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PubMed1985

Intracellular calcium handling in cystic fibrosis: normal cytosolic calcium and intracellular calcium stores in neutrophils.

We used quin2, a high affinity fluorescent calcium indicator to measure free cytosolic Ca++ in neutrophils isolated from fresh heparinized blood of 13 cystic fibrosis (CF) patients (age range 6-32 yr) and 11 healthy individuals (age range 25-41 yr) as controls. The mean value of free cytosolic Ca++ for CF patients was 137 +/- 19 nM (mean +/- SD) and 141 +/- 23 nM for controls (p = NS). Calcium stores released after exposure of the cells to ionomycin in Ca++-free medium were also similar in neutrophils of CF patients (51 +/- 34 pmol/10(6) neutrophils) and in the controls (50 +/- 15 pmol/10(6) neutrophils). Stimulation of the neutrophils with the chemotactic peptide f-mlp (10(-7) M) led to similar rises in free cytosolic Ca++ with mean peak levels of 718 +/- 230 nM in CF patients and 687 +/- 321 nM in another population of 10 healthy adults. In our group of CF patients free cytosolic Ca++, intracellular Ca++ stores, and the kinetics of intracellular Ca++ transients induced by f-mlp are similar to those of normals. These findings do not support the hypothesis of a generalized defect in calcium handling at the cellular level in CF.

Adolescent↗

PubMed1999

Calcium-induced calcium release contributes to action potential-evoked calcium transients in hippocampal CA1 pyramidal neurons.

Calcium-induced calcium release (CICR) is a mechanism by which local elevations of intracellular calcium (Ca2+) are amplified by Ca2+ release from ryanodine-sensitive Ca2+ stores. CICR is known to be coupled to Ca2+ entry in skeletal muscle, cardiac muscle, and peripheral neurons, but no evidence suggests that such coupling occurs in central neurons during the firing of action potentials. Using fast Ca2+ imaging in CA1 neurons from hippocampal slices, we found evidence for CICR during action potential-evoked Ca2+ transients. A low concentration of caffeine enhanced Ca2+ transient amplitude, whereas a higher concentration reduced it. Simultaneous Ca2+ imaging and whole-cell recordings showed that membrane potential, action potential amplitude, and waveform were unchanged during caffeine application. The enhancement of Ca2+ transients by caffeine was not affected by the L-type channel blocker nifedipine, the phosphodiesterase inhibitor IBMX, the adenylyl cyclase activator forskolin, or the PKA antagonist H-89. However, thapsigargin or ryanodine, which both empty intracellular Ca2+ stores, occluded this effect. In addition, thapsigargin, ryanodine, and cyclopiazonic acid reduced action potential-evoked Ca2+ transients in the absence of caffeine. These results suggest that Ca2+ release from ryanodine-sensitive stores contributes to Ca2+ signals triggered by action potentials in CA1 neurons.

Action Potentials↗

PubMed2003

Effects of calcium buffers and calbindin-D28k upon histamine-induced calcium oscillations and calcium waves in HeLa cells.

The effects of the artificial Ca(2+) buffers EGTA and BAPTA upon histamine-induced Ca(2+) oscillations and calcium waves were studied in HeLa cells. These events were also examined in HeLa cell lines transfected with the intracellular calcium-binding protein calbindin-D28k (CaBP; HeLa-CaBP) or the pCINeo vector alone (HeLa-pCINeo). High concentrations of the Ca(2+) indicators fluo-3 and fura-2 significantly influenced the oscillatory pattern of intracellular Ca(2+) in HeLa-pCINeo cells exposed to 1 microM histamine. Loading cells with low concentrations of the cell-permeant esters of the artificial Ca(2+)-buffers EGTA or BAPTA, resulted in fewer cells with a distinct "baseline" oscillatory pattern, and loading with higher concentrations of BAPTA almost completely abolished them. In HeLa-CaBP cells, stimulation with 1 microM histamine resulted in individual Ca(2+) spikes that had a flattened profile when compared to control cells; peak [Ca(2+)](i) was lowered, the rate of increase in [Ca(2+)](i) was slower and transients were prolonged. When compared to HeLa-pCINeo cells, loading with EGTA or BAPTA, or transfection of CaBP, significantly reduced the propagation velocity (by up to 60%) of Ca(2+) waves induced by exposure to 100 microM histamine. We conclude that intracellular Ca(2+) buffering exerts a significant influence on global Ca(2+) responses in HeLa cells and the propagation of Ca(2+) waves that underlie them. The relative effectiveness of different Ca(2+) buffers, including CaBP, appears to be particularly dependent upon the rapidity of their binding kinetics, with BAPTA being the most effective.

Aniline Compounds↗

PubMed2000

Elevation of mitochondrial calcium by ryanodine-sensitive calcium-induced calcium release.

Calcium is an important regulator of mitochondrial function. Since there can be tight coupling between inositol 1,4, 5-trisphosphate-sensitive Ca(2+) release and elevation of mitochondrial calcium concentration, we have investigated whether a similar relationship exists between the release of Ca(2+) from the ryanodine receptor and the elevation of mitochondrial Ca(2+). Perfusion of permeabilized A10 cells with inositol 1,4, 5-trisphosphate resulted in a large transient elevation of mitochondrial Ca(2+) to about 8 microm. The response was inhibited by heparin but not ryanodine. Perfusion of the cells with Ca(2+) buffers in excess of 1 microm leads to large increases in mitochondrial Ca(2+) that are much greater than the perfused Ca(2+). These increases, which average around 10 microm, are enhanced by caffeine and inhibited by ryanodine and depletion of the intracellular stores with either orthovanadate or thapsigargin. We conclude that Ca(2+)-induced Ca(2+) release at the ryanodine receptor generates microdomains of elevated Ca(2+) that are sensed by adjacent mitochondria. In addition to ryanodine-sensitive stores acting as a source of Ca(2+), Ca(2+)-induced Ca(2+) release is required to generate efficient elevation of mitochondrial Ca(2+).

Animals↗

PubMed2007

Dynamic assembly of TRPC1-STIM1-Orai1 ternary complex is involved in store-operated calcium influx. Evidence for similarities in store-operated and calcium release-activated calcium channel components.

Store-operated calcium entry (SOCE) is a ubiquitous mechanism that is mediated by distinct SOC channels, ranging from the highly selective calcium release-activated Ca2+ (CRAC) channel in rat basophilic leukemia and other hematopoietic cells to relatively Ca2+-selective or non-selective SOC channels in other cells. Although the exact composition of these channels is not yet established, TRPC1 contributes to SOC channels and regulation of physiological function of a variety of cell types. Recently, Orai1 and STIM1 have been suggested to be sufficient for generating CRAC channels. Here we show that Orai1 and STIM1 are also required for TRPC1-SOC channels. Knockdown of TRPC1, Orai1, or STIM1 attenuated, whereas overexpression of TRPC1, but not Orai1 or STIM1, induced an increase in SOC entry and I(SOC) in human salivary gland cells. All three proteins were co-localized in the plasma membrane region of cells, and thapsigargin increased co-immunoprecipitation of TRPC1 with STIM1, and Orai1 in human salivary gland cells as well as dispersed mouse submandibular gland cells. In aggregate, the data presented here reveal that all three proteins are essential for generation of I(SOC) in these cells and that dynamic assembly of TRPC1-STIM1-Orai1 ternary complex is involved in activation of SOC channel in response to internal Ca2+ store depletion. Thus, these data suggest a common molecular basis for SOC and CRAC channels.

Animals↗

PubMed1987

Regulation by dietary calcium of vitamin D-dependent calcium-binding protein and active calcium transport in the small intestine of lactating rats.

To test the hypothesis that vitamin D-dependent calcium-binding protein (CaBP) and active calcium (Ca) transport in the small intestine of vitamin D-replete lactating rats are regulated by dietary Ca intake, pregnant rats were given a high Ca (1.6% Ca and 1.4% phosphorus) or low Ca (0.1% Ca and 0.4% phosphorus) diet starting 3 days before delivery. Toward the end of lactation (days 16-23) the rats were killed, and active Ca transport (using everted gut sacs) and CaBP were determined in duodenum, jejunum, and ileum. The right tibiae were used for bone weight and ash determinations. The Ca transport ratios and CaBP concentrations in jejunum and ileum were significantly increased only in the low Ca group. In contrast, in the duodenum both parameters were equally high regardless of the diet. Nonlactating rats given the two diets for the same length of time had the expected increase in both parameters in the duodenum when fed the low Ca diet. Nonlactating rats, in contrast to lactating rats, had undetectable CaBP in jejunum and ileum regardless of diet. Lactating rats fed the high Ca diet had no net loss of bone at the end of lactation compared with rats on day 1 of lactation. In contrast, lactating rats fed the low Ca diet had a net loss of 44% of bone weight. Plasma 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] concentrations on the 21st day of lactation were (mean +/- SE) 538 +/- 96 and 46 +/- 18 pg/ml in rats consuming the low and high Ca diets, respectively. The comparable values for the nonlactating rats were 140 +/- 4 and 26 +/- 8 pg/ml. In conclusion, dietary Ca restriction during lactation can stimulate CaBP and active Ca transport in both jejunum and ileum, and both parameters appear to be modulated by dietary Ca via the circulating concentration of 1,25-(OH)2D3. In contrast, in the duodenum neither parameter appears to be related to dietary Ca, plasma 1,25-(OH)2D3 concentration, or lactation-associated bone loss.

Animals↗

PubMed1997

Extracellular calcium sensing by glial cells: low extracellular calcium induces intracellular calcium release and intercellular signaling.

Glial cells in primary mixed cultures or purified astrocyte cultures from mouse cortex respond to reduced extracellular calcium concentration ([Ca2+]e) with increases in intracellular calcium concentration ([Ca2+]i) that include single-cell Ca2+ oscillations and propagated intercellular Ca2+ waves. The rate and pattern of propagation of low [Ca2+]e-induced intercellular Ca2+ waves are altered by rapid perfusion of the extracellular medium, suggesting the involvement of an extracellular messenger in Ca2+ wave propagation. The low [Ca2+]e-induced Ca2+ response is abolished by thapsigargin and by the phospholipase antagonist U73122. The low [Ca2+]e-induced response is also blocked by replacement of extracellular Ca2+ with Ba2+, Zn2+, or Ni2+, and by 100 microM La3+. Glial cells in lowered [Ca2+]e (0.1-0.5 mM) show an increased [Ca2+]i response to bath application of ATP, whereas glial cells in increased [Ca2+]e (10-15 mM) show a decreased [Ca2+]i response to ATP. These results show that glial cells possess a mechanism for coupling between [Ca2+]e and the release of Ca2+ from intracellular stores. This mechanism may be involved in glial responses to the extracellular environment and may be important in pathological conditions associated with low extracellular Ca2+ such as seizures or ischemia.

Adenosine Triphosphate↗

PubMed2006

Calcium sensing receptors and calcium oscillations: calcium as a first messenger.

Calcium sensing receptors (CaR) are unique among G-protein-coupled receptors (GPCRs) since both the first (extracellular) and second (intracellular) messengers are Ca(2+). CaR serves to translate small fluctuations in extracellular Ca(2+) into intracellular Ca(2+) oscillations. In many cells and tissues, CaR also acts as a coincidence detector, sensing both changes in extracellular Ca(2+) plus the presence of various allosteric activators including amino acids, polyamines, and/or peptides. CaR oscillations are uniquely shaped by the activating agonist, that is, Ca(2+) triggers sinusoidal oscillations while Ca(2+) plus phenylalanine trigger transient oscillations of lower frequency. The distinct oscillation patterns generated by Ca(2+)versus Ca(2+) plus phenylalanine are the results of activation of distinct signal transduction pathways. CaR is a member of Family C GPCRs, having a large extracellular agonist binding domain, and functioning as a disulfide-linked dimer. The CaR dimer likely can be driven to distinct active conformations by various Ca(2+) plus modulator combinations, which can drive preferential coupling to divergent signaling pathways. Such plasticity with respect to both agonist and signaling outcomes allows CaR to uniquely contribute to the physiology of organs and tissues where it is expressed. This chapter will examine the structural features of CaR, which contribute to its unique properties, the nature of CaR-induced intracellular Ca(2+) signals and the potential role(s) for CaR in development and differentiation.

Allosteric Regulation↗

PubMed2003

2-Aminoethoxydiphenyl borate (2-APB) antagonises inositol 1,4,5-trisphosphate-induced calcium release, inhibits calcium pumps and has a use-dependent and slowly reversible action on store-operated calcium entry channels.

The action of 2-aminoethoxydiphenyl borate (2-APB) on Ca(2+) signalling in HeLa cells and cardiac myocytes was investigated. Consistent with other studies, we found that superfusion of cells with 2-APB rapidly inhibited inositol 1,4,5-trisphosphate (InsP(3))-mediated Ca(2+) release and store-operated Ca(2+) entry (SOC). In addition to abrogating hormone-evoked Ca(2+) responses, 2-APB could antagonise Ca(2+) signals evoked by a membrane permeant InsP(3) ester. 2-APB also slowed the recovery of intracellular Ca(2+) signals consistent with an effect on Ca(2+) ATPases. The inhibitory action of 2-APB on InsP(3) receptors (InsP(3)Rs), SOC channels and Ca(2+) pumps persisted for several minutes after washout of the compound. Application of 2-APB to unstimulated cells had no effect on subsequent Ca(2+) responses suggesting that it has a use-dependent action. Mitochondria in cells treated with 2-APB showed a rapid and slowly reversible swelling. 2-APB did not cause the mitochondria to depolarise, but it reduced the extent of mitochondrial calcium uptake. Although 2-APB has been demonstrated not to affect voltage-operated Ca(2+) channels or ryanodine receptors, we found that it gave a concentration-dependent long-lasting inhibition of Ca(2+) signalling in electrically-stimulated cardiac myocytes, where InsP(3)Rs and SOC channels do not play a significant role. Our data suggest that 2-APB has multiple cellular targets, a use-dependent action, is difficult to reverse and may affect Ca(2+) signalling in cell types where InsP(3) and SOC are not active.

Boron Compounds↗

PubMed1990

Osteoclast cytosolic calcium, regulated by voltage-gated calcium channels and extracellular calcium, controls podosome assembly and bone resorption.

The mechanisms of Ca2+ entry and their effects on cell function were investigated in cultured chicken osteoclasts and putative osteoclasts produced by fusion of mononuclear cell precursors. Voltage-gated Ca2+ channels (VGCC) were detected by the effects of membrane depolarization with K+, BAY K 8644, and dihydropyridine antagonists. K+ produced dose-dependent increases of cytosolic calcium ([Ca2+]i) in osteoclasts on glass coverslips. Half-maximal effects were achieved at 70 mM K+. The effects of K+ were completely inhibited by dihydropyridine derivative Ca2+ channel blocking agents. BAY K 8644 (5 X 10(-6) M), a VGCC agonist, stimulated Ca2+ entry which was inhibited by nicardipine. VGCCs were inactivated by the attachment of osteoclasts to bone, indicating a rapid phenotypic change in Ca2+ entry mechanisms associated with adhesion of osteoclasts to their resorption substrate. Increasing extracellular Ca2+ ([Ca2+]e) induced Ca2+ release from intracellular stores and Ca2+ influx. The Ca2+ release was blocked by dantrolene (10(-5) M), and the influx by La3+. The effects of [Ca2+]e on [Ca2+]i suggests the presence of a Ca2+ receptor on the osteoclast cell membrane that could be coupled to mechanisms regulating cell function. Expression of the [Ca2+]e effect on [Ca2+]i was similar in the presence or absence of bone matrix substrate. Each of the mechanisms producing increases in [Ca2+]i, (membrane depolarization, BAY K 8644, and [Ca2+]e) reduced expression of the osteoclast-specific adhesion structure, the podosome. The decrease in podosome expression was mirrored by a 50% decrease in bone resorptive activity. Thus, stimulated increases of osteoclast [Ca2+]i lead to cytoskeletal changes affecting cell adhesion and decreasing bone resorptive activity.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1984

Interactions between a "calcium channel agonist", Bay K 8644, and calcium antagonists differentiate calcium antagonist subgroups in K+-depolarized smooth muscle.

The proposal that calcium antagonists have different sites of action has been tested by attempting to reverse their inhibitory effects with a dihydropyridine which augments Ca2+ entry into cells, Bay K 8644. Bay K 8644 (1-1000 nmol/l) increased the sensitivity to Ca2+ of K+-depolarized taenia preparations from the guinea-pig caecum. Thus Bay K 8644 augmented established submaximal Ca2+-induced contractions and also shifted cumulative concentration-response curves to Ca2+ to the left, even in the presence of an optimal K+-depolarization. The inhibitory effects of nifedipine (10 nmol/l), verapamil (0.2 mumol/l), diltiazem (1 mumol/l) and diclofurime (1 mumol/l) on Ca2+-induced contractions were reversed by Bay K 8644 (1-1000 nmol/l). In contrast, Bay K 8644 did not reverse the effects of cinnarizine (1 mumol/l), flunarizine (1 mumol/l), fendiline (3 mumol/l), prenylamine (3 mumol/l), pimozide (1 mumol/l), bepridil (3 mumol/l), perhexiline (10 mumol/l) or the calmodulin antagonist W-7 (200 mumol/l). Bay K 8644 (1-100 nmol/l) was less effective at reversing the effects of nisoldipine (10 nmol/l), a slowly dissociating dihydropyridine, than the effects of nifedipine. However, preincubation with Bay K 8644 (1 mumol/l) protected the taenia from the inhibitory effects of nisoldipine (10 nmol/l). These findings are compatible with interactions of nisoldipine and Bay K 8644 at a common site. Taenia preparations incubated with Bay K 8644 (1 mumol/l) were protected from the inhibitory effects of nifedipine (10 nmol/l), nisoldipine (10 nmol/l) and to a lesser extent verapamil (0.2 mumol/l) and diltiazem (1 mumol/l).(ABSTRACT TRUNCATED AT 250 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMedDate not supplied

Subcellular calcium oscillators and calcium influx support agonist-induced calcium waves in cultured astrocytes.

We have analysed Ca2+ waves induced by norepinephrine in rat cortical astrocytes in primary culture using fluorescent indicators fura-2 or fluo-3. The temporal pattern of the average [Ca2+]i responses were heterogeneous from cell to cell and most cells showed an oscillatory response at concentrations of agonist around EC50 (200 nM). Upon receptor activation, [Ca2+]i signals originated from a single cellular locus and propagated throughout the cell as a wave. Wave propagation was supported by specialized regenerative calcium release loci along the length of the cell. The periods of oscillations, amplitudes, and the rates of [Ca2+]i rise of these subcellular oscillators differ from each other. These intrinsic kinetic properties of the regenerative loci support local waves when stimulation is continued over long periods of time. The presence of local waves at specific, invariant cellular sites and their inherent kinetic properties provide for the unique and reproducible pattern of response seen in a given cell. We hypothesize that these loci are local specializations in the endoplasmic reticulum where the magnitude of the regenerative Ca2+ release is higher than other regions of the cell. Removal of extracellular Ca2+ or blockade of Ca2+ channels by inorganic cations (Cd2+ and Ni2+) during stimulation of adrenergic receptors alter the sustained plateau component of the [Ca2+]i response. In the absence of Ca2+ release, due to store depletion with thapsigargin, agonist occupation alone does not induce Ca2+ influx in astrocytes. This finding suggests that, under these conditions, receptor-operated Ca2+ entry is not operative. Furthermore, our experiments provide evidence for local Ca2+ oscillations in cells which can support both wave propagation as well as spatially discrete Ca2+ signalling.

Animals↗

PubMed1992

The role of calcium in neutrophil migration: the effect of calcium and calcium-antagonists in electroporated neutrophils.

Chemotaxis by electroporated rabbit peritoneal neutrophils in the absence of Ca2+ is only slightly different from that in the presence of Ca2+. Pretreatment of neutrophils with quin2-AM causes inhibition of chemotaxis. Calcium antagonists as nitrendipine and verapamil are inhibitory in nanomolar concentrations, while 10(5) times higher concentrations are required for inhibition of chemotaxis by neutrophils which were not electroporated. The results support the hypothesis that Ca2+ from Ca(2+)-storing organelles is of importance for chemotaxis, but that chemotaxis is not dependent on changes in cytoplasmic Ca2+ concentrations.

Animals↗

PubMed1991

Calcium homeostasis in bovine somatotrophs: calcium oscillations and calcium regulation by growth hormone-releasing hormone and somatostatin.

The free intracellular calcium ion concentration ([Ca2+]i) was measured in single cells of a population containing 65-80% somatotrophs, using the fluorescent Ca(2+)-indicator Fura-2 and digital imaging microscopy. Spontaneous oscillations in [Ca2+]i ranging in frequency up to 1.5 oscillations per minute were observed in 30% of somatotrophs. These Ca2+ oscillations were blocked by the Ca2+ channel blocker CoCl2 and were thus proposed to be the result of influx of Ca2+ into the cell, possibly as the result of spontaneous electrical activity. GHRH (10-100 nM) increased [Ca2+]i in 61% of the cells studied, although the amplitude and dynamics of the response varied from cell to cell. Typically [Ca2+]i rose from 170 +/- 26 nM to 321 +/- 44 nM (n = 13) in response to a challenge with 66 nM GHRH. GHRH also increased the frequency of Ca2+ oscillations in a number of cells, and some previously quiescent cells showed Ca2+ oscillations following addition of GHRH. Forskolin, which raises cAMP levels in bovine anterior pituitary cells, also stimulated a sustained rise in [Ca2+]i in 10 out of 14 cells tested. Somatostatin (SS) (10-80 nM) rapidly reduced basal [Ca2+]i, blocked Ca2+ oscillations, and blocked the [Ca2+]i response to GHRH. The Ca2+ channel blocker CoCl2 (4 mM) had similar actions on [Ca2+]i to those of SS. These results suggest that GHRH and SS may regulate GH release by modulating Ca2+ entry into the cell through the cell membrane. The [Ca2+]i oscillations seen in a proportion of the somatotrophs were modulated in frequency by GHRH and SS, and are probably generated by influx of Ca2+ through channels in the cell membrane. Thus GH secretion may be regulated by changes in the mean level of [Ca2+]i, which in turn, may be influenced by the frequency of [Ca2+]i oscillations in bovine somatotrophs.

Animals↗

PubMed2000

Calcium binding properties of recombinant calcium binding protein 40, a major calcium binding protein of lower eukaryote Physarum polycephalum.

Calcium binding protein 40 (CBP40) is a Ca(2+)-binding protein abundant in the plasmodia of Physarum polycephalum. CBP40 consists four EF-hand domains in the COOH-terminal half and a putative alpha-helix domain in the NH(2)-terminal half. We expressed recombinant proteins of CBP40 in Escherichia coli to investigate its Ca(2+)-binding properties. Recombinant proteins of CBP40 bound 4 mol of Ca(2+) with much higher affinity (pCa(1/2) = 6.5) than that of calmodulin. When residues 1-196 of the alpha-helix domain were deleted, the affinity for Ca(2+) decreased to pCa(1/2) = 4.6. A chimeric calmodulin was generated by conjugating the alpha-helix domain of CBP40 with calmodulin. The affinity of Ca(2+) for the chimeric calmodulin was higher than that for calmodulin, suggesting that the alpha-helix domain is responsible for the high affinity of CBP40 for Ca(2+). CBP40 forms large aggregates reversibly in a Ca(2+)-dependent manner. A mutant protein with a deletion of NH(2)-terminal 32 residues, however, could not aggregate, indicating the importance of these residues for the aggregation. The aggregation occurs above micromolar levels of Ca(2+) concentration, so it may only occur when CBP40 is secreted out of the plasmodial cells.

Animals↗

PubMed1988

Calcium absorption from calcium carbonate and a new form of calcium (CCM) in healthy male and female adolescents.

Calcium absorption from two Ca salts was investigated in a crossover design using stable isotopic tracers in 12 healthy adolescents (6 males, 6 females). A Ca supplement in the form of Ca carbonate or Ca citric and malic acids (CCM) was ingested with a standardized breakfast and the order of administration was randomized. The oral supplement contained 250 mg elemental Ca, 21.8 mg of which was highly enriched 44Ca tracer. Thirty minutes later subjects received 3.6 mg 42Ca tracer intravenously. The molar concentrations of 42Ca and 44Ca tracers in a urine sample obtained 24 h after tracer administration were quantified by fast-atom-bombardment mass spectrometry and used to determine fractional absorption of the Ca from the supplement. Ca in the form of CCM had an increased fractional absorption (p less than 0.03) relative to Ca carbonate in healthy adolescents (36.2 vs 26.4%). This increase was not related to body size, sex, or indices of Ca metabolism.

Administration, Oral↗

PubMed1993

Spontaneous cytosolic calcium pulses in Xenopus melanotrophs are due to calcium influx during phasic increases in the calcium permeability of the cell membrane.

We recently discovered that melanotrophs of Xenopus laevis exhibit spontaneous pulse-like rises in cytosolic free calcium ([Ca2+]i) and that this cytosolic Ca pulsing is inhibited by the secreto-inhibitory transmitters dopamine, gamma-aminobutyric acid, and neuropeptide-Y and stimulated by the secretagogues CRF and TRH. Here we provide evidence for the factors responsible for the individual cytosolic Ca pulse and the repetitive behavior. Isolated melanotrophs of Xenopus were loaded with fura-2, and fluorescence was recorded from perifused single cells to measure [Ca2+]i and assess the patency of divalent cation channels by Mn quenching of fluorescence. Cytosolic Ca pulsing was arrested by omission of Ca and by the Ca channel blockers Ni and Co, but was unaffected by tetrodotoxin. Mn (0.3 mM) caused phasic quenching, each "quench" being synchronous with the rising phase of a cytosolic Ca pulse. Quenching was blocked by Ni, but was unaffected by tetrodotoxin. When introduced during the course of an individual Ca pulse, Ni aborted the pulse and [Ca2+]i collapsed. Extracellular K+ (10 mM) or K channel block with tetraethylammonium stimulated Ca pulsing. Threshold concentrations of Ni slowed Ca pulsing without reducing the amplitude of the individual pulses. The overshoots in quenching and Ca pulsing observed after exposure to Ni, dopamine, or baclofen are interpreted as off responses. It is concluded that each cytosolic Ca pulse is attributable, probably quantitatively, to Ca influx during a spontaneous increase in Ca permeability independent of Na-mediated action potentials, and that the pacemaker for the repetitive phenomenon is voltage sensitive and may involve Ca currents active around basal membrane potential.

Animals↗

PubMed1981

Utility and limitation of calciuric response to oral calcium load as a measure of intestinal calcium absorption: comparison with isotopic fractional calcium absorption.

The intestinal absorption of calcium (Ca), indirectly measured from the calciuric response to oral Ca load (1g), was compared to the more directly obtained isotopic fractional absorption, alpha (from the fecal recovery of orally administered 47Ca). In 17 normal subjects and 30 patients with absorptive hypercalciuria (AH), there was a significant (P less than 0.001) correlation of alpha with the Ca load responses, (r = 0.81). However, this correlation was not observed in patients with renal hypercalciuria (RH), and those with AH receiving thiazide or orthophosphate. In RH, 38 per cent of patients had elevated Ca load responses, despite normal values for alpha. The point correlating the calciuric response and alpha in these patients was below the 95 per cent confidence limit of the line correlating alpha and the load response. Thus, Ca load response often overestimated intestinal Ca absorption, because of the high basal (fasting) urinary Ca. Thiazide therapy in RH improved the correlation between the two tests of Ca absorption. However, thiazide therapy in AH produced normal Ca load responses despite persistently high alpha in 60 per cent of patients. Similarly, 50 per cent of patients with AH receiving orthophosphate had normal Ca load response, although alpha remained elevated. Thus, Ca load response underestimated Ca absorption when patients with AH took thiazide or orthophosphate, probably because these drugs augment renal tubular reabsorption of Ca. These data support the Ca load test as a valid indirect measure of intestinal Ca absorption in normal subjects and patients with AH, in whom fasting urinary Ca is not elevated. In conditions of renal Ca, leak or with various drugs known to alter renal Ca handling, there seen to be large deviations of Ca load response from alpha. Care should be exercised before reaching conclusions regarding the intestinal Ca absorption in these situations.

Administration, Oral↗