Possible deleterious effects of glucose on immature myocytes under hypothermic conditions

The purpose of the present study was to evaluate the functional and biochemical effects of glucose-based solutions in combination with potassium or insulin (or both) on immature myocytes under hypothermic conditions. Myocytes were isolated from neonatal rat ventricles and cultured for 4 days with MCDB 107 (University of Colorado solution). Initially, myocytes (12.5 x 10(5) myocytes/flask) were incubated at 4 degrees C for 6 hours in 5% glucose solution containing various potassium concentrations ranging from 0 to 80 mEq/L to evaluate the protective effects. Next, myocytes were incubated at 4 degrees C for 3, 6, 12, 18, and 24 hours in three types of solutions: normal saline solution (control), glucose-potassium solution, and glucose-insulin-potassium solution (glucose: 50 g/L; NaHCO3, 20 mEq; potassium, 20 mEq; insulin, 20 IU/L). After each incubation, creatine kinase and lactate dehydrogenase levels were measured in the incubation solutions. The myocytes then were cultured for an additional 24 hours at 37 degrees C to evaluate the recovery of myocyte beating rate. The 20-mEq potassium treatment showed significantly better beating rate recovery and lower enzymal release than the glucose-only control. The saline solution showed the best protection of all three solutions, both functionally and biochemically, by 12 hours. The greatest damage was observed with glucose-potassium solution, beginning at 3 hours of hypothermic incubation. Although potassium and insulin have additional protective effects on hypothermic preservation, the high concentration of glucose has noxious characteristics for immature myocytes that may not be suitable for cardiac preservation in the neonatal period.