Fluorescence polarization: A novel indicator of cardiomyocyte contraction

The changes measured in intracellular fluorescein fluorescence polarization (IFFP) are used as a new tool for tracing cytoplasmic effects during contractile cycles of cardiac myocytes (1-2-day-old rat hearts), in addition to the established Ca²⁺ monitoring and/or videometric methods of tracking cell-shortening. This novel method was found to be non-intrusive to the contraction cycles. The decay of the transient IFFP signal (from 0.220 ± 0.01 to 0.170 ± 0.013) seems to be closely related to the extended phase of contractile activation. This fact was further supported when Ca²⁺ exchanger inhibitor was introduced and significantly decreased (90%) the rate of beats of contraction and IFFP, but not the Ca²⁺ beat rate changes. This result suggests that the IFFP indicator is probably associated with the physiological activation, rather than with Ca²⁺ alterations. The IFFP measure monitors the average of effective changes in the micro-viscosity of the cytoplasm protein matrix, associated with cellular activation.