Fig. 6. Glucose increases cAMP levels in INS-1 cells. INS-1 cells were incubated for 24 hours at either 2 mM or 20 mM glucose with or without 10 µM nifedipine or 20 µM forskolin, as indicated. Measurements of the intracellular cAMP levels showed that glucose increased cAMP levels independently of Ca²⁺ entry. Data are means ± s.e.m. of three independent experiments made in duplicate. ^**, P<0.01; ^***, P<0.001.