Glucose d2

Plasma samples (50 μL) were mixed with 50 μL internal standard (100 μM myo-inositol-d₆, 12.5 mM glucose-d₂; Sigma) and 1 mL methanol, and centrifuged for 5 minutes, 10,000 g at room temperature. Supernatants were collected, dried by lyophilization, and stored at −70°C prior to analysis. Samples were derivatized by addition of 100 μL of 1 mg/ml 4-(dimethylamino)pyridine (Fluka) and 100 μL acetic anhydride, and incubation at 80°C for 30 min. A series of internal standards (0–200 μM MI and 0–25 mM glucose) were prepared and derivatized as above after addition of internal standard solution.

Solvents for extraction
were HLPC-grade methanol (MeOH) (Fisher Scientific), Milli-Q water
(H₂O) from Milli-Q Plus, and formic acid (Merck). For quantification
the internal standards oleic acid-d₉ (FA 18:1-d₉), LPC 19:0, PC 11:0/11:0, GABA-d₂, Acetylcholine-d₉, Glucose-d₂, and Cell Free Amino Acid Mixture-¹⁵N (Sigma-Aldrich) were prepared in MeOH:H₂O (9:1)
with 0.1% formic acid (Tables S1 and S2). INS-1 cells were exposed to a solution contained 138 mM NaCl,
5.6 mM KCl, 1.2 mM MgCl₂, 2.6 mM CaCl₂, and
1, 3, or 20 mM d-glucose at pH 7.4 and room temperature.