted actin ring formation in an extracellular Ca(2+)-dependent manner, and this disruption was prevented by Gd(3+). Moreover, Ca(2+) ionophore (ionomycin) also induced disruption of the actin rings. These results indicate that mechanical stress inhibits osteoclastic bone resorption activity, possibly via the elevation of [Ca(2+)](i) through stretch-activated, non-selective cation channels.

Vinblastine enhancement of hyposmosis-induced catecholamine release in cultured adrenal chromaffin cells: lack of relation to cell swelling and microtubule disruption

Exposure of chromaffin cells to hyposmotic solution has been shown to cause catecholamine release through the elevation of intracellular Ca2+ level. While cell volume change observed under hyposmotic conditions has been shown to be accompanied by the movement of various ions and suggested to be associated with the reorganization of cytoskeletons. In the present study, the effects of cytoskeleton-disrupting agents on hyposmosis-induced catecholamine release were examined to investigate a possible relationship between catecholamine release and cell volume change under hyposmotic conditions. Hyposmosis-induced catecholamine release was enhanced by pre-treatment of the cells with a microtubule-disrupting agent vinblastine, but not significantly altered by a microfilament-disrupting agent cytochalasin B. Vinblastine also caused an additional increase in the intracellular Ca2+ but failed to affect the cell volume change under hyposmotic conditions. In contrast, the hyposmosis-induced release was not significantly altered by either colchicine, another microtubule-disrupting agent, or taxol, a microtubule-stabilizing agent. These results indicate that vinblastine enhances hyposmosis-induced catecholamine release through an additional increase in the intracellular Ca2+ and furthermore suggest that this effect of vinblastine on the hyposmosis-induced release is unassociated with the disruption of the microtubule system, providing evidence for a lack of the direct relationship between catecholamine release and the cell volume change observed under hyposmotic conditions.

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.

Possible involvement of intracellular Ca2+ in hyposmosis-evoked catecholamine release from adrenal chromaffin cells

The influence of hyposmotic conditions on catecholamine release was studied using cultured adrenal chromaffin cells. Incubation of the cells in hyposmotic solution led to the enhancement of catecholamine release in a manner dependent on the reduction of osmolarity. Hyposmosis-evoked catecholamine release was similarly observed in the presence or absence of extracellular Ca2+, and was not significantly affected by organic and inorganic Ca2+ entry blockers. These results indicated that the hyposmosis-evoked release might be associated with a rise in the intracellular Ca2+ concentration. Further studies showed that neither ryanodine nor thapsigargin caused any significant effect on hyposmosis-evoked catecholamine release, whereas pretreatment of chromaffin cells with carbonyl cyanide m-chlorophenyl hydrazone significantly enhanced the hyposmosis-evoked release. Catecholamine release evoked by exposure to hyposmotic medium is therefore thought to be mediated through intracellular Ca2+, which may be mainly sequestered by the mitochondrial pools. Neither caffeine- nor inositol 1,4,5-trisphosphate-sensitive Ca2+ pools seems likely to be involved in hyposmosis-evoked catecholamine release, although the Ca2+ pools that contribute to the elevation of intracellular Ca2+ observed under hyposmotic conditions are not yet completely identified.

The role of swelling-induced anion channels during neuronal volume regulation

Regulation of cell volume is an essential function of most mammalian cells. In the cells of the central nervous system, maintenance of cell osmolarity and, hence, volume, is particularly crucial because of the restrictive nature of the skull. Cell volume regulation involves a variety of pathways, with considerable differences between cell types. One common pathway activated during hypo-osmotic stress involves chloride (Cl-) channels. However, hypo-osmotically stimulated anion permeability can be regulated by a diverse array of second messengers. Although neuronal swelling can occur in a number of pathological and nonpathological conditions, our understanding of neuronal volume regulation is limited. This article summarizes our current understanding of the role of anion channels during neuronal volume regulation.

Shrinkage activates a nonselective conductance: involvement of a Walker-motif protein and PKC

The ability of all cells to maintain their volume during an osmotic challenge is dependent on the regulated movement of salt and water across the plasma membrane. We demonstrate the phosphorylation-dependent gating of a nonselective conductance in Caco-2 cells during cellular shrinkage. Intracellular application of exogenous purified rat brain protein kinase C (PKC) resulted in the activation of a current similar to that activated during shrinkage with a Na(+)-to-Cl- permeability ratio of approximately 1.7:1. To prevent possible PKC- and/or shrinkage-dependent activation of cystic fibrosis transmembrane regulator (CFTR), which is expressed at high levels in Caco-2 cells, a functional anti-peptide antibody, anti-CFTR505-511, was introduced into the cells via the patch pipette. Anti-CFTR505-511, which is directed against the Walker motif in the first nucleotide binding fold of CFTR, prevented the PKC/shrink-age current activation. The peptide CFTR505-511 also induced current inhibition, suggesting the possible involvement of a regulatory element in close proximity to the channel that shares sequence homology with the first nucleotide binding fold of CFTR and whose binding to the channel is required for channel gating.

High glucose concentrations and protein kinase C isoforms in vascular smooth muscle cells

High extracellular glucose activates protein kinase C (PKC), a family of kinases vital to intracellular signaling. However, which PKC isoforms are involved and where in the cell they operate is unclear. We tested the hypothesis that only those PKC isoforms binding to diacylglycerol (DAG) are activated by high glucose. We also reasoned that the isoforms would translocate to different parts of the cell, where they presumably serve different functions. The PKC isoforms alpha, beta, delta, epsilon, and zeta were studied. Twenty mM glucose caused an increase in total PKC activity at six hours, which was maintained at 24 hours. High glucose decreased the angiotensin II-induced calcium signal. This effect was reversed by preincubating the cells with the PKC inhibitor staurosporine. Glucose induced a translocation of all PKC isoforms except PKC zeta by Western blot. Confocal microscopy showed that PKC alpha, beta, and epsilon were translocated into the nucleus. PKC delta showed strong association with cytoskeletal structures. The effects were sustained at 24 hours for PKC isoform beta and to a lesser extent for PKC delta and epsilon, but not for PKC alpha. Thus, PKC isoforms differ in their propensity to be activated by high glucose. Those isoforms binding to DAG are activated. Both cytoskeletal and nuclear signaling may be involved.

Combined magnetic fields increased net calcium flux in bone cells

Low energy electromagnetic fields (EMF) exhibit a large number of biological effects. A major issue to be determined is "What is the lowest threshold of detection in which cells can respond to an EMF?" In these studies we demonstrate that a low-amplitude combined magnetic field (CMF) which induces a maximum potential gradient of 10(-5) V/m is capable of increasing net calcium flux in human osteoblast-like cells. The increase in net calcium flux was frequency dependent, with a peak in the 15.3-16.3 Hz range with an apparent bandwidth of approximately 1 Hz. A model that characterizes the thermal noise limit indicates that non-spherical cell shape, resonant type dynamics, and signal averaging may all play a role in the transduction of low-amplitude EMF effects in biological systems.

Activation of protein kinase C during cell volume regulation in Ehrlich mouse ascites tumor cells

We have previously demonstrated that in Ehrlich cells a bumetanide-sensitive Na+,K+,2Cl- cotransporter is activated during regulatory volume decrease after cell shrinkage (hypertonic conditions) as well as during the late phase of regulatory volume decrease (hypotonic conditions). It is, however, quiescent under isotonic conditions. Using a protein kinase C assay system (Amersham, UK) it is here demonstrated that hypertonic cell shrinkage results in an increase in protein kinase C activity to 174% within the first minute, concomitant with the activation of the Na+,K+,2Cl- cotransporter. Hypotonic cell swelling results in a late activation of protein kinase C concomitant with a late activation of the Na+,K+,2Cl- cotransporter. The activation of protein kinase C during hypertonic as well as hypotonic conditions is inhibited by H-7. The more specific protein kinase C inhibitor chelerythrine inhibited protein kinase C as well as the Na+,K+,2Cl- cotransporter to the same extent as did H-7. These results indicate the involvement of protein kinase C in the regulation of the Na+,K+,2Cl- cotransporter in Ehrlich ascites tumor cells during cell volume regulation.