This report provides insight into parathyroid gland physiology and the pathophysiology of hyperparathyroidism (HPT). Increases in the extracellular calcium concentration constitute the primary physiological signal for inhibition of parathyroid hormone (PTH) release. Transduction of the external signal into a cellular response involves activation of a cation receptor mechanism on the plasma membrane with rapid rise in the cytoplasmic calcium concentration of the cells. This recently discovered parathyroid calcium receptor has been characterized as a glycoprotein of unusually high molecular weight, which may play a key role in calcium homeostasis since it is also expressed in the kidney and placenta. Binding of external calcium to the receptor is associated with mobilization of intracellular calcium as well as calcium influx into the cells and phosphoinositol hydrolysis. These events rapidly interfere with the release process through essentially unknown mechanisms and probably also at sustained stimulation inhibit PTH gene transcription. The relative calcium insensitivity of the PTH release in HPT is associated with a deranged regulation of cytoplasmic calcium within pathological parathyroid cells. The molecular basis for this disturbance comprises down regulation of the cation receptor, whereby external calcium is translated into abnormally low levels of cytoplasmic calcium and insufficient inhibition of PTH release. Studies on expression of the functionally important cation sensing glycoprotein and its associated cellular signal systems may provide novel means for interference with the pathophysiological derangements of HPT.

Animals↗

PubMedDate not supplied

Comparison of the effects of leucines, non-metabolizable leucine analogues and other insulin secretagogues on the activity of glutamate dehydrogenase.

Glutamate dehydrogenase (GLDH) from bovine liver was employed in model system for testing a possible role of GLDH in insulin release. The ability of different insulin secretagogues to stimulate the activity of the diethylstilbestrol-inhibited enzyme was tested. The two insulin-releasing amino acids, L-leucine and its non-metabolizable analogue 2-aminobicyclo(2, 2, 1)heptane-2-carboxylic acid [b(--)-BCH], were the best stimulators of GLDH activity. The non-secreting stereoisomers, D-leucine and b(+)-BCH, were less effective. Glucose, L-arginine and the leucine metabolite alpha-ketoisocaproic acid lacked significant effects on GLDH activity. Small and diverging effects were obtained with sulfonvlurea compounds: whereas carbutamide caused slight stimulation, tolbutamide and glipizide had no effect, and glibenclamide was an inhibitor. The specificity of the insulin-releasing amino acids L-leucine and b(--)-BCH in stimulating GLDH activity makes it tempting to speculate about a connection between allosteric regulation of pyridine nucleotide-dependent enzymes and insulin release.

Amino Acids↗

PubMedDate not supplied

Cyclic AMP raises cytoplasmic calcium in pancreatic alpha 2-cells by mobilizing calcium incorporated in response to glucose.

The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in individual guinea-pig pancreatic alpha 2-cells exposed to modulators of glucagon release. Addition of the stimulatory amino acid arginine resulted in a sustained increase in [Ca2+]i, whereas the inhibitor glucose had the opposite effect. Epinephrine, the beta-adrenergic agonist isoproterenol, the adenylate cyclase activator forskolin and 8-bromo-cAMP transiently raised [Ca2+]i provided that the cells had been pretreated with glucose. However, simultaneous presence of glucose was not required and the effect occurred even in the absence of extracellular Ca2+. Carbachol, the alpha 2-adrenergic agonist clonidine and the sulfonylurea tolbutamide lacked effects on [Ca2+]i. In addition to providing support for the concept that glucagon release is positively modulated by [Ca2+]i, the results demonstrate that cAMP raises [Ca2+]i in the alpha 2-cells by mobilizing calcium incorporated in response to glucose.

Animals↗

PubMedDate not supplied

Glucose induces temperature-dependent oscillations of cytoplasmic Ca2+ in single pancreatic beta-cells related to their electrical activity.

Glucose induces large amplitude oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells. The effects of temperature on these oscillations were examined by monitoring [Ca2+]i continuously in single beta-cells from ob/ob-mice using dual wavelength microfluorometry. The oscillations of [Ca2+]i disappeared when the temperature was increased above 42 degrees C and were reversibly inhibited below 30 degrees C. However, cooling did not prevent a glucose response in terms of the average rise of [Ca2+]i. Since patch clamp studies of single beta-cells have indicated a random occurrence of glucose-induced action potentials at room temperature, it was important to explore how the sugar affected the electrical activity at 37 degrees C. Using the cell-attached configuration of the patch clamp technique for such analyses, the action potentials were found to occur in bursts with durations similar to the large amplitude oscillations of [Ca2+]i.

Action Potentials↗

PubMedDate not supplied

Propagation of cytoplasmic Ca2+ oscillations in clusters of pancreatic beta-cells exposed to glucose.

Digital image analysis was employed for resolving the temporal and spatial variations of the cytoplasmic Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with the Ca(2+)-indicator Fura-2. Glucose-stimulated individual beta-cells exhibited large amplitude oscillations of [Ca2+]i with a mean frequency of 0.33 min-1. When Ca2+ diffusion was restricted by increasing the Ca2+ buffering capacity, the sugar-induced rise of [Ca2+]i preferentially affected the peripheral cytoplasm. When glucagon was present glucose also caused less prominent oscillations with about a 10-fold higher frequency superimposed on an elevated [Ca2+]i. In small clusters of 6-14 cells the average frequency of the large amplitude oscillations increased to 0.60 min-1. The clusters were found to contain micro-domains of electrically coupled cells with synchronized oscillations. After increasing the glucose concentration, adjacent domains became functionally coupled. The oscillations originated from different cells in the cluster. Also the fast glucagon-dependent oscillations were synchronized between cells and had different origins. The results indicate that coupling of beta-cells leads to an increased frequency of the large amplitude oscillations, and that the oscillatory characteristics are determined collectively among electrically coupled beta-cells rather than by particular pacemaker cells. In the light of these data it is necessary to reconsider the previous ideas that glucose-induced oscillations of membrane potential and [Ca2+]i require coupling between many beta-cells, and that the peak [Ca2+]i values reached during oscillations should increase with the size of the coupled cluster.

Animals↗

PubMedDate not supplied

Early ionic events associated with phorbol ester induced differentiation and inhibition of cell growth in hematopoietic tumor cell lines.

The effects of 12-O-tetradecanoylphorbol-13 acetate (TPA) on DNA synthesis, phenotypic expression, cytoplasmic Ca2+ (Ca2+i), intracellular pH (pHi) and membrane potential were studied in the monoblastic U-937 and the erythroleukemic K-562 cells. In both cell lines DNA synthesis was inhibited and in the U-937 cells this was accompanied by the appearance of macrophage differentiation markers. The erythroid characteristics of K-562 cells, on the other hand, were markedly suppressed. Intracellular pH (pHi) was increased by TPA treatment; however, while the alkalinization of K-562 cells was dependent on the presence of extracellular Na+, the response of U-937 cells was unaffected by the removal of this cation. In each cell type the protein kinase C (PKC) inhibitor H-7 largely attenuated the TPA induced increase of pHi. Moreover, the alpha-stereoisomer of TPA, which does not activate PKC, had no effects. TPA caused a dose-dependent decrease in Ca2+i which was more pronounced in U-937 cells. Measurements of membrane potential revealed a marked TPA depolarization of the K-562 cells, but no such effects were observed in the U-937 cell line. The depolarizing response of K-562 cells could be abolished by substituting extracellular Na+ with choline+, indicating the presence of a TPA sensitive Na+ permeability. The results show that the phorbol ester TPA, which inhibits proliferation and causes phenotypic modulation, also induced a number of early, apparently PKC dependent and cell type specific, changes of intracellular ion activities. The possible role of intracellular ion fluxes in the regulation of cell growth and differentiation is discussed.

Calcium↗

PubMedDate not supplied

The role of Ca2+ in the release of pancreatic islet hormones.

The role of Ca2+ in initiating exocytosis of granule-bound secretory products was evaluated with respect to pancreatic islet hormones. Apart from stimulating the transfer of the granules to the plasma membrane and their subsequent extrusion, a rise of the cytoplasmic Ca2+ concentration ([Ca2+]i) may under certain conditions also have depressive effects on insulin release. Glucose has a bidirectional action on [Ca2+]i by stimulating both the entry of the ion and its removal by organelle sequestration and outward transport. The recognition of glucose as a secretory stimulus is based on sudden transitions between oscillatory and steady-state [Ca2+]i at threshold concentrations of the sugar characteristic for the individual beta-cell. The intrinsic ability of each beta-cell to generate oscillations of [Ca2+]i and the subsequent synchronization of these signals result in a pulsatile release of insulin from isolated islets. Glucose regulation of this process is manifested as alterations of the amplitudes of the insulin pulses without effects on the frequency. It is suggested that electrical signalling from the beta-cells in combination with direct effects of glucose are important for regulating the release of glucagon and somatostatin.

Animals↗