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Biomedical subjects

D G Pipeleers

Publications and source records attributed to D G Pipeleers.

At least 109 records · Page 6Linked to original sources
PubMed1984

Evidence against the presence of tight junctions in normal endocrine pancreas.

Tight junctional fibrils were absent in freeze-fracture replicas of rat and human islets in situ, but were easily discerned in collagenase-isolated islets. Disruption of the pancreatic gland and its exposure to trypsin were each found to induce tight junction formation in rat islets. The amount of tight junctions between islet cells declined progressively during culture, but tight junctional structures remained detectable after 1 day of culture. It is suggested that rather than being involved in normal islet cell function tight junctions provide an adaptive mechanism intended to seal and hence to protect islet microdomains against sudden perturbations in local interstitial fluid.

Animals↗
PubMed1982

Selective uptake of alloxan by pancreatic B-cells.

Alloxan rapidly binds to or accumulates in pancreatic B-cells as distinct from non-B-cells. The selective uptake of this cytotoxic agent by the insulin-producing B-cells might account for its well-known diabetogenic effect.

Alloxan↗
PubMed1982

Determinants of the selective toxicity of alloxan to the pancreatic B cell.

The diabetogenic agent alloxan exerts a preferential cytotoxic effect on the pancreatic B cell. The determinants of such a tissue specificity were investigated. Alloxan accumulated rapidly in liver and pancreatic islets but much more slowly in muscle. The activity of glutathione peroxidase and the resistance to exogenous peroxide were approximately 20 times higher in liver and kidney than in islets, intermediate values being found in exocrine pancreas and muscle. These findings suggest that the selective cytotoxicity of alloxan to the pancreatic B cell is attributable to the conjunction of two features: a rapid cellular uptake of the drug and an exquisite sensitivity of the B cell to peroxide.

Alloxan↗
PubMed1982

Effect of temperature, nutrients, calcium, and cAMP on motility of human spermatozoa.

The motility of human spermatozoa and its regulation were examined on cells isolated from other seminal components and purified into fractions of uniform progressive motility. The percent motile cells and estimates of their translational speed were determined by visual inspection, by stroboscopy, and by photon correlation spectroscopy; microcinematography and gradient centrifugation were occasionally used to clarify discrepancies. The motility of isolated spermatozoa could be maintained for periods up to 24 h at 4 or 37 degrees C; the presence of seminal fluid was not required and even provoked a reversible inhibition at 4 degrees C. Albumin facilitated cell movement between microscopic glass plates but had no effect on progressive motility per se, as evidenced by other techniques. During incubations of up to 2 h, progressive cell motility occurred independently of extracellular glucose and calcium but responded to variations in adenosine 3',5'-cyclic monophosphate and calcium. Dibutyryl cAMP increased forward motility, whereas ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid reversibly immobilized the spermatozoa in a calcium-dependent manner; phosphodiesterase inhibition resulted in increased vibration of sperm heads without any effect on progressive motility. Longer incubation periods required the presence of extracellular nutrients. These experiments further demonstrate that several motility measuring techniques should be used in parallel to distinguish the various components of cell movement, to exclude aspecific effects, and to supplement the shortcomings of each individual technique. Such procedure could clarify the various discrepancies that have been reported so far and should lead to a better understanding of the regulation of human sperm motility.

Calcium↗
PubMed1981

A method for the purification of single A, B and D cells and for the isolation of coupled cells from isolated rat islets.

A method has been developed for the purification of single A, B and D cells and for the isolation of coupled islet cells. Isolated rat islets were dissociated by repeated pipetting in the presence of trypsin and ethylene glycol bis (beta-aminoethyl ether) N,N' tetra-acetic acid (EGTA), and then filtered through nylon and Percoll; the cell preparation consisted of 70% single and 30% coupled cells. Sizing of the cells led to the recognition of a small-islet cell population (35%; cell volume 200-600 micrometer 3) composed of A and D cells, and a large-islet cell population (65%; cell volume 600-1500 micrometer 3) identified as B cells. Differences in sedimentation velocity formed the basis for the islet cell separation by counterflow elutriation. Single islet cells eluted prior to coupled cells and were distributed over A and D cell-enriched fractions I and II (65% A, 25% B, 10% D) and the B cell-enriched fraction III (93% B). The slightly different densities of A (d = 1.068), B (d = 1.065) and D (d = 1.070) cells allowed a subsequent purification by density gradient centrifugation resulting in a final 10- to 30-fold enrichment in either A, B or D cells. Most coupled islet cells were recovered in fraction IV, occurred mainly as doublets and were composed of 90% B cells and 7% D cells; the multiple pseudopods, which characterize isolated D cells, might contribute to the coupling tendency of the D cells. It is concluded that the purified A, B and D cell fractions and the coupled islet cell preparations offer a direct approach to the study of individual islet cell types and their intercellular communication.

Animals↗
PubMed1981

A rapid method for the fractionation of human spermatozoa according to their progressive motility.

A rapid and gentle procedure has been developed for the separation of spermatozoa from other seminal cells and for the preparation of purified spermatozoa with uniform progressive velocity. The method consists of the filtration of semen through an isotonic. Percoll layer and the subsequent centrifugation of the cell pellet on a preformed continuous Percoll gradient; after fractionation of the density gradient, immotile spermatozoa are recovered at density 1.06 to 1.09, whereas an increase in progressive velocity is measured for spermatozoa collected at higher densities. The usefulness of this simple method as a diagnostic aid in men with infertility and urogenital disease was illustrated by the analysis of two abnormal semen samples. This method for the preparation of homogeneous sperm cell populations might facilitate further (sub)cellular studies under normal and pathologic conditions.

Cell Separation↗
PubMed1980

Microtubule interactions in islets of Langerhans.

Morphologic and functional studies have implicated islet microtubules in the transport of the B-cell secretory product form the endoplasmic reticulum to the peripheral pool of secretory vesicles. The participation of the microtubular apparatus in the insulin release mechanism appears to be mediated through an increased rate of tubulin synthesis and of tubulin polymerization, two possible sites for a physiologic and pharmacologic regulation of hormone discharge. It is conceivable that cytoplasmic microtubules from either a rigid cytoskeleton which facilitates hormone transport by establishing n intracellular organization or act as a motion generating system along which the secretory vesicles are actively transported to the cell periphery. The existence of an eventual interaction between secretory vesicles and islet microtubules has been examined by measuring I123-tubulin binding to various subcellular fractions. In working out the experimental procedure on liver tissue, tubulin was found to bind to all subcellular fractions being most pronounced in the microsomial fraction; in the cytosol, tubulin was incorporated into high molecular weight complexes. Similar results were obtained with islet subcellular fractions, binding per microgram protein being tenfold higher than in liver tissue. In view of the calcium-induced increase in tubulin binding to islet subcellular fractions, and of the high affinity of tubulin and secretory vesicles for calcium, it is suggested that a calcium stimulated bridge - eventually microfilamentous in nature - might link the growing microtubules to the secretory vesicles and could, as such, participate in the intracellular transport of the secretory product.

Animals↗
PubMed1978

The stimulus-secretion coupling of glucose-induced insulin release XXVI. Are the secretory and fuel functions of glucose dissociable by iodoacetate?

Iodoacetate inhibits glyceraldehyde-3-phosphate dehydrogenase activity in pancreatic islets and causes a time- and dose-related inhibition of glucose oxidation and lactate output by the islets. High concentrations of the drug (0.3 mM or more) fail to affect Ba2+-induced insulin secretion but inhibit glucose-stimulated proinsulin biosynthesis, 45Ca net uptake and insulin release. A mixture of fumarate, glutamate, and pyruvate, the oxidation of which is only partially reduced by iodoacetate, fails to protect the B-cell against the inhibitory effect of the drug. These findings are compatible with the view that glycolysis plays an essential role in the process of glucose-induced insulin release. At low concentrations of iodoacetate (up to 0.2 mM), the reduction in glucose metabolism coincides with a partial inhibition of proinsulin biosynthesis. However, the expected reduction in 45Ca net uptake and subsequent insulin release is masked by a concomitant facilitating action of iodoacetate, possibly due to interference with native ionophoretic processes. It is concluded that iodoacetate is not an adequate tool to dissociate, if they are dissociable, the fuel and secretory functions of glucose.

Animals↗
PubMed1977

A sensitive method for measuring polymerized and depolymerized forms of tubulin in tissues.

A rapid method for measuring polymerized and depolymerized forms of tubulin in tissues has been developed. The procedure consists of homogenization and centrifugation of the tissue in a microtubule-stabilizing solution and depolymerization of the precipitated microtubules; polymerized and depolymerized forms of tubulin are quantitated by a colchicine-binding assay. The validity of the technique was assessed by electron microscopy and recovery studies with labeled and unlabeled preparations of polymerized and depolymerized forms of rat brain tubulin. The sensitivity of this technique allows quantitation of tubulin in 150 micrograms of tissue, wet weight. The method demonstrated that both the polymerized and depolymerized forms of tubulin were present in rat liver cells, and that in the fed state 31.3 +/-0.7% of the total tubulin pool was in the polymerized form.

Animals↗
PubMed1977

Physiological regulation of total tubulin and polymerized tubulin in tissues.

Polymerized and depolymerized forms of tubulin were measured in rat and mouse liver, rat islets, human lymphocytes, and platelets. The percent of the total tubulin present in the polymerized form varied from 30.3 +/- 1.5% in the liver of the fed rat to 89.2 +/- 0.2% in human platelets. Fasting decreased the total tubulin and to a greater extent the polymerized form of tubulin in both rat and mouse liver. Glucose feeding increased the polymerized tubulin without affecting the total tubulin content in rat liver. Phytohemagglutinin-stimulated lymphocytes exhibited at least a three-fold increase in total tubulin (expressed in terms of DNA content), which during the initial 48 h of incubation was accounted for in toto by an increase in polymerized tubulin. It is suggested that the lectin not only accelerates tubulin synthesis but also stimulated the polymerization process. Storage of platelets at 4 degrees C for 6 days resulted in a marked decrease in total tubulin and an even greater reduction in the polymerized form. It is concluded that both the total tubulin content and its degree of polymerization can be modulated independently by a wide variety of physiological factors.

Animals↗
PubMed1976

Microtubule assembly and the intracellular transport of secretory granules in pancreatic islets.

Polymerized and depolymerized tubulin were measured in pancreatic islets under various physiological and pharmacological conditions. Variations in insulin release from islets of fed or fasted rats were accompanied by concomitant changes in tubulin polymerization. Glucose induced the formation of microtubules in vitro independent of extracellular calcium. Total and polymerized tubulin content were decreased by fasting and restored by glucose feeding.

Animals↗
PubMed1976

Calcium-antagonists and islet function. IV. Effect of D600.

D600 (2 to 20 muM; alpha-isopropyl-alpha [(N-methyl-N-homoveratril)-gamma-aminopropyl]-3,4,5-trimethoxyphenyl-acetonitril) caused a dose-related, rapid and reversible inhibition of glucose-induced insulin release. It also suppressed the insulinotropic action of a sulphonylurea but failed to affect the enhancing action of theophylline upon glucose-induced release. The inhibitory effect of D600 was enhanced at low extracellular Ca2+ concentration. D600 reduced both basal and glucose-stimulated 45calcium net uptake, whilst failing to affect the efflux of 45calcium from perifused islets. The recognition of glucose by the B-cell was also unaffected by D600 as judged by the effect of the sugar upon both 45calcium efflux and net uptake in the isolated islets. These findings are compatible with the hypothesis that the primary mode of action of D600 is to inhibit Ca2+ entry in the B-cell.

Animals↗
PubMed1976

Regulation of tubulin synthesis in islets of Langerhans.

Tubulin represents a major protein in rat pancreatic islets, which averages 0.5% of the total protein content and 6% of the noninsulin protein synthesized under basal metabolic conditions. Glucose increases the synthesis of tubulin twofold to threefold. Tubulin synthesis is also stimulated by adenosine 3':5'-cyclic monophosphate in both the absence and presence of glucose; this effect of adenosine 3':5'-cyclic monophosphate occurs preferential to noninsulin protein synthesis at physiological glucose concentrations. Tubulin synthesis was decreased more than 75% by fasting, an effect prevented by maintaining animals on glucose exclusively. The fasting-induced reduction in tubulin synthesis is corrected in vitro by increasing adenosine 3':5'-cyclic monophosphate levels. These findings parallel changes previously reported in insulin release and suggest that physiological agents can exert their insulin secretory action through an effect upon the rate of tubulin synthesis.

Animals↗
PubMed1976

Metabolic and morphologic studies in intraportal-islet-transplanted rats.

The intraportal injection of 350 to 1,000 isolated islets into streptozotocin-diabetic rats immediately normalized (approximately 24 hours) fasting plasma glucose and insulin levels. Polyuria, polydipsia, and hyperglucagonemia disappeared more gradually over a 2-to-12-week period--the time required for normalization varying with the severity of the diabetes and the number of islets transplanted. In long-term islet-transplanted rats (greater than five months), the hepatic insulin and glucagon reserves averaged 50 per cent and 25 per cent, respectively, of the corresponding normal pancreatic hormone content. Glucagon was increased slightly in the pancreas of streptozotocin-diabetic rats and decreased considerably in transplanted animals. However, total pancreatic glucagon (i.e. pancreatic and hepatic reserves) in transplanted animals was the same as the pancreatic content of normal control rats, indicating the presence of feedback control mechanism(s) in the regulation of pancreatic glucagon reserves. Long-term transplanted islets demonstrated well-granulated A-, B-, and D-cell movement out of the vascular space and the formation of narrow intercellular spaces and junctional complexes with surrounding hepatocytes.

Animals↗
PubMed1975

In vitro biosynthesis and release of three immuno-reactive insulin-like components by a human insulinoma.

Pieces of a human insulinoma were incubated for 120 or 180 min in media containing labelled leucine. In addition to insulin and proinsulin, the tumoral tissue was found to synthesize and release a third immunoreactive insulin-like component of higher molecular weight. No significant amount of the latter component was detected in normal rat islets under identical experimental conditions. The rate of secretion of insulin-like peptides was abnormally high relative to their concentration in the tumoral tissue, and was apparently unaffected by various insulinotropic or inhibitory agents. Only cycloheximide was found to significantly reduce the synthesis and subsequent release of insulin-like peptides by the tumoral tissue. It is suggested that the secretion of large amounts of both the high molecular weight component and proinsulin relative to that of insulin might be due, in part at least, to the inability of the tumoral cells to prevent the release of newly synthetized hormonal peptides, leading to the depletion of the tissue in hormonal products, as judged by both immunological and ultrastructural criteria.

Adenoma, Islet Cell↗