Optical Methods to Evaluate the Contractile Function of Unloaded Isolated Cardiac Myocytes

The enzymatically isolated cardiac myocyte has achieved prominence as an experimental model of myocardial excitation contraction coupling. Considerable effort has been directed to investigating the shortening behaviour of unattached and externally unloaded contracting cardiomyocytes – measured both at the level of the whole cell and at the level of the sarcomere. The purpose of this review is to provide a survey of these methodological approaches and to identify the crucial optical issues involved in striation imaging and whole-cell end-detection procedures. Factors limiting the precision with which measurements of cardiomyocyte shortening can be made, and sources of error attributable to unfavourable optical conditions and myocyte geometry are discussed. Original data are presented, examining the relationship between sarcomere and whole-cell shortening dynamics recorded simultaneously in the same cells. These results confirm that the measurement of cell length can provide an adequate index of sarcomere dynamics if the data are obtained under carefully controlled conditions where the entire cell is monitored to ensure minimal geometric or optical non-linearities. The role of the myocyteʹs internal load in modulating the observed contractile responses at differing levels of inotropic status is discussed. Finally, the validity of interpreting the shortening response of an untethered isolated cardiomyocyte as an index of myocardial contractility, reflecting the inotropic status, is considered.