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Biomedical subjects

E Gylfe

Publications and source records attributed to E Gylfe.

At least 127 records · Page 7Linked to original sources
PubMed1986

Normalizing effect of Ca2+ ionophore on cytoplasmic Ca2+ and parathyroid hormone release of dispersed parathyroid cells from patients with hyperparathyroidism.

The effects of the Ca2+ ionophore A23187 on parathyroid hormone (PTH) secretion and cytoplasmic free Ca2+ concentration (Ca2+i) were measured at different extracellular Ca2+ concentrations using dispersed cells from patients with hyperparathyroidism (HPT). The addition of a low concentration of the Ca2+ ionophore to quin2-loaded cell preparations resulted in the apparent normalization of calcium-regulated Ca2+i. At all extracellular calcium concentrations Ca2+i reached significantly higher values in the presence of the ionophore and the dose-response relationship was shifted to the left. Under similar conditions calcium-regulated PTH release was correspondingly corrected with an increased suppressibility and left-shifted dose-response relationship. The data render strong support for a disturbed regulation of Ca2+i as a major factor in the pathophysiology of HPT.

Aminoquinolines↗
PubMed1986

Mobilization of different intracellular calcium pools after activation of muscarinic receptors in pancreatic beta-cells.

Exposure to carbachol resulted in a biphasic stimulation of 45Ca efflux when beta-cell-rich pancreatic islets from ob/ob mice were perifused with a Ca2+-deficient medium. The pattern of stimulated 45Ca efflux was markedly modified by glucose. Whereas the initial carbachol-stimulated phase was conditional on previous exposure to glucose, the subsequent phase was completely suppressed by 20 mmol/l of the sugar. The stimulatory could be clearly separated also on the basis of a Na+ dependence. Removal of extracellular Na+ resulted in a disappearance of the second phase, but it was still possible to induce a prominent initial peak if depletion of intracellular K+ was prevented when Na+ was omitted. It is concluded that activation of muscarinic receptors in the pancreatic beta-cells results in mobilization of calcium from more than one intracellular pool. Whereas the second phase of stimulated efflux can be explained in terms of an increased entry of Na+ into the beta-cells, the initial stimulation may be due to receptor-mediated breakdown of polyphosphoinositides.

Animals↗
PubMed1986

Inositol 1,4,5-trisphosphate mobilizes glucose-incorporated calcium from pancreatic islets.

Mobilization of intracellular calcium from beta-cell-rich pancreatic islets of ob/ob-mice was studied by measuring unidirectional 45Ca efflux at 37 degrees and 18 degrees C during perifusion with a K+-rich medium deficient in Ca2+ and Na+. Addition of 100 microM carbachol induced a prominent peak of Ca2+ efflux from islets preexposed to glucose. After cell permeabilization with digitonin D-myo-inositol 1,4,5-trisphosphate (IP3) caused glucose-dependent mobilization of calcium. In demonstrating that not only carbachol but also IP3 can mobilize calcium incorporated in response to glucose, the present data suggests that the endoplasmic reticulum participates in glucose-induced lowering of cytoplasmic Ca2+ activity in the pancreatic beta-cells.

Animals↗
PubMed1985

Paradoxical effects of K+ and D-600 on parathyroid hormone secretion and cytoplasmic Ca2+ in normal bovine and pathological human parathyroid cells.

The effects of K+ and the Ca2+ channel blocker D-600 on parathyroid hormone (PTH) release and cytoplasmic Ca2+ activity (Ca2+i) were measured at different Ca2+ concentrations in dispersed parathyroid cells from normal cattle and from patients with hyperparathyroidism. When the extracellular Ca2+ concentration was raised within the 0.5-3.0 mM range Ca2+i increased and PTH secretion was inhibited. There was also a stimulatory effect of Ca2+ on secretion as indicated by a parallel decrease of Ca2+i and PTH release when extracellular Ca2+ was reduced to less than 25 nM. Addition of 30-50 mM K+ stimulated PTH release and lowered Ca2+i. The effect of K+ was less pronounced in the human cells with a decreased suppressability of PTH release. The Ca2+ channel blocker D-600 had no effect on Ca2+i and PTH release in the absence of extracellular Ca2+. However, at 0.5-1.0 mM Ca2+, D-600 increased Ca2+i and inhibited PTH release, whereas the opposite effects were obtained at 3.0 mM Ca2+. The transition from inhibition to stimulation occurred at a higher Ca2+ concentration in the human cells and the right-shift in the dose-effect relationship for Ca2+-inhibited PTH release tended to be normalized by D-600. It is suggested that K+ stimulates PTH release by increasing the intracellular sequestration of Ca2+ and that the reduced response in the parathyroid human cells is due to the fact that Ca2+i already is lowered. D-600 appears to have both Ca2+ agonistic and antagonistic actions in facilitating and inhibiting Ca2+ influx into the parathyroid cells at low and high concentrations of extracellular Ca2+, respectively. D-600 and related drugs are considered potentially important for the treatment of hyperparathyroidism.

Animals↗
PubMed1985

Stimulus-secretion coupling of parathyroid hormone release: studies of 45Ca and 86Rb fluxes.

Pieces of rat parathyroid glands were used to study fluxes of 45Ca and 86Rb. The uptake of 45Ca increased with the extracellular Ca2+ concentration up to at least 5 mM. A rise of extracellular Ca2+ had dual effects on 45Ca efflux in terms of an initial stimulation and a subsequent inhibition. However, K+ depolarization neither affected the uptake nor the efflux of 45Ca indicating a lack of voltage-dependent Ca2+ channels. The depolarization obtained with exposure to Ca2+ cannot be attributed to a decreased K+ permeability, since the 86Rb concentrating ability diminished and the efflux of the isotope increased when parathyroid pieces were exposed to a raised Ca2+ concentration. A stimulation of 86Rb efflux by the Ca2+ ionophore A-23187 indicated that the parathyroid cells possess a K+ permeability activated by cytoplasmic Ca2+. It is suggested that Ca2+ fluxes through channels sensitive to activation by Ca2+ are important both for the membrane potential and the cytoplasmic Ca2+ activity.

Animals↗
PubMed1985

The dual action of glucose on the cytosolic Ca2+ activity in pancreatic beta-cells. Demonstration of an inhibitory effect of glucose on insulin release in the mouse and man.

The cytosolic Ca2+ activity was measured with the fluorescent indicator quin-2 in pancreatic beta-cells obtained from obese-hyperglycemic mice. When present at a concentration of 20 mmol/l in a medium physiologically balanced in cations, glucose induced a rise of cytosolic Ca2+ after a delay of 1--3 min. At lower concentrations of extracellular Ca2+ this effect was not only prevented but the sugar promptly reduced the cytosolic Ca2+ activity. The dual effect of glucose on cytosolic Ca2+ had its counterpart in the release of insulin. Whereas 20 mmol/l of glucose stimulated the release of insulin from mouse islets previously stored in a Ca2+-deficient medium, the sugar was clearly inhibitory when present at a concentration of 6 mmol/l. Intravenous glucose tolerance tests revealed a temporary glucose depression of the serum concentrations of insulin and C-peptide in several patients with diabetes. In a mentally retarded girl with hyperinsulinemia associated with acanthosis nigricans the glucose suppression of circulating insulin was prolonged and sufficiently pronounced to suggest an almost complete inhibition of the secretory activity of the pancreatic B-cells.

Aminoquinolines↗
PubMed1984

Dual effects of glucose on the cytosolic Ca2+ activity of mouse pancreatic beta-cells.

The cytosolic Ca2+ activity of mouse pancreatic beta-cells was studied with the intracellular fluorescent indicator quin2 . When the extracellular Ca2+ concentration was 1.20 mM, the basal cytosolic Ca2+ activity was 162 +/- 9 nM. Stimulation with 20 mM glucose increased this Ca2+ activity by 40%. In the presence of only 0.20 mM Ca2+ or after the addition of the voltage-dependent Ca2+ -channel blocker D-600, glucose had an opposite and more prompt effect in reducing cytosolic Ca2+ by about 15%. It is concluded that an early result of glucose exposure is a lowering of the cytosolic Ca2+ activity and that this effect tends to be masked by a subsequent increase of the Ca2+ activity due to influx of Ca2+ through the voltage-dependent Ca2+ channels.

Animals↗
PubMed1984

Glucose inhibits 45Ca efflux from pancreatic beta-cells also in the absence of Na+-Ca2+ countertransport.

During perifusion with medium deprived of Ca2+, addition of glucose or omission of Na+ resulted in prompt and quantitatively similar inhibitions of 45Ca efflux from beta-cell rich pancreatic islets microdissected from ob/ob mice. Glucose had no additional inhibitory effect when Na+ was isoosmotically replaced by sucrose or choline+. When K+ was used as a substitute for Na+, the inhibitory effect of Na+ removal on 45Ca efflux became additive to that of glucose. The observation that glucose can be equally effective in inhibiting 45Ca efflux in the presence or absence of Na+ is difficult to reconcile with the postulate that the Na+-Ca2+ countertransport mechanism is a primary site of action for glucose.

Animals↗
PubMed1984

Defective regulation of the cytosolic Ca2+ activity in parathyroid cells from patients with hyperparathyroidism.

The parathyroid hormone (PTH) release and cytosolic Ca2+ activity were determined in normal bovine parathyroid cells and parathyroid cells obtained from patients with hyperparathyroidism (HPT). There was a sigmoid relation between the cytosolic Ca2+ activity and the extracellular calcium concentration between 0.5 and 6.0 mmol/l. The PTH release was inhibited in parallel with the rise in the cytosolic Ca2+ activity. Both the hormone release and the cytosolic Ca2+ activity were lower in cells from human adenomas and hyperplastic glands, and in comparison with the bovine preparations these cells had higher set points for the cytosolic Ca2+ activity and PTH release. There was a close correlation between the individual set points for the cytosolic Ca2+ activity and PTH release in a material containing both normal and pathological cells. The results indicate that the abnormal PTH release characteristic of HPT is due to a defective regulation of the cytosolic Ca2+ activity.

Adenoma↗
PubMed1984

Effect of hypoglycemic sulfonylureas on Ca2+ fluxes across lipid bilayers.

Black lipid membranes and liposomes loaded with Ca2+ or 5,6-carboxyfluorescein were used for exploring the mechanism of action of insulin-releasing sulfonylureas. Unlike the Ca2+/H+ exchanging ionophore A-23187, tolbutamide did not stimulate the net efflux of Ca2+ from the liposomes. Glibenclamide caused a sustained release of Ca2+, but this effect could be attributed to labilization of the liposomal membrane as indicated by a quantitatively similar loss of the stability marker 5,6-carboxyfluorescein. Unlike the neutral ionophore nonactin or the channel forming quasi-ionophore gramicidin A, the sulfonylureas did not alter the conductance of black lipid membranes in medium containing Na+, K+, Ca2+, Mg2+, and Cl-. It is concluded that the sulfonylureas tested lack ionophore properties but that glibenclamide can labilize membranes.

Biological Transport↗
PubMed1984

Evidence for glucose stimulation of intracellular buffering of calcium in the pancreatic beta-cell.

Glucose-induced movements of Ca2+ in pancreatic beta-cells were analysed using islets isolated from ob/ob mice and insulin-releasing cells from a clonal cell line (RINm5F). Addition of glucose to a perifusion medium resulted in an inhibited efflux of 45Ca from the islets both when the extracellular Ca2+ concentration was lower or higher than that in the cytosol. Glucose inhibition of 45Ca efflux was seen also after altering the Na+ gradient across the plasma membrane provided that the cytosolic K+ activity was maintained. The glucose suppression of 45Ca efflux corresponded to a stimulated net uptake of Ca2+ in the RINm5F cells. Also in this case the glucose effect was maintained when Na+ was replaced with K+ and suppressed after substitution with choline. During perifusion of islets with a Ca2+-deficient medium the removal of glucose resulted in a temporary stimulation of insulin release. The results suggest that glucose, in addition to stimulating the entry of Ca2+, also promotes active sequestration of the ion in intracellular stores. The balance between these processes will determine the activity of cytosolic Ca2+ and consequently the rate of insulin release.

Animals↗
PubMed1983

Depolarization-independent net uptake of calcium into clonal insulin-releasing cells exposed to glucose.

Insulin release, net fluxes of Ca2+, and glucose metabolism were studied in a clonal cell line (RINm5F) established from a transplantable rat islet tumor. The insulin content amounted to only 0.03% of that of the total protein and decreased even further with subsequent passages. The insulin secretion was as high as 10 to 20% of the total hormone content per hour. Insulin release was stimulated by K+ depolarization but not by exposure to glucose. In contrast to this secretory pattern, glucose but not K+ stimulated the net uptake of Ca2+ at micromolar concentrations of the ion. The glucose effect was not mimicked by 20 mM 3-O-methylglucose. It was as pronounced at 1 mM as at 20 mM of the sugar and corresponded to an uptake of 119 fmol cm-2 s-1. Glucose metabolism was typical for tumor cells with a high glycolytic flux and an oxidation-to-utilization ratio as low as 0.05-0.15. Maximal oxidative degradation was attained already at 1 mM. This concentration was also equivalent to the Km for glucose utilization, indicating a substantial left-hand shift of the normal dose-response curve. It is suggested that glucose induces a depolarization-independent net uptake of Ca2+ by favouring intracellular buffering of the cation.

Adenoma, Islet Cell↗
PubMed1983

Reduction of the cytosolic calcium activity in clonal insulin-releasing cells exposed to glucose.

The cytosolic Ca2+ activity of insulin-releasing clonal cells (RINm5F) was studied with the intracellular fluorescent indicator quin-2. When the extracellular Ca2+ concentration was 1 mM, the basal cytosolic Ca2+ activity was 101 +/- 5 nM. Depolarization with 25 mM K+ increased this Ca2+ activity to at least 318 nM, an effect completely reversed by the voltage-dependent channel blocker D-600. In the presence of K+ alone these channels appeared to have a half-life of 6.7 +/- 0.8 min. In contrast to the action of K+, exposure of the RINm5F cells to 4 mM glucose resulted in a reduction of the cytosolic Ca2+ activity. This effect was observed during K+ depolarization but was more pronounced under basal conditions when it amounted to 20%. The data provide the first direct evidence that glucose can decrease the cytosolic Ca2+ activity in beta-cells. Unlike the case in normal beta-cells the glucose effect on the voltage-dependent Ca2+ channels in the RINm5F cells is apparently not sufficient to overcome the intracellular buffering of Ca2+. A defective depolarization is therefore a probable cause of the failing insulin secretion of RINm5F cells exposed to glucose.

Adenoma, Islet Cell↗
PubMed1983

Interactions between magnesium and calcium in beta-cell-rich pancreatic islets.

Calcium-magnesium interactions, total amounts of intracellular magnesium, and insulin release were studied in beta-cell-rich pancreatic islets from ob/ob mice. Mg2+ inhibited the uptake of intracellular 45Ca and insulin release induced by glucose or high concentrations of potassium. Omission of Mg2+ from a Ca2+-deficient medium resulted in an increased efflux of 45Ca, whereas the characteristic glucose inhibition of the efflux was diminished. After addition of Mg2+ to a Mg2+-depleted medium, the glucose-stimulated 45Ca efflux was markedly reduced. Mg2+ inhibited the basal efflux of 45Ca, and this effect was preceded by a transient stimulation. Ca2+ but not Mg2+ stimulated 45Ca efflux in a medium depleted of Ca2+, Mg2+, and Na+. The data indicate that Mg2+ interferes with Ca2+ entry through voltage-dependent Ca2+ channels. Mg2+ may also inhibit the outward transport of Ca2+ from the cells at a site different from the Na+-Ca2+ countertransport mechanism. The total amount of intracellular magnesium remained unaffected by glucose and was not changed unless the ionic composition of the mediums were changed grossly. Under physiological conditions it is therefore unlikely that fluctuations in the intracellular Mg2+ concentration are part of the mechanism by which the functionally important Ca2+ is regulated.

Animals↗
PubMed1983

Significance of serum for the preservation of insulin secretion during culture.

The effects of serum and serum fractions on the maintenance of glucose-stimulated insulin secretion during culture of pancreatic islets were studied. The basal insulin release was independent of previous culture with serum but glucose-stimulated secretion increased with serum concentrations up to 0.3% which was sufficient for maximal effect. Although a high molecular size fraction (greater than 30,000 daltons) possessed the full activity of serum it is possible that the active principles are lighter compounds bound to serum proteins. Whereas the islet content of insulin was unaffected by culture with 3% serum, the presence of serum tended to increase the levels of pyridine nucleotides and the uptake of intracellular 45Ca in response to glucose. The sites of action for the serum factors are, therefore, the stimulus-secretion coupling and hormone discharge mechanisms rather than insulin biosynthesis.

Animals↗
PubMed1982

Glucose inhibits insulin release induced by Na+ mobilization of intracellular calcium.

45Ca2+ incorporated in response to glucose was selectively mobilized from the beta-cell-rich pancreatic islets of ob/ob-mice after raising the intracellular Na+ by removal of K+ or addition of ouabain or veratridine. Also studies of insulin release indicated opposite effects of glucose and Na+ on the intracellular sequestration of calcium. The fact that glucose inhibits insulin release induced by raised intracellular Na+ indicates that this sugar can lower the cytoplasmic [Ca2+]. The concept of a dual action of glucose on the cytoplasmic [Ca2+]. The concept of a dual action of glucose on the cytoplasmic [Ca2+] might well explain previous observations of an inhibitory component in the glucose action on the 45Ca2+ efflux.

Animals↗