Ca2+ sensitivity of skinned ventricular cardiac muscle and expression of cardiac myosin heavy chain isoforms in hibernating versus active frogs

Hibernation is a biological phenomenon enabling animals to endure hypothermia. This study was designed to find the changes in myosin heavy chain (MHC) isoforms and their relationship to calcium sensitivity in frog ventricular muscle during hibernation. MHC profile was determined by SDS-4% polyacrylamide gel electrophoresis in active and hibernating frogs for different intervals of time (10, 20, 30, and 60 days). Calcium sensitivity of Triton-skinned ventricular muscle fiber bundles was determined. Compared with active and hibernated frogs for 10 and 20 days, hibernated frogs for 30 and 60 days expressed significant amount (p < 0.05) of MHC type β (15. 3 ± 3.8%, 35.6 ± 1.5%, respectively). This change in myosin heavy chain profile seen in long period of hibernation was associated with significant decrease (p < 0.05) in sensitivity of skinned myofilaments to Ca2+ where skinned ventricular muscle taken from hibernating frogs for 30 and 60 days p Ca value were 5.52 and 5.50, respectively, while active frog showed p Ca = 5.67. In conclusion, expression of increased amount of β-MHC in hibernated frog for 30 days and more was associated with a decrease in contractile responses of skinned ventricular myocardium to calcium.