2.1. Cell culture

The rat insulinoma-derived cell line RINm5F (RRID: CVCL_0501; ATCC CRL-11605, Manassas, VA, USA) was cultured according to Ref. [30]. The rat insulinoma-derived cell line INS-1E (RRID:CVCL_0351) [31] was cultured in RPMI 1640 medium (Roswell Park Memorial Institute) supplemented with 10 mM glucose, 10% (v/v) fetal calf serum, 10 mM HEPES, 1 mM pyruvate, 1 mM l-glutamine and 50 μM 2–mercaptoethanol. Rat pancreatic islets were isolated from 250 to 300 g adult male Lewis rats by collagenase digestion, separated by Ficoll gradient, handpicked under a stereo microscope and cultivated according to Ref. [9]. All culture media were supplemented with penicillin (100 U/ml) and streptomycin (0.1 mg/ml) and cells were cultured in a humidified atmosphere at 37 °C and 5% CO₂. Experiments were performed in cell culture medium or Krebs-Ringer solution (KR) as indicated composed of (in mM) 115 NaCl, 10 HEPES, 4.7 KCl, 2.5 CaCl₂, 1.2 KH₂PO₄, 1.2 MgSO₄, 20 NaHCO₃, 5 d-glucose and 0.1% albumin or in a physiological salt solution indicated composed of (in mM) 138 NaCl, 10 HEPES, 5 KCl, 1 MgCl₂, 2 CaCl₂ and 5 d-glucose, both adjusted to pH 7.4 with NaOH.