Efficacy of Mitiglinide Combined with Dapagliflozin in Streptozotocin-nicotinamide-induced Type 2 Diabetic Rats and in Zucker Fatty Rats.

The efficacy of the combination of the rapid-acting insulin secretagogue mitiglinide and the sodium-glucose cotransporter 2 (SGLT2) inhibitor dapagliflozin was explored in streptozotocin-nicotinamide-induced type 2 diabetic (STZ-NA) rats and in Zucker fatty (ZF) rats. The STZ-NA rats were prepared at 8 weeks of age. At 9 weeks of age, the combination study was conducted by oral glucose tolerance test (OGTT). At 13 weeks of age, ZF rats were dosed orally with dapagliflozin once daily up to the 22(nd) day. At days 15 and 22, the combination study was conducted by OGTT. In 2 different animal models, plasma glucose levels were strongly suppressed by the combination of mitiglinide and dapagliflozin as compared with either drug alone. The urinary glucose excretion was drastically elevated in the dapagliflozin group, but the combination with mitiglinide suppressed it about 50%. In STZ-NA rats, the plasma insulin secretion by the combination of both drugs was about at the same level as in the mitiglinide group. In ZF rats, the plasma insulin secretion by the combination of both drugs was less than mitiglinide group. Thus, in 2 different animal models, the combination of mitiglinide and dapagliflozin showed stronger antihyperglycemic action accompanied by less insulin secretion than mitiglinide alone, and reduced the urinary glucose excretion as compared with dapagliflozin used alone. These results indicate that the combination of mitiglinide and dapagliflozin can be a promising combination for the treatment of diabetic patients.

Inhibitory effect of oxcarbazepine on high-frequency firing in peripheral nerve fibers.

We assessed the effects of oxcarbazepine, an antiepileptic derivative of carbamazepine, on discharges in single cutaneous afferent fibers produced by repetitive high-frequency stimulation (mimicking the abnormal excitation of peripheral nerves in neuropathic pain and paresthesia). After intravenous administration of oxcarbazepine, the later responses in the train dropped out without the earlier ones being affected and, thus, the total number of spikes decreased. The latency of the responses to an individual pulse was unchanged. These results, which indicate that oxcarbazepine inhibits the generation of high-frequency firing without affecting impulse conduction, suggest that this drug may be useful against neuropathic pain and paresthesia.

Suppressive effects of oxcarbazepine on tooth pulp-evoked potentials recorded at the trigeminal spinal tract nucleus in cats.

1. The purpose of the present study was to evaluate the antinociceptive effect of oxcarbazepine, a keto derivitive of carbamazepine (an anticonvulsant), in an animal model. To evoke a nociceptive response, we electrically stimulated the maxillary canine tooth pulp (MCTP) in anaesthetized (allobarbital-urethane), spontaneously breathing cats. 2. The evoked potentials were recorded from the superficial layers of the caudal part of the trigeminal spinal tract nucleus (5ST). We examined a slow component with a large amplitude (the P3 component) in evoked compound potentials; its mean conduction velocity was 1.7 m/s, suggesting a response mediated by C-fibres. 3. To confirm that the P3 component was related to pain sensation, we used morphine, a most efficacious antinociceptive agent, in the present study. The P3 component was significantly suppressed by intravenous administration of morphine (3 mg/kg) and was also suppressed by microinjection of morphine (2 microg) into the recording site of the 5ST. These results suggest that the P3 component is involved in the transmission of nociceptive information. 4. We compared the effect of oxcarbazepine with mexiletine; both are known to block neuronal Na+ channels. Intravenous administration of mexiletine suppressed the P3 component at a dose of 5 mg/kg. Microinjection of mexiletine (10 microg) into the recording site of the 5ST tended to suppress the P3 component, but this effect was not significant. 5. Intravenous administration of oxcarbazepine (1-10 mg/kg) caused a dose-dependent inhibition of the P3 component, which was significantly suppressed at 10 mg/kg oxcarbazepine. Intravenous administration of 10,11-dihydro-10-hydroxy-5H-dibenz[b,f]azepine-5-carboxamide (MHD), a metabolite of oxcarbazepine, at doses of 3-30 mg/kg caused a dose-dependent inhibition of the P3 component. Oxcarbazepine was not available for the microinjection study because it is not water soluble. We used MHD for the microinjection study instead of oxcarbazepine, because MHD can be dissolved in water up to 3 mg/mL. Microinjections of MHD (6 microg) into the recording site of the 5ST suppressed the P3 component. These results indicate that oxcarbazepine has an antinociceptive action.

Metabolism ofL-cysteine via transamination pathway (3-mercaptopyruvate pathway).

We have studied the transamination pathway (3-mercaptopyruvate pathway) ofL-cysteine metabolism in rats. Characterization of cysteine aminotransferase (EC 2.6.1.3) from liver indicated that the transamination, the first reaction of this pathway, was catalyzed by aspartate aminotransferase (EC 2.6.1.1). 3-Mercaptopyruvate, the product of the transamination, may be metabolized through two routes. The initial reactions of these routes are reduction and transsulfuration, and the final metabolites are 3-mercaptolactate-cysteine mixed disulfide [S-(2-hydroxy-2-carboxyethylthio)cysteine, HCETC] and inorganic sulfate, respectively. The study using anti-lactate dehydrogenase antiserum proved that the enzyme catalyzing the reduction of 3-mercaptopyruvate was lactate dehydrogenase (EC 1.1.1.27). Formation of HCETC was shown to depend on low 3-mercaptopyruvate sulfurtransferase (EC 2.8.1.2) activity. Results were discussed in relation to HCETC excretion in normal human subjects and patients with 3-mercaptolactate-cysteine disulfiduria. Incubation of liver mitochondria withL-cysteine, 2-oxoglutarate and glutathione resulted in the formation of sulfate and thiosulfate, indicating that thiosulfate was formed by transsulfuration of 3-mercaptopyruvate and finally metabolized to sulfate.

Metabolism of 3-mercaptopyruvate in rat tissues.

Metabolism of 3-mercaptopyruvate was investigated using homogenates of rat heart, liver and kidney. When 3-mercaptopyruvate was incubated with heart homogenate, L-cysteine, L-alanine, S-(2-hydroxy-2-carboxyethylthio)-L-cysteine and 3-mercaptolactate were produced. At the same time, a decrease in the amounts of L-glutamate and L-aspartate was demonstrated. These results indicate that 3-mercaptopyruvate was converted to L-cysteine by cysteine aminotransferase (EC 2.6.1.3), to 3-mercaptolactate by lactate dehydrogenase (EC 1.1.1.27), and to pyruvate by 3-mercaptopyruvate sulfurtransferase (EC 2.8.1.2), and that HCETC and L-alanine were formed from these products. In the presence of liver homogenate, 3-mercaptopyruvate was mainly metabolized by 3-mercaptopyruvate sulfurtransferase; production of L-cysteine was small and HCETC was not formed. The metabolism of 3-mercaptopyruvate in the presence of kidney homogenate was intermediate between heart and liver: a fair amount of L-cysteine was formed, but HCETC was not produced. A peak which corresponds to L-cysteine-glutathione disulfide on the chromatogram of amino acid analysis was present when 3-mercaptopyruvate was incubated with heart or liver homogenate, but not with kidney homogenate.

Biosynthesis of S-(2-hydroxy-2-carboxyethylthio)-L-cysteine (3-mercaptolactate-cysteine disulfide) by the rat heart.

Incubation of 3-mercaptopyruvate with rat heart homogenate resulted in the formation of S-(2-hydroxy-2-carboxy-ethylthio)-L-cysteine (HCETC, 3-mercaptolactate-cysteine disulfide), L-cysteine and 3-mercaptolactate with the concomitant decrease in glutamate and aspartate. These results indicate that a part of 3-mercaptopyruvate was converted to L-cysteine by transamination, a part was reduced to 3-mercaptolactate, and HCETC was formed from these two products. Another peak which corresponds to L-cysteine-glutathione disulfide on amino acid analysis was also produced during the incubation.