Physiol. Res. 50: 536-546, 2001
`
`MINIREVIEW
`
`The Mechanism of Alloxan and Streptozotocin Action in
`B Cells of the Rat Pancreas
`T. SZKUDELSKI
`
`Department of Animal Physiology and Biochemistry, University of Agriculture, Poznan, Poland
`
`Received November 2, 2000
`Accepted March 20, 2001
`
`Summary
`Alloxan and streptozotocin are widely used to induce experimental diabetes in animals. The mechanism of their action
`in B cells of the pancreas has been intensively investigated and now is quite well understood. The cytotoxic action of
`both these diabetogenic agents is mediated by reactive oxygen species, however, the source of their generation is
`different in the case of alloxan and streptozotocin. Alloxan and the product of its reduction, dialuric acid, establish a
`redox cycle with the formation of superoxide radicals. These radicals undergo dismutation to hydrogen peroxide.
`Thereafter highly reactive hydroxyl radicals are formed by the Fenton reaction. The action of reactive oxygen species
`with a simultaneous massive increase in cytosolic calcium concentration causes rapid destruction of B cells.
`Streptozotocin enters the B cell via a glucose transporter (GLUT2) and causes alkylation of DNA. DNA damage
`induces activation of poly ADP-ribosylation, a process that is more important for the diabetogenicity of streptozotocin
`than DNA damage itself. Poly ADP-ribosylation leads to depletion of cellular NAD+ and ATP. Enhanced ATP
`dephosphorylation after streptozotocin treatment supplies a substrate for xanthine oxidase resulting in the formation of
`superoxide radicals. Consequently, hydrogen peroxide and hydroxyl radicals are also generated. Furthermore,
`streptozotocin liberates toxic amounts of nitric oxide that inhibits aconitase activity and participates in DNA damage.
`As a result of the streptozotocin action, B cells undergo the destruction by necrosis.
`
`Key words
`Alloxan • Streptozotocin • Pancreatic B cells • Mechanism of action • Diabetes
`
`The induction of experimental diabetes in the rat
`using chemicals which selectively destroy pancreatic
`B cells is very convenient and simple to use. The most
`usual substances to induce diabetes in the rat are alloxan
`and streptozotocin. The understanding of changes in
`
`B cells of the pancreas as well as in the whole organism
`after alloxan or streptozotocin treatment is essential for
`using these compounds as diabetogenic agents. The
`metabolic disturbances in alloxan- and streptozotocin-
`treated rats were described recently by Szkudelski et al.
`
` ISSN 0862-8408
`PHYSIOLOGICAL RESEARCH
` 2001 Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic
` Fax+420 224920590
`E-mail physres@biomed.cas.cz
` http://www.biomed.cas.cz/physiolres/s.htm
`
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`
`(1998). This review focuses on the elucidation of the
`mechanism of cytotoxic action of alloxan and
`streptozotocin in B cells of the rat pancreas.
`
`1. The mechanism of alloxan action
`
`Alloxan (2,4,5,6-tetraoxypyrimidine; 5,6-dioxyura-
`cil) was first described by Brugnatelli in 1818. Wöhler
`and Liebig used the name “alloxan” and described its
`synthesis by uric acid oxidation (for review see Lenzen
`and Panten 1988). The diabetogenic properties of this
`drug were reported many years later by Dunn, Sheehan
`and McLethie (1943), who studied the effect of its
`administration in rabbits and reported a specific necrosis
`of pancreatic islets. Since then, alloxan diabetes has been
`commonly utilized as an animal model of insulin-
`dependent diabetes mellitus (IDDM).
`Alloxan exerts its diabetogenic action when it is
`administered parenterally:
`intravenously,
`intraperito-
`neally or subcutaneously. The dose of alloxan required
`for inducing diabetes depends on the animal species,
`route of administration and nutritional status. Human
`islets are considerably more resistant to alloxan than
`those of the rat and mouse (Eizirik et al. 1994). The most
`frequently used intravenous dose of this drug to induce
`diabetes in rats is 65 mg/kg b.w. (Gruppuso et al. 1990,
`Boylan et al. 1992). When alloxan is given intraperito-
`nealy or subcutaneously its effective dose must be 2-3
`times higher. The intraperitoneal dose below 150 mg/kg
`b.w. may be insufficient for inducing diabetes in the rat
`(Katsumata et al. 1992, 1993). Fasted animals are more
`susceptible to alloxan (Katsumata et al. 1992, Szkudelski
`et al. 1998), whereas increased blood glucose provides
`partial protection (Bansal et al. 1980, Szkudelski et al.
`1998).
`The mechanism of alloxan action has been
`intensively studied, predominantly in vitro, and is now
`characterized quite well. Using isolated islets (Weaver et
`al. 1978b) and perfused rat pancreas (Kliber et al. 1996)
`it was demonstrated that alloxan evokes a sudden rise in
`insulin secretion in the presence or absence of glucose.
`This phenomenon appeared just after alloxan treatment
`and was not observed after repetitive exposure of islets to
`this diabetogenic agent (Weaver et al. 1978b). The
`sudden rise in blood insulin concentration was also
`observed in vivo just after alloxan injection to rats
`(Szkudelski et al. 1998). Alloxan-induced insulin release
`is, however, of short duration and is followed by
`complete suppression of the islet response to glucose,
`
`even when high concentrations (16.6 mM) of this sugar
`were used (Kliber et al. 1996).
`Alloxan is a hydrophilic and unstable substance. Its
`half-life at neutral pH and 37 °C is about 1.5 min and is
`longer at lower temperatures (Lenzen and Munday 1991).
`On the other hand, when a diabetogenic dose is used, the
`time of alloxan decomposition is sufficient to allow it to
`reach the pancreas in amounts that are deleterious.
`The action of alloxan in the pancreas is preceded by
`its rapid uptake by the B cells (Weaver et al. 1978a,
`Boquist et al. 1983). Rapid uptake by insulin-secreting
`cells has been proposed to be one of the important
`features determining alloxan diabetogenicity. Another
`aspect concerns the formation of reactive oxygen species
`(Heikkila et al. 1976). A similar uptake of alloxan also
`takes place in the liver. However, the liver and other
`tissues are more resistant to reactive oxygen species in
`comparison to pancreatic B cells and this resistance
`protects them against alloxan toxicity (Malaisse et al.
`1982, Tiedge et al. 1997). The formation of reactive
`oxygen species is preceded by alloxan reduction. In
`B cells of the pancreas its reduction occurs in the
`presence of different reducing agents. Since alloxan
`exhibits a high affinity to the SH-containing cellular
`compounds, reduced glutathione (GSH), cysteine and
`protein-bound
`sulfhydryl
`groups
`(including SH-
`containing enzymes) are very susceptible to its action
`(Lenzen and Munday 1991). However, other reducing
`agents such as ascorbate may also participate in this
`reduction (Zhang et al. 1992). Lenzen et al. (1987)
`proposed that one of the SH-containing compounds
`essential for proper glucose-induced insulin secretion is
`glucokinase (EC 2.7.1.2), being very vulnerable to
`alloxan. Alloxan reacts with two -SH groups in the sugar-
`binding side of glucokinase resulting in the formation of
`the disulfide bond and inactivation of the enzyme.
`Glucose can protect glucokinase against the inactivation
`hindering the access of alloxan to the -SH groups of the
`enzyme (Lenzen et al. 1987, 1988, Lenzen and Mirzaie-
`Petri 1991).
`Dialuric acid is formed as a result of alloxan
`reduction. It
`is
`then re-oxidized back
`to alloxan
`establishing a redox cycle for
`the generation of
`superoxide
`radicals
`(Munday 1988). The
`reaction
`between alloxan and dialuric acid is a process in which
`intermediate alloxan radicals (HA•) and an unidentified
`"compound 305" (maximum absorption at 305 nm) is
`formed. The latter appears when alloxan is reduced by
`GSH (Sakurai and Ogiso 1991). Superoxide radicals are
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`able to liberate ferric ions from ferritin and reduce them
`to ferrous ions. Fe3+ can also be reduced by alloxan
`radicals (Sakurai and Ogiso 1995). Moreover, superoxide
`radicals undergo dismutation to hydrogen peroxide:
`•− + O2
`•− + 2 H+ → H2O2 + O2
`O2
`This reaction may occur spontaneously or may be
`catalyzed by superoxide dismutase
`(EC 1.15.1.1)
`
`(Malaisse 1982). In the presence of Fe2+ and hydrogen
`peroxide, highly reactive hydroxyl radicals are then
`formed according to the Fenton reaction (Fig. 1):
`Fe2+ + H2O2 → Fe3+ + OH− + OH•−
`The action of hydroxyl radicals following alloxan
`treatment was demonstrated in vitro (Grankvist 1981,
`Munday 1988) and in vivo (Kurahashi et al. 1993).
`
`Fig. 1. The mechanism of alloxan-induced
`reactive oxygen species generation in B cells of
`rat pancreas. GKa, GKi – glucokinase active
`and inactive, respectively; HA• – alloxan
`radicals; [Ca2+]i –
`intracellular calcium
`concentration.
`
`One of the targets of the reactive oxygen species
`is DNA of pancreatic islets. Its fragmentation takes place
`in B cells exposed to alloxan (Takasu et al. 1991a,
`Sakurai and Ogiso 1995). DNA damage stimulates poly
`ADP-ribosylation, a process participating in DNA repair.
`Some inhibitors of poly ADP-ribosylation can partially
`restrict alloxan
`toxicity. This effect
`is, however,
`suggested to be due to their ability to scavenge free
`radicals rather than to a restriction of poly ADP-
`ribosylation initiated by alloxan (Sandler and Swenne
`1983, LeDoux et al. 1988). Superoxide dismutase,
`catalase (EC 1.11.1.6) (Grankvist et al. 1979, Grankvist
`1981, Jörns et al. 1999) and non-enzymatic scavengers of
`hydroxyl radicals (Ebelt et al. 2000) were also found to
`protect against alloxan toxicity. Therefore, chemicals
`rendering anti-oxidative properties and inhibiting poly
`ADP-ribosylation can attenuate alloxan toxicity.
`It has been argued that glucose counteracts
`alloxan cytotoxicity in vitro and in vivo. This ability,
`however, is not only the result of the protection of
`glucokinase. The protective effect of glucose against
`necrotic death of B cells may be due to interaction of the
`sugar with the glucose transporter GLUT2 resulting in
`limited alloxan uptake (Jörns et al. 1997).
`
` It has been previously proposed that the action
`of glucose is also related to its metabolism and to the
`increased generation of reducing equivalents (NADH and
`NADPH) accelerating the recirculation of glutathione.
`GSH is known to provide protection against free radicals
`(Donnini et al. 1996). It may thus divert hydrogen
`peroxide from the pathway leading to the formation of
`hydroxyl radicals (Malaisse 1982, Malaisse-Lagae et al.
`1983, Pipeleers and van de Winkel 1986):
`GSSG + 2 NADPH → 2 GSH + 2 NADP+
`H2O2 + 2 GSH → GSSG + 2 H2O
`Moreover, Sakurai and Ogiso (1991) observed
`that the in vitro generation of hydroxyl radicals in the
`presence of alloxan
`strongly depends on GSH
`concentration. GSH in low concentrations potentiated the
`formation of
`these
`radicals, whereas
`the oxygen
`consumption, autoxidation of dialuric acid and formation
`of hydroxyl radicals were significantly inhibited in higher
`concentrations. GSH at high concentrations can also
`inhibit HA• generation and directly neutralize hydroxyl
`radicals. Thiyl radicals (GS•) formed in this reaction are
`then converted to GSSG:
`GSH + OH•− → GS• + H2O
`GS• + GS• → GSSG
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`Indeed, in rat islets incubated with alloxan the
`GSH content and GSH/GSSG ratio were decreased
`(Malaisse et al. 1982), whereas glucose evoked the
`opposite effect.
`In the in vivo experiment, glucose given to rats
`20 min prior to alloxan partially restricted alloxan-
`induced increase in the activity of glutathione peroxidase
`(EC 1.11.1.9) and mitigated the drop of liver nonprotein
`-SH groups (especially reduced glutathione) (Szkudelski
`et al. 1998). The protective action of this sugar is,
`however, strongly glucose and alloxan dose-dependent
`(Harman and Fischer 1982, Gorray et al. 1983).
`in
`It has been proposed
`that disturbances
`intracellular calcium homeostasis constitute an important
`step in the diabetogenic action of alloxan. This concept
`was confirmed by in vitro and in vivo experiments
`demonstrating that alloxan elevates cytosolic free Ca2+
`concentration in pancreatic B cells (Kim et al. 1994, Park
`et al. 1995). This effect arises from several events:
`alloxan-induced calcium influx from extracellular fluid,
`exaggerated calcium mobilization from
`intracellular
`stores and its limited elimination from the cytoplasm. The
`calcium influx may result from the ability of alloxan to
`depolarize pancreatic B cells (Dean and Matthews 1972).
`Depolarization of the cell membrane opens voltage-
`dependent calcium channels and enhances calcium entry
`into cells. Alloxan was also found to exert a stimulatory
`effect on mitochondrial Ca2+ efflux with simultaneous
`inhibitory action on Ca2+ uptake by mitochondria (Nelson
`and Boquist 1982, Lenzen et al. 1992). The restriction of
`calcium removal from the cells due to alloxan-induced
`inhibition of liver plasma membrane Ca2+-ATPase was
`also reported (Seckin et al. 1993). The effect of alloxan
`on intracellular calcium concentration seems to be
`mediated, at least partially, by H2O2 since hydrogen
`peroxide
`itself exerts a similar effect on calcium
`concentration in B cells (Park et al. 1995).
`Thus, the previously mentioned sudden rise in
`insulin release from B cells treated with alloxan (Weaver
`et al. 1978b, Kliber et al. 1996) may be one of the effects
`in cytosolic Ca2+
`of alloxan-induced augmentation
`concentration (Weaver et al. 1978b, Kim et al. 1994).
`The exaggerated concentration of this ion contributes to
`supraphysiological insulin release and, together with
`reactive oxygen species, causes damage of pancreatic
`B cells.
`
`The results of experiments with calcium channel
`antagonists have confirmed
`the
`important role of
`cytosolic calcium in the cytotoxic action of alloxan.
`
`the
`Pretreatment of rats with verapamil prevented
`alloxan-induced increase in B cell Ca2+ concentration and
`abolished the stimulatory effect of alloxan on insulin
`release (Kim et al. 1994). The calcium channel
`antagonists (verapamil and diltiazem) also suppressed
`hyperglycemia and the onset of alloxan diabetes in rats
`(Katsumata et al. 1992, Kim et al. 1994).
`Summing up, the toxic action of alloxan on
`pancreatic B cells, described many years ago by Dunn et
`al. (1943), are the sum of several processes such as
`oxidation of essential
`-SH groups,
`inhibition of
`glucokinase, generation of free radicals and disturbances
`in intracellular calcium homeostasis.
`Many investigators suggested that the selectivity
`of alloxan action is not quite satisfactory. Recent
`experiments confirmed this objection. The diabetogenic
`dose of alloxan was found to decrease -SH groups
`accompanied by a simultaneous rise in glutathione
`peroxidase activity in the rat liver two minutes after its
`administration (Szkudelski et al. 1998). At the same time,
`the blood insulin concentration rose dramatically. This
`exaggerated insulinemia did not evoke, however, any
`significant
`reduction of blood glucose suggesting
`impaired peripheral insulin sensitivity in the short time
`after alloxan treatment (Szkudelski et al. 1998). It was
`also observed
`that alloxan
`intensified basal and
`epinephrine-induced lipolysis in isolated rat adipocytes
`and insulin failed to restrict this effect (Kandulska et al.
`1999).
`
`Thus, using alloxan to evoke diabetes, animals
`should be examined after proper period of time to
`minimize side effects of alloxan action. It should also be
`emphasized that the range of the diabetogenic dose of
`alloxan is quite narrow and even light overdosing may be
`generally toxic causing the loss of many animals. This
`loss is most likely due to kidney tubular cell necrotic
`toxicity, in particular when too high doses of alloxan are
`administered (Lenzen et al. 1996).
`
`2. The mechanism of streptozotocin action
`
`2-deoxy-2-(3-(methyl-3-
`(STZ,
`Streptozotocin
`nitrosoureido)-D-glucopyranose)
`is
`synthesized by
`Streptomycetes achromogenes and is used to induce both
`insulin-dependent and non-insulin-dependent diabetes
`mellitus (IDDM and NIDDM, respectively).
`The range of the STZ dose is not as narrow as in the
`case of alloxan. The frequently used single intravenous
`dose in adult rats to induce IDDM is between 40 and 60
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`mg/kg b.w. (Ganda et al. 1976), but higher doses are also
`used. STZ
`is also efficacious after
`intraperitoneal
`administration of a similar or higher dose, but single dose
`below 40 mg/kg b.w. may be ineffective (Katsumata et
`al. 1992). For instance, when 50 mg/kg b.w. STZ are
`injected
`intravenously
`to
`fed
`rats, blood glucose
`(determined 2 weeks after treatment) can reach about 15
`mM (Szkudelski, unpublished observations).
`STZ may also be given in multiple low doses.
`Such treatment is used predominantly in the mouse and
`the induction of IDDM is mediated by the activation of
`immune mechanisms. However, Ziegler et al. (1984) and
`Wright and Lacy (1988) demonstrated that the non-
`specific activation of the immune system via complete
`Freund's adjuvant prior to STZ injections allows to
`reduce its diabetogenic dose even in the rat.
`NIDDM can easily be induced in rats by
`intravenous or intraperitoneal treatment with 100 mg/kg
`b.w. STZ on the day of birth. This method of NIDDM
`induction was described for the first time by Portha et al.
`(1974). At 8-10 weeks of age and thereafter, rats
`neonatally
`treated with STZ manifest mild basal
`hyperglycemia, an impaired response to the glucose
`tolerance test (Portha et al. 1979) and a loss of B cell
`sensitivity to glucose (Giroix et al. 1983).
`Streptozotocin action in B cells is accompanied
`by characteristic alterations in blood insulin and glucose
`concentrations. Two
`hours
`after
`injection,
`the
`hyperglycemia is observed with a concomitant drop in
`blood insulin. About six hours later, hypoglycemia occurs
`with high levels of blood insulin. Finally, hyperglycemia
`develops and blood insulin levels decrease (West et al.
`1996). These changes in blood glucose and insulin
`concentrations reflect abnormalities in B cell function.
`STZ impairs glucose oxidation (Bedoya et al. 1996) and
`decreases insulin biosynthesis and secretion (Bolaffi et al.
`1987, Nukatsuka et al. 1990b). It was observed that STZ
`at first abolished
`the B cell response
`to glucose.
`Temporary return of responsiveness then appears which
`is followed by its permanent loss and cells are damaged
`(West et al. 1996).
`STZ is taken up by pancreatic B cells via
`glucose transporter GLUT2. A reduced expression of
`GLUT2 has been found to prevent the diabetogenic
`action of STZ (Schnedl et al. 1994, Thulesen et al. 1997).
`Wang and Gleichmann (1995, 1998) observed that STZ
`itself restricts GLUT2 expression in vivo and in vitro
`when administered in multiple doses.
`Intracellular action of STZ results in changes of
`DNA in pancreatic B cells comprising its fragmentation
`
`(Yamamoto et al. 1981, Morgan et al. 1994). Recent
`experiments have proved that the main reason for the
`STZ-induced B cell death is alkylation of DNA (Delaney
`et al. 1995, Elsner et al. 2000). The alkylating activity of
`STZ is related to its nitrosourea moiety, especially at the
`O6 position of guanine. After STZ injection to rats,
`different methylated purines were found in tissues of
`these animals (Bennett and Pegg 1981).
`Since STZ is a nitric oxide (NO) donor and NO
`was found to bring about the destruction of pancreatic
`islet cells, it was proposed that this molecule contributes
`to STZ-induced DNA damage (Kröncke et al. 1995,
`Morgan et al. 1994). The participation of NO in the
`cytotoxic effect of STZ was confirmed in several
`experiments (Turk et al. 1993, Kröncke et al. 1995).
`Pancreatic B cells exposed to STZ manifested changes
`characteristic for NO action, i.e. increased activity of
`guanylyl cyclase and enhanced formation of cGMP (Turk
`et al. 1993). STZ is, however, not a spontaneous nitric
`oxide donor (Kröncke et al. 1995). This molecule is
`liberated when STZ is metabolized inside cells, but NO
`synthase is not required for this effect (Kröncke et al.
`1995). On
`the other hand,
`the
`lowering of NO
`concentration in pancreatic islet cells by inhibition of the
`inducible
`form of nitric oxide synthase partially
`counteracted DNA cleavage induced by STZ (Bedoya et
`al. 1996). A similar effect can be attained by NO
`scavengers (Kröncke et al. 1995). However, the results of
`several experiments provide the evidence that NO is not
`the only molecule responsible for the cytotoxic effect of
`STZ. STZ was found to generate reactive oxygen species,
`which also contribute to DNA fragmentation and evoke
`other deleterious changes in the cells (Takasu et al.
`1991b, Bedoya et al. 1996). The formation of superoxide
`anions results from both STZ action on mitochondria and
`increased activity of xanthine oxidase (EC 1.1.3.22). It
`was demonstrated that STZ inhibits the Krebs cycle (Turk
`et al. 1993) and substantially decreases oxygen
`consumption by mitochondria (Nukatsuka et al. 1990b).
`These effects
`strongly
`limit mitochondrial ATP
`production and cause depletion of this nucleotide in B
`cells (Nukatsuka et al. 1990b, Sofue et al. 1991).
`Restriction of mitochondrial ATP generation is partially
`mediated by NO. This molecule was found to bind to the
`iron-containing aconitase
`inhibiting enzyme activity
`(Welsh and Sandler 1994).
`Augmented ATP dephosphorylation increases
`the supply of substrate for xanthine oxidase (B cells
`possess high activity of this enzyme) and enhances the
`production of uric acid – the final product of ATP
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`degradation (Nukatsuka et al. 1990a). Then, xanthine
`oxidase catalyses reaction in which the superoxide anion
`is formed (Nukatsuka et al. 1988). As a result of
`superoxide anion generation hydrogen peroxide and
`hydroxyl radicals are formed (Nukatsuka et al. 1990a,
`Takasu et al. 1991b). The inhibition of xanthine oxidase
`by allopurinol restricts the cytotoxic effect of STZ in
`vitro. Pretreatment of B cells with this inhibitor prevented
`the STZ-induced decrease of insulin secretion (Nakatsuka
`et al. 1990a).
`It can be stated that potent alkylating properties
`of STZ are the main reason of its toxicity. However, the
`synergistic action of both NO and reactive oxygen
`species may also contribute to DNA fragmentation and
`other deleterious changes caused by STZ. NO and
`reactive oxygen species can act separately or form the
`highly toxic peroxynitrate (ONOO; Fig. 2). Therefore,
`intracellular antioxidants or NO scavengers substantially
`attenuate STZ toxicity.
`
`Fig. 2. The mechanism of streptozotocin (STZ)-induced
`toxic events
`in B cells of rat pancreas. MIT –
`mitochondria; XOD – xanthine oxidase
`
`References
`
`STZ-induced DNA damage activates poly ADP-
`ribosylation (Sandler and Swenne 1983). This process
`leads to depletion of cellular NAD+, further reduction of
`the ATP content (Heller et al. 1994) and subsequent
`inhibition of insulin synthesis and secretion (Nukatsuka
`et al. 1990b). The concept of unfavorable consequences
`of augmented poly ADP-ribosylation as a result of STZ
`action was confirmed by experiments revealing that the
`inhibition of this process prevents the toxicity of this
`diabetogenic agent. It was found that 3-aminobenzamide,
`a strong
`inhibitor of poly(ADP-ribose)
`synthase,
`protected against the action of STZ in rats, even when
`this substance was administered 45-60 min after STZ
`(Masiello et al. 1985, 1990). Another inhibitor of
`poly(ADP-ribose) synthase, nicotinamide, which is also
`scavenging oxygen free radicals, exerted best protection
`when it was administered shortly after STZ (Masiello et
`al. 1990). The failure of protective action of nicotinamide
`administered after STZ is probably due to a potent
`reduction of the cellular ATP content by STZ since
`nicotinamide uptake is ATP-dependent (Sofue et al.
`1991). The protective effect of 3-aminobenzamide and
`nicotinamide was also confirmed in vitro (Masiello et al.
`1990).
`
`It has been suggested that some inhibitors of
`poly ADP-ribosylation may also exert a protective effect
`due to their hydroxyl radical scavenging properties
`(LeDoux et al. 1988). However, in the case of STZ,
`recent investigations in poly(ADP-ribose) polymerase-
`deficient mice demonstrated that the inhibition of poly
`ADP-ribosylation itself prevents STZ-induced B cell
`damage and hyperglycemia (Pieper et al. 1999). Thus, it
`can be stated that the activation of poly ADP-ribosylation
`is of greater importance for the diabetogenicity of STZ
`than generation of free radicals and DNA damage per se.
`Calcium, which may also induce necrosis, does
`not seem to play a significant role in the necrosis evoked
`by STZ since calcium channel antagonists do not protect
`B cells against streptozotocin, as they do in the case of
`alloxan (Katsumata et al. 1992).
`
`BANSAL R, AHMAD N, KIDWAI JR: Alloxan-glucose interaction: effect of incorporation of 14C-leucine into
`pancreatic islets of rats. Acta Diabetol Lat 17: 135-143, 1980.
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`BEDOYA FJ, SOLANO F, LUCAS M: N-monomethyl-arginine and nicotinamide prevent streptozotocin-induced
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`BENNETT RA, PEGG AE: Alkylation of DNA in rat tissues following administration of streptozotocin. Cancer Res
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`BOLAFFI JL, NAGAMATSU S, HARRIS J, GRODSKY GM: Protection by thymidine, an inhibitor of polyadenosine
`diphosphate ribosylation, of streptozotocin inhibition of insulin secretion. Endocrinology 120: 2117-2122,
`1987.
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