Aluminum Chloride Pretreatment of Elastin Inhibits Elastolysis by Matrix Metalloproteinases and Leads to Inhibition of Elastin-Oriented Calcification

Calcification of elastin occurs in many pathological cardiovascular diseases including atherosclerosis. We have previously shown that purified elastin when subdermally implanted in rats undergoes severe calcification and aluminum chloride (AlCl(3)) pretreatment of elastin inhibits calcification. In the present study we investigated whether matrix metalloproteinase (MMP) binding to elastin and elastin degradation is prevented by AlCl(3) pretreatment. Subdermal implantation of AlCl(3)-pretreated elastin showed significantly lower MMP-9 and MMP-2 activity surrounding the implant as compared to the control implants. AlCl(3) pretreatment also significantly inhibited elastin implant calcification at the seven-day implant period (AlCl(3)-pretreated 4.07 +/- 1.27, control 23.82 +/- 2.24 microg/mg; p<0.0001). Moreover, elastin gel zymography studies showed that gel pretreatment with AlCl(3) inhibited elastolysis by MMP-9. We also demonstrate significant suppression of MMP-2 activity in aortic wall segments of AlCl(3)-pretreated porcine bioprosthetic heart valve implants as compared to control valve implants in sheep mitral valve replacement studies. AlCl(3) pretreatment also significantly inhibited calcification of elastin in this model. Thus, we conclude that aluminum ion binding to elastin prevents MMP-mediated elastolysis and thus prevents elastin calcification.     

Bisphosphonate derivatized polyurethanes resist calcification

Calcification of polyurethane cardiovascular implants is an important disease process that has the potential to compromise the long-term function of devices such as polymer heart valves and ventricular assist systems. In this study we report the successful formulation and characterization of bisphosphonate-derivatized polyurethanes, hypothesized to resist implant calcification based on the pharmacologic activity of the immobilized bisphosphonate. Fully polymerized polyurethanes (a polyurea-polyurethane and a polycarbonate polyurethane) were modified (post-polymerization) with bromoalkylation of the hard segments followed by attachment of a bisphosphonate group at the bromine site. These bisphosphonate-polyurethanes resisted calcification in rat 60 day subdermal implants compared to nonmodified control polyurethane implants, that calcify. Bisphosphonates-modified polyurethanes were also studied in circulatory implants using a pulmonary valve cusp replacement model in sheep. Polyurethane cusps modified with bisphosphonate did not calcify in 90 day implants. compared to control polyurethane cusps implants, that demonstrated nodular surface oriented calcific deposits. It is concluded that bisphosphonate modified polyurethanes resist calcification both in subdermal implants and in the circulation. This novel biomaterial approach offers great promise for long-term blood stream implantation with calcification resistance.     

Initiation of mineralization in bioprosthetic heart valves: studies of alkaline phosphatase activity and its inhibition by AlCl3 or FeCl3 preincubations

The principal cause of the clinical failure of bioprosthetic heart valves fabricated from glutaraldehyde-pretreated porcine aortic valves is calcification. Other prostheses composed of tissue-derived and polymeric biomaterials also are complicated by deposition of mineral. We have previously demonstrated that: (a) Failure due to calcification of clinical bioprosthetic valves can be simulated by either a large animal circulatory model or subdermal implants in rodents. (b) Calcification of bioprosthetic tissue has complex host, implant, and mechanical determinants. (c) The initial calcification event in the rat subdermal model is the mineral deposition in devitalized cells intrinsic to the bioprosthetic tissue within 48 to 72 h, followed later by collagen mineralization. (d) Initiation of bioprosthetic tissue mineralization, like that of physiological bone formation, has "matrix vesicles" as early nucleation sites. (e) Alkaline phosphatase (AP), an enzyme also associated with matrix vesicles involved in bone mineral nucleation, is present in both fresh and fixed bioprosthetic tissue at sites of initial mineralization. (f) Certain inhibitors of bioprosthetic tissue calcification (e.g., Al3+, Fe3+) are localized to the sites at which alkaline phosphatase is present. On the basis of these results, we hypothesize that alkaline phosphatase is a key element in the pathogenesis of mineralization of bioprosthetic tissue. In the present studies, we focused on the relationship of AP to early events in calcification, and the inhibition of both calcification and AP activity by FeCl3 and AlCl3 preincubations. Subdermal implants of glutaraldehyde pretreated bovine pericardium (GPBP) were done in 3-week-old rats. AP was characterized by enzymatic hydrolysis of paranitrophenyl phosphate (pnpp), and by histochemical studies. Calcification was evaluated chemically (by atomic adsorption spectroscopy) and morphologically (by light microscopy). The results of these studies are as follows: (a) Extractable AP activity is present in fresh but not glutaraldehyde-pretreated bovine pericardial tissue. However, histochemical studies reveal active AP within the intrinsic devitalized cells of GPBP, despite extended glutaraldehyde incubation. (b) Extrinsic AP is rapidly adsorbed following implantation, with peak activity at 72 h (424 +/- 67.2 nm pnpp/mg protein/min enzyme activity [units]), but markedly lesser amounts at 21 days (96.8 +/- 3.9 units). (c) Simultaneously to the AP activity maximum, bulk calcification is initiated, with GPBP calcium levels rising from 1.2 +/- 0.1 (unimplanted) to 2.4 +/- 0.2 micrograms/mg at 72 h, to 55.6 +/- 3.1 micrograms/mg at 21 days, despite a marked decline in AP activity at this later time.(ABSTRACT TRUNCATED AT 400 WORDS)     

猪主动脉生物瓣远期损坏的病理学观察

对３６ 例植入人体内１０７ 个月以上的损坏瓣膜作病理学观察。基本病损：９４－４４ ％ 为钙化、３６－１１％ 为撕裂、１３－２９％ 为穿孔。结果表明,首先瓣叶应力集中区胶原纤维变性产生钙化,新的应用集中区的形成,导致瓣叶撕裂与穿孔,相互促进。早期损坏以钙化为主,而后期常以钙化和撕裂、钙化和穿孔等多种病损因素的复合结果为主。     

Glutaraldehyde-fixed kangaroo aortic wall tissue: histology,crosslink stability and calcification potential

Stentless aortic heart valve substitutes, manufactured from biological tissues, are fixed with glutaraldehyde to cross-link collagen, reduce antigenicity, and sterilize the tissue. Despite improved cross linking, reduced antigenicity, and various anticalcification measures, the aortic wall tissue present in these prostheses tends to calcify. The aim of this study was to assess the morphology, collagen cross-link stability, and calcification potential of glutaraldehyde-preserved kangaroo aortic wall tissue as opposed to porcine aortic wall tissue. Porcine and kangaroo aortic wall tissues were fixed in 0.625% buffered glutaraldehyde. Histology and cross-link stability were examined. Calcification potential was determined in the subcutaneous rat model. Kangaroo aortic wall tissue was significantly (p < 0.01) less calcified than porcine aortic wall tissue (26.67 +/- 6.53 versus 41.959 +/- 2.75 microg/mg tissue) at 8 weeks. In conclusion, the histological differences between kangaroo and porcine aortic wall tissue correlate well with the reduced calcification potential of kangaroo aortic wall tissue. The reduced calcification potential could result in improved long-term durability of stentless kangaroo heart valves as bioprostheses.     

Role of glutaraldehyde in calcification of porcine aortic valve fibroblasts

Glutaraldehyde-treated porcine aortic valve xenografts frequently fail due to calcification. Calcification in the prostheses begins intracellularly. In a previous study, various types of cell injury to canine valvular fibroblasts, including glutaraldehyde treatment, led to calcification. An influx of extracellular Ca2+ into the phosphate-rich cytosol was theorized to be the mechanism of calcification. To test the Ca2+ influx theory, cytosolic Ca2+ and Pi concentrations were assessed in glutaraldehyde-treated porcine aortic valve fibroblasts, and their relationship to a subsequent calcification was studied. Glutaraldehyde caused an immediate and sustained massive cytosolic Ca2+ increase that was dose dependent and a several-fold increase in Pi. Calcification of cells followed within a week. The earliest calcification was observed in blebs formed on glutaraldehyde-treated cells. Live control cells or cells fixed with glutaraldehyde in Ca2+-free solution did not calcify under the same conditions. Concomitant increases in Ca2+ and Pi in glutaraldehyde-treated cells appear to underlie the mechanism of calcification, and the presence of extracellular Ca2+ during glutaraldehyde fixation promotes calcification.     

New treatments using alginate in order to reduce the calcification of bovine bioprosthetic heart valve tissue

Calcification limits the functional lifetime of cardiac valve substitutes fabricated from glutaraldehyde preserved bovine pericardium. Host factors, mainly younger age, and implant factors, mainly glutaraldehyde cross-linking, are implicated in the calcification process. Glutaraldehyde cross-linking is believed to activate the potential sites in the tissues for biocalcification. In the present work, we investigated the possibility of using alginate azide (AA) instead of glutaraldehyde for the preservation of pericardial tissues in order to enhance the durability of bioprosthetic heart valves. Grafting with poly(GMA-BA) copolymer to the alginate azide cross-linked pericardial (AACPC) tissue was carried out to obtain better stability, strength, and anticalcification properties. The strength property and thermal stability of the AA cross-linked tissues were studied. Calcification studies in rat subdermal models reveal that AA cross-linking reduces the calcification to negligible levels. After 30 days implantation, the calcium content was found to be 10.4 ± 1.2 and 6.1 ± 0.3 μg mg^-1 for untreated AACPC and polymer grafted AACPC, respectively, compared to a value of 100 ± 1.2 μg mg^-1 calcium recorded for control glutaraldehyde cross-linked pericardial (GCPC) tissues.     

Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering

Various research groups around the world are actively investigating cardiovascular prostheses of biological origin. This review article discusses the need for such bioprosthetics and the potential role for natural tissues in cardiovascular applications such as cardiac valves and vascular grafts. Upon implantation, unmodified natural materials are subject to chemical and enzymatic degradation, seriously decreasing the life of the prosthesis. Therefore, methods such as glutaraldehyde and polyepoxide crosslinking treatments and dye-mediated photooxidation have been developed to stabilize the tissue while attempting to maintain its natural mechanical properties. Also, residual cellular components in a bioprosthetic material have been associated with undesired effects, such as calcification and immunological recognition, and thus have been the motivation for various decellularization processes. The effects of these stabilization and decellularization treatments on mechanical, biological and chemical properties of treated tissues have been investigated, specifically with regard to calcification, immunogenicity, and cytotoxicity concerns. Despite significant advances in the area of cardiovascular prostheses, there has yet to be developed a completely biocompatible, long-lasting implant. However, with the recent advent of tissue engineering, the possibility of applying selective cell seeding to naturally derived bioprosthetics moves us closer to a living tissue replacement.     

Controlled release of diphosphonates from synthetic polymers to inhibit calcification

Calcification is the most frequent cause of the failure of bioprosthetic heart valves fabricated from glutaraldehyde pretreated porcine aortic valves or bovine pericardium. Formulation and evaluation of controlled-release drug delivery system to inhibit bioprosthetic heart valve calcification is reviewed.     

LIGHT AND ELECTRON MICROSCOPIC OBSERVATION ON THE PROCESS OF TISSUE CALCIFICATION IN A CASE OF PARATHYROID ADENOMA

Metastatic calcification in various organs in an autopsy case of parathyroid adenoma was studied by light and electron microscopy. Calcification was observed in kidney, lung, liver, heart, stomach, and thyroid. In the kidney, calcification was found in the tubules and glomeruli. In the liver, calcification was mainly found in Disse's space. The kidney, lung, liver, and heart were examined by electron microscopy. Four forms of early calcification were observed: 1) amorphous dense bodies within the mitochondria; 2) needle-shaped hydroxyapatite; 3) concentric laminated structure; and 4) cytoplasmic vesicles originating in hepatocytes. Calcium phosphate precipitates within the mitochondria were seen as amorphous dense bodies. When calcium phosphate precipitated in the cytoplasmic colloid gel, concentric laminated structures were formed due to Liesegang's phenomenon. Needle-shaped crystals in the amorphous dense bodies of the mitochondria were interpreted as a phenomenon of epitaxy. Calcification in Disse's space was thought to start in cytoplasmic vesicles which were derived from hepatocytes. ACTA PATHOL. JPN. 37: 1621 - 1635, 1987.     

Light and electron microscopic observation on the process of tissue calcification in a case of parathyroid adenoma

Metastatic calcification in various organs in an autopsy case of parathyroid adenoma was studied by light and electron microscopy. Calcification was observed in kidney, lung, liver, heart, stomach, and thyroid. In the kidney, calcification was found in the tubules and glomeruli. In the liver, calcification was mainly found in Disse's space. The kidney, lung, liver, and heart were examined by electron microscopy. Four forms of early calcification were observed: 1) amorphous dense bodies within the mitochondria; 2) needle-shaped hydroxyapatite; 3) concentric laminated structure; and 4) cytoplasmic vesicles originating in hepatocytes. Calcium phosphate precipitates within the mitochondria were seen as amorphous dense bodies. When calcium phosphate precipitated in the cytoplasmic colloid gel, concentric laminated structures were formed due to Liesegang's phenomenon. Needle-shaped crystals in the amorphous dense bodies of the mitochondria were interpreted as a phenomenon of epitaxy. Calcification in Disse's space was thought to start in cytoplasmic vesicles which were derived from hepatocytes.     

A new hypothesis on the mechanism of calcification formed on a blood-contacted polymer surface

Calcification on a blood-contacting polymer surface in an artificial heart is one of the most serious problems. Recently, we maintained a goat with a total artificial heart (AH) for 532 days without systemic anticoagulation. Sactype blood pumps coated with segmented polyurethane and incorporating jellyfish valves, thin polymer membrane valves, were used in the experiment. The pump was exchanged for a new one on the 312th day on the left side and the 414th day on the right side. They were analyzed with a scanning electron microscope (SEM) and an X-ray microanalyzer. The valve membrane after 312 days of pumping revealed plastic deformation expanding toward upstream between the spokes by creep fatigue with blood pressure difference when the valve closed. Calcification on the membrane was concentrated in the limited portions that received a strong stretching force: the upstream side of the membrane between the spokes and downstream side of the membrane on the spokes. Slight or no calcification was observed on the opposite side of the membrane that received a compression force, and no calcification was found on nonmoving parts such as the center of the membrane and spokes. A new hypothesis on the mechanism of calcification at the portion that received repeated stretching force was raised. The repeated stretching force would extend the polymer membrane, causing some loosening between polymer molecules and generating microgaps. The blood protein and phospholipid would invade into these microgaps, which would then attract Ca ions followed by phosphate ions to make their complexation. The hypothesis could well explain the calcification phenomena on a blood-contacting polymer surface, and gave a good clue on how to protect from calcification.     

In vivo behavior of epoxy-crosslinked porcine heart valve cusps and walls

Calcification limits the long-term durability of xenograft glutaraldehyde-crosslinked heart valves. In this study, epoxy-crosslinked porcine aortic valve tissue was evaluated after subcutaneous implantation in weanling rats. Non-crosslinked valves and valves crosslinked with glutaraldehyde or carbodiimide functioned as control. Epoxy-crosslinked valves had somewhat lower shrinkage temperatures than the crosslinked controls, and within the series also some macroscopic and microscopic differences were obvious. After 8 weeks implantation, cusps from non-crosslinked valves were not retrieved. The matching walls were more degraded than the epoxy- and control-crosslinked walls. This was observed from the higher cellular ingrowth with fibroblasts, macrophages, and giant cells. Furthermore, non-crosslinked walls showed highest numbers of lymphocytes, which were most obvious in the capsules. Epoxy- and control-crosslinked cusps and walls induced lower reactions. Calcification, measured by von Kossa-staining and by Ca-analysis, was always observed. Crosslinked cusps calcified more than walls. Of all wall samples, the non-crosslinked walls showed the highest calcification. It is concluded that epoxy-crosslinked valve tissue induced a foreign body and calcification reaction similar to the two crosslinked controls. Therefore, epoxy-crosslinking does not represent a solution for the calcification problem of heart valve bioprostheses.     

Prevention of bioprosthetic heart valve tissue calcification by charge modification: effects of protamine binding by formaldehyde

Calcification is the principal cause of the clinical failure of bioprosthetic heart valves (BHV). Calcification occurs through an interaction of host and implant factors, mainly younger age and glutaraldehyde pretreatment, respectively. The hypothesis of this work was that an impaired balance between positively and negatively charged amino acids, due to the reaction with Lys and Hyl tissue-collagen residues, expose affinity sites to Ca++. We further hypothesized that regardless of the cause(s) of BHV calcification, positive charge modification of the tissues will prevent their propensity to calcify. Modification of BHV tissue was obtained by covalently binding protamine sulfate, a polybasic peptide, via formaldehyde and subsequent glutaraldehyde tissue crosslinking. Protamine-bound tissue exhibited stability properties (shrinkage temperature and resistance to collagenase digestion) similar to BHV tissue. Protamine-treated tissue was less permeable to Ca++, and reduced staining was observed with positively charged dyes, indicating the presence of positively charged functional groups in the modified tissue. Significant prevention of calcification was exhibited by the p-bound tissue in comparison to BHV tissue, 30.9 and 109 micrograms/mg calcium, respectively, after 30 days of subdermal implants in rats. The modification procedure resulted in stable, covalent links of approximately 10% w/w protamine with undiminished anticalcification properties, even after 1 year storage. The results support our hypotheses, and orthotopical heart valve replacements are required in order to completely evaluate the treatment efficacy and biocompatibility.     

Inhibition of matrix metalloproteinase activity attenuates tenascin-C production and calcification of implanted purified elastin in rats

Elastin, a major extracellular matrix protein present in arterial walls provides elastic recoil and resilience to arteries. Elastin is prone to calcification in a number of cardiovascular diseases including atherosclerosis and bioprosthetic heart valve mineralization. We have recently shown that purified elastin when implanted subdermally in rats undergoes severe calcification. In the present study, we used this elastin implant model to investigate the molecular mechanisms underlying elastin calcification. Intense matrix metalloproteinase (MMP-2) and tenascin-C (TN-C) expression were seen in the proximity of the initial cal-cific deposits at 7 days. Gelatin zymography studies showed both MMP-2 (latent and active form) and MMP-9 expression within the implants. To investigate the role of MMPs in calcification, rats were administered a MMP inhibitor, (2S:-allyl-N:-hydroxy-3R:-isobutyl-N:-(1S:-methylcarbamoyl-2-ph enylet hyl)-succinamide (BB-1101) by daily injection, either systemically or at the implant site. The site-specific BB-1101 administration almost completely suppressed TN-C expression, as shown by immunohistochemical staining, within the implants. The systemic BB-1101 injections also significantly reduced TN-C expression within the elastin implants. Moreover, calcification of elastin implants was significantly reduced in the site-specific administration group (5.43 +/- 1.03 microg/mg Ca for BB-1101 group versus 21.71 +/- 1.19 for control group, P: < 0.001). Alizarin Red staining clearly showed that the elastin fibers were heavily calcified in the control group, whereas in BB-1101 group the calcification was scarce with few fibers showing initial calcification deposits. The systemic administration of BB-1101 also significantly reduced elastin calcification (28.07 +/- 5.81 control versus 16.92 +/- 2.56 in the BB-1101 group, P: < 0.05), although less than the site-specific administration. Thus, the present studies indicate that MMPs and TN-C play a role in elastin-oriented calcification.     

Morphologic findings and causes of failure in 24 explanted Ionescu-Shiley low-profile pericardial heart valves.

From 1981 to 1987 just over 608 Ionescu-Shiley low-profile bovine pericardial bioprostheses were implanted at the Toronto Hospital. Twenty-four prostheses (11 aortic and 13 mitral) were surgically explanted from 1988 to 1990 from 20 adults (10 men and 10 women). Prosthesis failure was caused by primary tissue failure in 17 valves or by other mechanisms in seven valves. Variable degrees of tissue failure were also seen in four of the seven valves from the latter group. Primary tissue failure was characterized by fluid insudation between collagen bundles, para stent post tears (alignment stitch related, 20 valves), cusp perforation with prolapse, and calcification. The earliest cusp tears occurred at 28 months. Calcification (10 of 24 cases) was minimal in seven of 10 valves (occurring primarily at the margins of the torn cusp), moderate in two, and severe in one. Tissue overgrowth (pannus) was seen in all but three prostheses. Like its predecessor, the Ionescu-Shiley standard pericardial valve, this prosthesis failed at 2 to 5 years largely due to design-related (alignment stitch) causes and tissue degeneration. Calcification was less prominent, while tissue overgrowth (pannus) was more marked.     

Acellular scaffold implantation--no alternative to tissue engineering

OBJECTIVE: Degradation mechanisms of cardiovascular bioprostheses may play an important role in bioartificial implants. The fate of acellular implanted and cellular cardiovascular scaffolds was examined in an in vivo model. METHODS: Decellularized or native ovine carotid artery (CA, n=42) and aorta (AO, n=42) were implanted subcutaneously into rats for 2, 4 and 8 weeks. Immunohistochemical methods were used to monitor repopulation. Desmin-vimentin, CD31-, CD4- and CD18-antibodies for myocytes, endothelium, and inflammatory cell-infiltration, respectively. Calcification was detected by von-Kossa staining. Cell density was quantified by DNA-isolation. RESULTS: Acellular AO and CA matrices showed progressive calcification. Cellular AO and CA matrices trigger a strong inflammatory reaction which subsides after two weeks. CA scaffolds are revascularized progressively, whereas AO biocomposites degenerate. Calcification is less pronounced in cellular AO scaffolds and lacking in CA. CONCLUSION: Acellular bioartificial implants demonstrate degradation mechanisms similar to currently applied cardiovascular bioprostheses. Cellularized viable implants are promising clinical alternatives.     

Comparative study of calcified changes in aortic valvular diseases.

Calcification of the aortic valve leads to stenosis or regurgitation or both. To clarify the mechanism of heart valve calcification, comparative studies using histological and ultrastructural examinations were performed of calcified aortic valves. These valves were obtained at valve replacement surgery from 11 patients with rheumatic aortic valvular disease (RAVD), 10 patients with degenerative aortic valve disease (DAVD), and 10 patients with congenitally bicuspid aortic valves (CBAV). For electron microscopic study, 5 cases were selected from each group. In RAVD, histological examination revealed calcification in a degenerated amorphous area at the center of fibrous thickened regions and in laminar fibrous thickened areas near the valve surface. In DAVD, calcification was observed mainly in the fibrosa near the valve ring. In CBAV, basic pathological changes were similar to those in DAVD; however, additional severe calcification of the raphe was observed, if the raphe was present. Ultrastructural examinations showed deposition of electron-dense materials in two patterns in all three groups; one pattern was observed in the interfibrillar spaces of collagen fibrils, and the other pattern was widespread macular deposition unrelated to the preexisting structure. In RAVD, microfibril-like fibrillar structures were found in the areas of deposition of electron-dense materials. These findings suggest that newly formed connective tissue degraded and became necrotic because of nutritional deprivation, especially in the thickened central area, causing calcium deposition. In DAVD and CBAV, numerous lipid vacuoles were found in the electron-dense deposition areas similar to lipid deposition in aortic atherosclerosis. Localized calcium deposition in the fibrosa suggests that the stress of valvular motion and pressure load induces sclerotic changes with the degeneration of collagen fibers, providing a core for calcification. In CBAV, the raphe was the main location of calcification, wherein spiraled collagen fibrils were observed. Increasing the hemodynamic load with abnormal structure might influence calcification. The ultrastructural pattern of calcification of the valve is common; however, additional findings suggest that the cause and mechanism are different in each type of heart valve disease.     

Covalent binding of aminopropanehydroxydiphosphonate to glutaraldehyde residues in pericardial bioprosthetic tissue: stability and calcification inhibition studies

Calcification has limited the clinical utility of bioprosthetic heart valves fabricated from either glutaraldehyde-pretreated bovine pericardium or porcine aortic valves. Aminopropanehydroxydiphosphonate (APDP), covalently bound to residual aldehyde groups in glutaraldehyde-treated pericardial bioprosthetic tissue, has been shown to inhibit cardiovascular calcification in the rat subdermal model. Using 3H-labeled glutaraldehyde (GLUT) at a concentration of 0.02 M and 0.14 M 14C-labeled APDP, we assessed the effects of GLUT incubation temperature (4 degrees or 25 degrees C) and pH of the GLUT incubation solution (pH 4.0, 7.4, or 10.0) on the GLUT incorporation step and subsequent APDP binding (24 hr 25 degrees C) into bioprosthetic valve (BPV) tissue (bovine pericardium). Increased incorporation of GLUT and APDP occurred at lower GLUT incubation temperature (GLUT, 346.05 +/- 1.9 nM/mg, 4 degrees C vs 259.76 +/- 1.39 nM/mg, 25 degrees C; APDP, 57.56 +/- 4.43 nM/mg, 4 degrees C vs 36.36 +/- 0.46 nM/mg, mean +/- standard error, at 25 degrees C). There also was a greater incorporation of GLUT but not APDP at the higher glutaraldehyde pretreatment pH (GLUT, pH 10.0, 213.73 +/- 73 nM/mg vs pH 4, 132.08 +/- 43 nM/mg; APDP, pH 10.0, 51.41 +/- 12 nM/mg vs pH 4.0, 49.97 +/- 6 nM/mg). In vivo studies revealed that all groups with treated BPV implanted for 21 days in male 3-week-old CD rats demonstrated a loss of both GLUT (12-50%) and APDP (48-64%) compared to preimplant content. BPV implant calcification was significantly inhibited in all groups treated with APDP compared to control Ca2+ (5.54 +/- 2.1-9.64 + 1.2 micrograms/mg, APDP pretreated, vs 93.64 +/- 11.65 micrograms/mg, control; P less than or equal to 0.001) despite the progressive loss of both GLUT and APDP with time. It is concluded that preincubation of BPV tissue in GLUT at lower temperature (4 degrees C) and higher pH (10.0) enhanced BPV GLUT uptake and subsequent APDP covalent binding. In addition, in the rat subdermal model, BPV tissue calcification was markedly inhibited by APDP, despite a significant loss of bound drug.     

Inhibition of ectopic calcification of glutaraldehyde crosslinked collagen and collagenous tissues by a covalently bound diphosphonate (APD)

Calcification of collagen-derived prosthesis, such as glutaraldehyde crosslinked porcine heart valves or heart valves assembled out of bovine pericardium, presents a major clinical problem. Their subcutaneous implantation into young rats provides us with a reproducible method of assessing this form of ectopic calcification. Long-term implantation is essential, since some materials which do not calcify within the first month frequently exhibit a delayed calcific response. Crosslinked pericardium is much more likely to calcify than crosslinked tendon or reconstituted crosslinked pepsin extracted bovine type I collagen. The covalent binding of a diphosphonate to collagen and collagen-rich tissues can prevent calcification. The binding of this diphosphonate and its ability to inhibit calcification can be enhanced by increasing the number of amino groups on the collagen molecule. The degree of calcification is inversely related to the number of diphosphonate molecules covalently bound to collagen. Under standard conditions, chemical modifications appear to occur primarily on the surface of the collagen fibrils, as evidenced by the relationship between the number of molecules of APD bound and fibril diameter. The bound diphosphonate seems to interfere with crystal growth and prevent the formation of highly insoluble hydroxyapatite on the surface and interstices of the collagen fibrils.