Search PubMedSearch

PubMed · 7808098

Hypocalcemic emergencies.

Abstract

Chronic hypocalcemia occurs frequently, although emergent hypocalcemia does not. When hypocalcemia is suspected, verification of ionized hypocalcemia is required and an etiopathologic search warranted. Etiology-specific therapy is suggested, although at times emergent intravenous calcium is indicated. Long-term nonspecific therapy includes oral calcium and vitamin D supplementation.

Explore related subjects

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

P M Reber, H Heath. 1995. Hypocalcemic emergencies.. https://doi.org/10.1016/s0025-7125(16)30086-4

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

Effect of calcium, other ions, and pH on the reactions of barley peroxidase with hydrogen peroxide and fluoride. Control of activity through conformational change.

Transient-state kinetic analysis of compound I formation for barley grain peroxidase (BP 1) has revealed properties that are highly unusual for a heme peroxidase but which may be relevant to its biological function. The enzyme shows very little reaction with H2O2 at pH > 5 and exhibited saturation kinetics at higher H2O2 concentrations (kcatapp increases from 1.1 s-1 at pH 4.5 to 4.5 s-1 at pH 3.1 with an enzyme-linked pKa < 3.7 (Rasmussen, C.B., Bakovic, M., Welinder, K. G., and Dunford, H. B. (1993) FEBS Lett. 321, 102-105)). In the present paper, it is shown that the presence of Ca2+ gives rise to biphasic kinetics for compound I formation, with a slow phase as described above and a fast phase that exhibits a second order rate constant more typical of a classical peroxidase (K1app = 1.5 x 10(7) M-1 S-1, which is pH-independent between 3.3 and 5.0). The amount of enzyme reacting in the fast phase increases with Ca2+ concentration (Kd = 4 +/- 1 mM at pH 4.0), although it is also moderately inhibited by Cl-. The absorption spectrum of BP 1, which appears to be a five-coordinate high spin ferric in the resting state changes insignificantly in the presence of Ca2+. In the presence of Cl-, it becomes six-coordinate high spin (Kd approximately 60 mM at pH 4.0) but only if Ca2+ is also present. Fluoride binds to BP 1 with monophasic kinetics in the presence of 0-5 mM Ca2+. The activating effect of Ca2+ can be mimicked only by replacing it with Sr2+ and Ba2+ ions. Comparing these data with the crystal structure of the inactive neutral form of BP 1 (Henriksen, A., Welinder, K. G., and Gajhede, M. (1997) J. Biol. Chem. 273, 2241-2248) and similar data for wild-type and mutant peroxidases of plant and fungal origin suggests (i) a proton-induced conformational change from an inactive BP 1 at neutral pH to a low activity BP 1 form with a functional distal histidine and (ii) a Ca(2+)-induced slow conformational change (at least compared with compound I formation) of this low activity form to a high activity BP 1 with a typical peroxidase reactivity. BP 1 is the first example of a plant peroxidase whose activity can be reversibly controlled at the enzyme level by pH- and Ca(2+)-induced conformational changes.

Calcium

Intercellular calcium signaling via gap junction in connexin-43-transfected cells.

In excitable cells, intracellular Ca2+ is released via the ryanodine receptor from the intracellular Ca2+ storing structure, the sarcoplasmic reticulum. To determine whether this released Ca2+ propagates through gap junctions to neighboring cells and thereby constitutes a long range signaling network, we developed a cell system in which cells expressing both connexin-43 and ryanodine receptor are surrounded by cells expressing only connexin-43. When the ryanodine receptor in cells was activated by caffeine, propagation of Ca2+ from these caffeine-responsive cells to neighboring cells was observed with a Ca2+ imaging system using fura-2/AM. Inhibitors of gap junctional communication rapidly and reversibly abolished this propagation of Ca2+. Together with the electrophysiological analysis of transfected cells, the observed intercellular Ca2+ wave was revealed to be due to the reconstituted gap junction of transfected cells. We next evaluated the functional roles of cysteine residues in the extracellular loops of connexin-43 in gap junctional communication. Mutations of Cys54, Cys187, Cys192, and Cys198 to Ser showed the failure of Ca2+ propagation to neighboring cells in accordance with the electrical uncoupling between transfected cells, whereas mutations of Cys61 and Cys68 to Ser showed the same pattern as the wild type. [14C]Iodoacetamide labeling of free thiols of cysteine residues in mutant connexin-43s showed that two pairs of intramolecular disulfide bonds are formed between Cys54 and Cys192 and between Cys187 and Cys198. These results suggest that intercellular Ca2+ signaling takes place in cultured cells expressing connexin-43, leading to their own synchronization and that the extracellular disulfide bonds of connexin-43 are crucial for this process.

Calcium

Macrophage-derived chemokine is a functional ligand for the CC chemokine receptor 4.

Macrophage-derived chemokine (MDC) is a recently identified member of the CC chemokine family. MDC is not closely related to other chemokines, sharing most similarity with thymus- and activation-regulated chemokine (TARC), which contains 37% identical amino acids. Both chemokines are highly expressed in the thymus, with little expression seen in other tissues. In addition, the genes for MDC and TARC are encoded by human chromosome 16. To explore this relationship in greater detail, we have more precisely localized the MDC gene to chromosome 16q13, the same position reported for the TARC gene. We have also examined the interaction of MDC with CC chemokine receptor 4 (CCR4), recently shown to be a receptor for TARC. Using a fusion protein of MDC with secreted alkaline phosphatase, we observed high affinity binding of MDC-secreted alkaline phosphatase to CCR4-transfected L1.2 cells (Kd = 0.18 nM). MDC and TARC competed for binding to CCR4, while no binding competition was observed for six other chemokines (MCP-1, MCP-3, MCP-4, RANTES (regulated on activation normal T cell expressed and secreted), macrophage inflammatory protein-1 alpha, macrophage inflammatory protein-1 beta). MDC was tested for calcium mobilization in L1.2 cells tranfected with seven different CC chemokine receptors. MDC induced a calcium flux in CCR4-transfected cells, but other receptors did not respond to MDC. TARC, which also induced calcium mobilization in CCR4 transfectants, was unable to desensitize the response to MDC. In contrast, MDC fully desensitized a subsequent response to TARC. Both MDC and TARC functioned as chemoattractants for CCR4 transfectants, confirming that MDC is also a functional ligand for CCR4. Since MDC and TARC are both expressed in the thymus, one role for these chemokines may be to attract CCR4-bearing thymocytes in the process of T cell education and differentiation.

Calcium