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 A as 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. (C) 2002 Elsevier Science (USA). All rights reserved.