Alloxan-induced alteration of insulin release, rubidium efflux and glucose metabolism in rat islets stimulated by various secretagogues

Abstract

Summary.Insulin release and 86Rb efflux were studied in perifused rat islets exposed in vitro to alloxan (2 mmol/1) for 5 min. At a low glucose concentration, alloxan transiently increased 86Rb efflux. Alloxan immediately and completely abolished the secretory response to glucose (15 mmol/1) and markedly delayed the reduction in 86Rb efflux normally produced by the sugar. 3-0-methylglucose (20 mmol/ 1) provided complete protection against the alteration of 86Rb efflux and partial protection against the inhibition of insulin release. Immediately after alloxan treatment, glyceraldehyde, ct-ketoisocaproic acid and tolbutamide still induced a rapid release of insulin, but the late phase normally stimulated by glyceraldehyde and a-ketoisocaproic acid was inhibited. If islets were exposed to glyceraldehyde or tolbutamide 15 min after alloxan treatment, the rapid insulin release was also markedly impaired. Alloxan failed, however, to affect the ability of these three stimuli to reduce 86Rb efflux from islet cells. Glucose oxidation and utilization were decreased in alloxantreated islets and 3-0-methylglucose protected against this effect. The results show that the glucose recognition system in B-cells is the most rapidly and severely affected by alloxan. The drug also alters the response to other secretagogues, the insulin releasing properties of which can be impaired without alteration of their ability to reduce 86Rb efflux.Key words: Isolated rat islets, alloxan, perifusion, insulin release, rubidium efflux, glucose metabolism, glucose, glyceraldehyde, a-ketoisocaproic acid, tolbutamide.Alloxan has been widely used to induce experimental diabetes [1], but the exact mechanisms by which the drug produces a relatively selective destruction of pancreatic B-cells remain elusive. Elucidation of the molecular mechanisms of its action would certainly be of more than pharmacological interest. Indeed, observations that glucose can protect B-cells against alloxan toxicity [2, 3] make the drug a potential probe of the physiological stimulus-secretion coupling.Present experimental evidence, recently discussed in detail [4], suggests that the B-cell membrane may be an important site of the action of alloxan. In mouse islets alloxan produces depolarization of Bcells [5] and inhibition of the univalent cation pump [6]. In rat islets it produces a loss of membraneassociated particles [7]. In toadfish islets [8], but not in rat islets [9], it increases the membrane permeability to mannitol, which is normally restricted to the extracellular space.Recent studies have shown that glucose rapidly and markedly decreases potassium efflux from rat islet cells [10]. This supports the suggestion [11] that the depolarization of B-cells produced by the sugar [12-13] might be due to a reduction in K-permeability. The importance of these changes in K-permeability is underlined by the evidence that pharmacological agents which potentiate the effect of glucose on K-permeability also potentiate its insulin releasing effect, whereas those which augment...

A New Hypothesis for Alloxan Diabetes

“…3), indicating an inhibition of the release of insulin stored in granules in alloxan-treated mice. This agrees with the observation that alloxan inhibits glucose-induced insulin secretion ( Gunnarsson & Hellerstrom 1973, Henquin et al 1979. This inhibition is abolished by glucose in vitro (Tornita & Lacy 1972).…”

“…1975, Weaver et (11. 19781, or for an intracellular action, besides plasma membrane effects (Henquin et al 1979). Autoradiography has revealed that mouse islets accumulate alloxan Wammurstrom et al 1967).…”

Oxidative stress and beta-cell dysfunction

“…The diabetes-inducing substance alloxan [120,121] mainly acts via the liberation of H 2 O 2 [122,123]. Consequently, alloxan has very similar effects on islets as H 2 O 2 with respect to V m , K ATP current and [Ca 2+ ] c [74].…”

Alloxan: history and mechanism of action

“…/ /<'-... tent protect glucose-induced insulin secretion [66] and pancreatic B cell glucokinase (S.Lenzen, unpublished observation) against alloxan inhibition. Insulin secretion, in response to other fuel insulin secretagogues such as leucine and 2-ketoisocaproate and the hypoglycaemic sulfonylurea drugs, which are not recognised as insulin secretagogues by the pancreatic B cell through interaction with the glucokinase [41, 67, 681, is not immediately and completely inhibited by alloxan [69,70]. 8.…”