G. H. J. Wolters

Improved biocompatibility but limited graft survival after purification of alginate for microencapsulation of pancreatic islets

Summary Graft failure of alginate-polylysine microencapsulated islets is often interpreted as the consequence of a non-specific foreign body reaction against the microcapsules, initiated by impurities present in crude alginate. The aim of the present study was to investigate if purification of the alginate improves the biocompatibility of alginate-polylysine microcapsules. Alginate was purified by filtration, extraction and precipitation. Microcapsules prepared from crude or purified alginate were implanted in the peritoneal cavity of normoglycaemic AO-rats and retrieved at 1, 2, 3, 6, 9, and 12 months after implantation. With crude alginate, all capsules were overgrown within 1 month after implantation. With purified alginate, however, the portion of capsules overgrown was usually less than 10 %, even at 12 months after implantation. All recipients of islet allografts in capsules prepared of purified alginate became normoglycaemic within 5 days after implantation, but hyperglycaemia reoccurred after 6 to 20 weeks. With intravenous and oral glucose tolerance test, all recipients had impaired glucose tolerance and insulin responses were virtually absent. After graft failure, capsules were retrieved (80–100 %) by peritoneal lavage. Histologically, the percentage of overgrown capsules was usually less than 10 % and maximally 31 %. This small portion cannot explain the occurrence of graft failure. The immunoprotective properties of the capsules were confirmed by similar if not identical survival times of encapsulated islet allo- and isografts. Our results show that purification of the alginate improves the biocompatibility of alginate-polylysine microcapsules. Nevertheless, graft survival was still limited, most probably as a consequence of a lack of blood supply to the encapsulated islets. [Diabetologia (1997) 40: 262–270]

Significance of the peri-insular extracellular matrix for islet isolation from the pancreas of rat, dog, pig, and man

SummaryThe presence and distribution in the peri-insular region of extracellular matrix, and in particular basement membrane, was investigated in a comparative study comprising pancreata of rat, dog, pig, and man. Basement membrane markers, collagen type-IV and laminin, were determined immunohistochemically. Additional information pertaining to the structural relationships between endocrine and exocrine pancreas, in particular cell-to-cell and cell-to-matrix contacts, was obtained by electron microscopy. In pig, very little periinsular capsule is present, and the structural integration of the porcine islet in the exocrine pancreas almost exclusively depends on cell-to-cell adhesion. In the canine pancreas, the islets are almost completely encapsulated with very little direct exocrine-to-endocrine cell-to-cell contact. In rat and man, the situation is intermediate with a tendency towards predominance of cell-to-matrix adhesion. The intra-insular adhesion mechanisms depend largely on cell-to-cell adhesion in all four species. The ultrastructural results suggest that collagenase preparations employed in islet isolation procedures should be of high purity as to preserve the protease-sensitive intra-islet cell-to-cell adhesion. Under these conditions, however, the endocrine-to-exocrine cell-to-cell contacts will be conserved also, resulting in an exocrine-tissue contamination of the islets of Langerhans. Consequently, additional steps for the effective removal of exocrine tissue and the purification of islets are required.

An analysis of the role of collagenase and protease in the enzymatic dissociation of the rat pancreas for islet isolation.

SummaryCrude Clostridium histolyticum collagenase is widely used for the enzymatic degradation of pancreatic extracellular matrix in order to isolate the islets of Langerhans. The variable enzymatic composition of crude collagenases is a critical issue which contributes to the poor reproducibility of islet isolation procedures. In this study, the separate contributions of collagenase and protease to the islet isolation process were analysed by testing various combinations of purified collagenase and purified protease in rat pancreas dissociations under conditions which eliminated all other proteolytic activity. Under these conditions, complete tissue dissociation by purified collagenase required 99±10 min, whereas increasing amounts of protease progressively reduced this time to a minimum of 36±1 min. Histochemical analysis of the dissociation process showed that protease enhanced the degradation of all four major components of the extracellular matrix: collagen was degraded more completely, while proteoglycans, glycoproteins and elastin were degraded at a higher rate. Pancreas dissociation under the present, strictly controlled conditions resulted in a high yield of viable islets: 4.2–5.0 μl islet tissue volume (3,300–3,800 islets) were isolated per g pancreas in the presence of a high or low protease concentration, respectively. Prolonged dissociation in the presence of protease resulted in a dramatic decrease in islet yield which correlated with the observation that the enzyme accelerated islet disintegration. It is concluded that the collagenase-induced dissociation of the extracellular matrix is facilitated by protease. Our study shows that high yields of viable islets can be obtained under controlled enzymatic conditions, provided that the exposure of islets to protease is limited.

Distribution of collagens type I, type III and type V in the pancreas of rat, dog, pig and man

The presence of collagens type I, type III and type V was determined immunohistochemically in pancreatic tissue of rat, pig, dog and man. The reaction to anti-collagen type I is weak (pig, dog) or moderate (rat, man) in the peri-insular region and in the lobar, lobular and acinar septa, whereas the reaction to anti-collagen type III is well developed. In rat and dog, the latter reaction deposit on the lobar and acinar septa is prominent. These elements only show a moderate reaction intensity in pig and man. The peri-insular region displays a weak (rat, dog, man) or very weak (pig) reaction against collagen type III. Anti-collagen type V reacts moderately (rat, dog, man) or weakly (pig) in the lobar and lobular septa. The acinar septa show a moderate (rat, dog, man) or very weak (pig) reaction. This information regarding the types and distribution of the collagenous compounds in pancreatic extracellular matrix could lead to differentiated enzymatic pancreas dissociation and, ultimately, increased islet yield and improved reproducibility of pancreatic islet isolation procedures for transplantation purposes.

Glucose tolerance and plasma insulin response to intravenous glucose infusion and test meal in rats with microencapsulated islet allografts

SummaryAlbino Oxford rats made diabetic with 75 mg/kg streptozotocin were intraperitoneally transplanted with 2500–2900 alginate-polylysine microencapsulated Lewis islets (n=9, total islet tissue volume 8.0–11.0 μl), or a similar volume of non-encapsulated Lewis islets (n=5). All rats with microencapsulated islets became normoglycaemic, and remained normoglycaemic for 5–16 weeks. In rats with non-encapsulated islet grafts, only a temporary decrease in blood glucose was observed, and all were again severely hyperglycaemic at 1 week after implantation. At 5–6 weeks after transplantation, glucose tolerance in rats with microencapsulated islets was tested by intravenous glucose infusion (10 mg/min over 20 min) and test meal administration (n=4). During glucose infusion, maximum glucose levels were 13.0±0.4 mmol/l in rats with microcapsules and 8.9±0.4 mmol/l in healthy control rats (p<0.01). Concomitant maximum plasma insulin levels were 215±17 pmol/l in rats with microcapsules and 715±85 pmol/l in controls (p<0.001). After the test meal, maximum blood glucose was 10.6±0.9 mmol/l in rats with microcapsules and 6.2±0.1 mmol/l in controls (p<0.001), with concomitant maximum plasma insulin levels of 247±11 pmol/l and 586±59 pmol/l, respectively (p<0.001). In conclusion, although the glucose tolerance is impaired and plasma insulin responses to intravenous glucose-load and test-meal are reduced, the alginate-polylysine membrane does provide adequate immunoisolation for the prolongation of allograft survival, resulting in prolonged normoglycaemia in streptozotocin diabetic rats.

Insulin secretion by rat islet isografts of a defined endocrine volume after transplantation to three different sites

SummaryWe have analysed the graft function of rat islet isografts of identical and well-defined endocrine volumes after transplantation to three different sites (kidney, liver and spleen). Graft endocrine mass was determined by measuring the total islet volume prior to transplantation and was chosen to be similar to the endocrine volume in the normal adult rat pancreas. Graft function was tested in unanaesthetized, unstressed rats by the responses to glucose infusion and to a meal. All transplanted animals returned to normoglycaemia within one week after transplantation. At one month, basal glucose and insulin levels were similar to controls in rats with grafts to the spleen, but higher in rats with grafts to the kidney or liver. Irrespective of the transplantation site, recipients had higher glucose and lower insulin levels than controls in response to glucose infusion, but in response to a meal these differences from normal were less obvious. Finally, recipients showed both an acute insulin response to glucose infusion as well as a pre-absorptive insulin release after food ingestion, irrespective of the transplantation site. Our findings indicate that the insulin response to glucose infusion and to a meal is quantitatively reduced, but qualitatively intact after transplantation to the kidney, liver or spleen.

The effect of glucose stimulation on 45calcium uptake of rat pancreatic islets and their total calcium content as measured by a fluorometric micro-method

SummaryGlucose-stimulated 45calcium uptake and total calcium content of rat pancreatic islets has been studied, using a new fluorometric micro-method to estimate total calcium. Extracellular calcium was separated from incubated tissue by a rapid micro-filtration procedure. Islets incubated up to 60 min with calcium chloride 2.5 mmol/l and glucose 2.5 mmol/l maintained the same calcium content (670±7.5 pmol/μg DNA). When the glucose concentration was raised to 15 mmol/l no change in the total calcium content could be detected. On incubation with glucose 2.5 mmol/l in the absence of calcium, the calcium content decreased to 488±27 pmol/μg DNA. On incubation with 45calcium chloride 2.5 mmol/l for 5 or 30 min at 2.5 mmol/l glucose, islets exchanged 21 ±2 and 28±1% of their total calcium content and, at 15 mmol/l glucose, 30±3 and 45±2%, respectively. Thus, islet calcium has a high turn-over rate. Glucose stimulation results in an increase of the calcium uptake without enhancing the total calcium content and hence must increase the calcium-exchangeable pool.

Changes in histochemically detectable calcium and zinc during tolbutamide-induced degranulation and subsequent regranulation of rat pancreatic islets

SummarySecretory granules of pancreatic B-cells contain high concentrations of zinc and calcium. The effect of gradual degranulation (induced by tolbutamide over a period of 3 days) and the subsequent regranulation (over a period of 4 days) on the histochemically detectable zinc (Zn) and calcium (Ca) content of B-cells was investigated. Zn was stained by dithizone, Ca by glyoxal-bis-(2-hydroxyanil), (GBHA), and B-granules by aldehyde fuchsin (AF). The staining intensities were determined cytophotometrically. A decrease of the granulation by 50% causes a comparable decrease of the Zn content. Almost complete degranulation, however, hardly further diminished the Zn content. Regranulation restores the Zn content parallel to the granulation. The presence of 40% of the initial Zn content in degranulated B-cells suggests the existence of a non-granular Zn fraction. The Zn content of B-cells may be partly involved in the storage of insulin as a Zn-insulin complex in the secretory vesicles. A-cells, however, contain even more (+30%) Zn than B-cells. Degranulation of B-cells is accompanied by a moderate decrease of the zinc content of the A-cells. The function of Zn in A-cells is completely unknown. Degranulation of B-cells causes the GBHA-Ca content to decrease to a very low level parallel to the AF-positive granulation. During regranulation the GBHA-Ca content restores parallel to the granulation and reaches after complete regranulation a slightly higher level than in untreated control rats. Almost complete disappearance of CBHA-Ca in the B-cells is accompanied by a decrease of the total islet calcium content of 33%. The results indicate that GBHA stains a Ca fraction which is mainly localized to the secretory granules. The stainability of granular Ca by GBHA is probably based on: a) the high Ca concentration in the granules, b) the presence of ionized Ca in the granules, due to the low intragranular pH, and c) on the properties of GBHA, which stains, under conditions used, only ionized (possibly also readily ionizable) Ca.

Relationship between Extracellular Na+ and the Total Ionized Ca2+ Content of Rat Pancreatic Islets

It is assumed that sodium ions, Na+, are involved in a Na-Ca exchange process at the β-cell membrane and in the mobilization of calcium ions, Ca2+, from intracellular stores4,5. Na-Ca exchange, which exchanges extracellular Na+ for intracellular Ca2+, has been observed in several cell types and has been characterized in detail in cardiac cells and membrane vesicles6. Na-Ca exchange depends on the electrochemical gradient of Na+ (and of Ca2+) across the cell membrane. This process is sensitive to changes in the membrane potential and the extracellular Na+ concentration, [Na+]o 6. If this process exists in β-cells, depolarization of the membrane will inhibit the exchange and enhance the intracellular Ca2+ concentration, [Ca2+]i, in the β-cells.