Normalization of Cytoplasmic Calcium Response in Pancreatic β-Cells of Spontaneously Diabetic GK Rat by the Treatment with T-1095, a Specific Inhibitor of Renal Na⁺-Glucose Co-Transporters

Abstract

Chronic hyperglycemia is known to lead to a progressively further impaired insulin response and to hasten the development of complications in patients with type 2 diabetes, a notion referred as glucose toxicity. T-1095, a derivative of phlorizin, is a newly developed oral hypoglycemic agent that acts as a specific inhibitor of renal Na⁺-glucose co-transporters, reducing circulating blood glucose levels by promoting glucose excretion into urine. The effects of glycemic improvement by T-1095 on secretory function and cytoplasmic calcium response in pancreatic β-cells were investigated using spontaneously diabetic GK rats. After four weeks of treatment with T-1095 (age 4 to 8 week rats), serum glucose and HbA1c levels were significantly improved (serum glucose level, GK vs. GK T-1095, 277.3 ± 11.8 vs. 204.7 ± 6.4 mg/dl; HbA1c level, GK vs. GK T-1095, 6.2 ± 0.2 vs. 4.8 ± 0.1 %). Insulin secretion induced by 16.7 mM glucose was also significantly increased in the T-1095-treated group compared to the untreated group. The [Ca²⁺]i response induced by 16.7 mM glucose in GK β-cells was characterized by the loss of the steep first peak of [Ca²⁺]i elevation, and the lost first peak of [Ca²⁺]i reappeared in T-1095-treated β-cells in 32 of 34 observations. In T-1095-treated β-cells, the time lag to peak [Ca²⁺]i levels in the 16.7 mM glucose stimulation was significantly reduced (259.1 ± 15.3 sec, p < 0.01) compared to untreated GK rats (524.7 ± 52.9 sec). Thus, improvement of hyperglycemia by T-1095 ameliorates β-cell function by relieving [Ca²⁺]i response.

Key words

Type 2 Diabetes - Insulin Secretion - Fura-2 AM - Pancreatic Beta-Cell - Phlorizin

References

• 1 Pfeiffer M A, Halter J B, Porte D. Insulin secretion in diabetes mellitus.  Am J Med. 1981;  70 579-588
• 2 Ward W K, Beard J C, Halter J B, Pfeiffer M A, Porte D. Pathophysiology of insulin secretion in non-insulin-dependent diabetes mellitus.  Diabetes care. 1984;  7 491-502
• 3 Leahy J L. Natural history of beta cell dysfunction in NIDDM.  Diabetes Care. 1990;  13 992-1010
• 4 Tsuji K, Taminato T, Usami M, Ishida H, Kitano N, Fukumoto H, Koh G, Kurose T, Yamada Y, Yano H, Seino Y, Imura H. Characteristic features of insulin secretion in the streptozotocin-induced NIDDM rat model.  Metabolism. 1988;  37 1040-1044
• 5 Giroix M H, Portha B, Kergoat M, Bailbe D, Picon L. Glucose insensitivity and amino-acid hypersensitivity of insulin release in rats with non-insulin-dependent diabetes.  Diabetes. 1983;  32 445-451
• 6 Rossetti L, Giaccari A, DeFronzo R A. Glucose toxicity.  Diabetes Care. 1990;  13 610-630
• 7 Leahy J L, Cooper H E, Weir G C. Impaired insulin secretion associated with near normoglycemia: study in normal rats with 96-h in vivo glucose infusions.  Diabetes. 1987;  36 459-464
• 8 Davalli A M, Ricordi C, Socci C, Braghi S, Bertuzzi F, Fattor B, Carlo V D, Pontiroli A E, Pozza G. Abnormal sensitivity to glucose of human islets cultured in a high glucose medium: partial reversibility after an additional culture in a normal glucose medium.  J Clin Endocrinol Metab. 1991;  72 202-208
• 9 Grodsky G M. A new phase of insulin secretion: How will it contribute to our understanding of beta cell function?.  Diabetes. 1989;  38 673-678
• 10 Rossetti L, Shulman G I, Zawalich W, DeFronzo R A. Effect of chronic hyperglycemia on in vivo insulin secretion in partially pancreatectomized rats.  J Clin Invest. 1987;  80 1037-1044
• 11 Prentki M, Matschinsky F M. Ca²⁺, cAMP and phospholipid-derived messengers in coupling mechanisms of insulin secretion.  Physiol Rev. 1987;  67 1185-1248
• 12 Tsuji K, Taminato T, Ishida H, Okamoto Y, Tsuura Y, Kato S, Kurose T, Okada Y, Imura H, Seino Y. Selective impairment of the cytoplasmic Ca²⁺ response to glucose in rat pancreatic beta cells of streptozotocin-induced non-insulin-dependent diabetic rats.  Metabolism. 1993;  42 1424-1428
• 13 Kato S, Ishda H, Tuura T, Tsuji K, Nishimura M, Horie M, Taminato T, Ikehara S, Odaka H, Ikeda H, Okada Y, Seino Y. Alternations in basal and glucose-stimulated voltage-dependent Ca²⁺ channel activities in pancreatic? cells of non-insulin-dependent diabetes mellitus GK rats.  J Clin Invest. 1996;  97 2417-2425
• 14 Okamoto Y, Ishida H, Taminato T, Tsuji K, Kurose T, Tsuura Y, Kato S, Imura H, Seino Y. Role of cytosolic Ca²⁺ in impaired sensitivity to glucose of rat pancreatic islets exposed to high glucose in vitro.  Diabetes. 1992;  41 1555-1561
• 15 Tsujihara K, Hongu M, Saito K, Inamasu M, Arakawa K, Oku A, Matsumoto M. Na⁺-glucose cotransporter inhibitors as antidiabetics. I. Synthesis and pharmacological properties of 4^,-dehydroxyphlorizin derivatives based on a new concept.  Chem Pharm Bull. 1996;  44 1174-1180
• 16 Hongu M, Tanaka T, Funami N, Saito K, Arakawa K, Matsumoto M, Tsujihara K. Na⁺-glucose cotransporter inhibitors as antidiabetics. II. Synthesis and structure-activity relationships of 4’-dehydroxyphlorizin derivatives.  Chem Pharm Bull. 1998;  46 22-33
• 17 Fujimoto S, Ishida H, Kato S, Okamoto Y, Tsuji K, Mizuno N, Ueda S, Mukai E, Seino Y. The novel insulinotropic mechanism of pimobendan: direct enhancement of the exocytotic process of insulin secretory granules by increased Ca²⁺ sensitivity in beta cells.  Endocrinology. 1998;  139 1133-1140
• 18 Williams D A, Fogarty K E, Tsien R Y, Fay F S. Calcium gradients in single smooth muscle cells revealed by the digital imaging microscope using Fura-2.  Nature. 1985;  318 (6046) 558-561
• 19 Pipeleers D G, In’t-Veld P A, Van-de-Winkel M, Maes E, Schuit F C, Gepts W. A new in vitro model for the study of pancreatic A and B cells.  Endocrinology. 1985;  117 (3) 806-816
• 20 Pralong W F, Bartley C, Wollheim C B. Single islet beta-cell stimulation by nutrients: relationship between pyridine nucleotides, cytosolic Ca²⁺ and secretion.  EMBO-J. 1990;  9 (1) 53-60
• 21 Adachi T, Yasuda K, Okamoto Y, Shihara N, Oku A, Ueta K, Kitamura K, Saito A, Iwakura T, Yamada Y, Yano H, Seino Y, Tsuda K. T-1095, renal Na⁺-glucose transporter inhibitor, improves hyperglycemia in streptozotocin-induced diabetic rats.  Metabolism. 2000;  49 990-995
• 22 Oku A, Ueta K, Arakawa K, Ishihara T, Nawano M, Kuronuma Y, Matsumoto M, saito A, Tsujihira K, Anai M, Asano T, Kanai Y, Endou H. T-1095, an Inhibitor of Renal Na⁺-Glucose Cotransporters, May Provide a Novel Approach to Treating Diabetes.  Diabetes. 1999;  48 1794-1800
• 23 Portha B, Serradas P, Bailbe D, Suzuki K, Goto Y, Giroix M H. Beta cell insensitivity to glucose in the GK rat, a spontaneous nonobese model for type 2 diabetes.  Diabetes. 1991;  40 486-491
• 24 Okamoto Y, Ishida H, Tsuda K, Yasuda K, Kato S, Matsubara H, Nishimura M, Mizuno N, Ikeda H, Seino Y. Hyperresponse in calcium-induced insulin release from electrically permeabilized pancreatic islets of diabetic GK rats and its defective augumentation by glucose.  Diabetologia. 1995;  38 772-778
• 25 Ishida H, Kato S, Nishimura M, Mizuno N, Fujimoto S, Mukai E, Kajikawa M, Yamada Y, Odaka H, Ikeda H, Seino Y. Beneficial effect of long-term combined treatment with voglibose and pioglitazone on pancreatic islet function of genetically diabetic GK rats.  Horm Metab Res. 1998;  30 673-678
• 26 Ashcroft F M, Rorsman P. Electrophysiology of the pancreatic beta cell.  Prog Biophys Mol Biol. 1989;  54 87-143
• 27 Eto K, Suga S, Wakui M, Tsubamoto Y, Terauchi Y, Taka J, Aizawa S, Noda M, Kumura S, Kasai H, Kadowaki T. NADH shuttle system regulates KATP channel-dependent pathway and steps distal to cytosolic Ca2+ concentration elevation in glucose-induced insulin secretion.  J Biol Chem. 1999;  274 25 386-25 392
• 28 Tsuura Y, Isida H, Okamoto Y, Kato S, Horie M, Ikeda H, Seino Y. Reduced sensitivity of dihydroxyacetone on ATP-sensitive K⁺ channels of pancreatic beta cells in GK rats.  Diabetologia. 1994;  37 1082-1087
• 29 Ostenson C G, Abdel-Halim S M, Rsschaert J, Malaisse-Lagae F, Meuris S, Sener A, Efendic S, Malaisse W J. Deficient activity of FAD-linked glycerophosphate dehydrogenase in islets of GK rats.  Diabetologia. 1993;  36 722-726
• 30 MacDonald M J, Efendic S, Ostenson C G. Normalization by insulin treatment of low mitochondrial glycerol phosphate dehydrogenase and pyruvate carboxylase in pancreatic islets of the GK rat.  Diabetes. 1996;  45 886-890