Cytoplasmic changes in cardiac cells during a contraction cycle detected by fluorescence polarization

Intracellular structural changes, occurring in a cardiac myocyte during a contraction cycle, were investigated by means of intracellular fluorescein fluorescence polarization (IFFP), in comparison to cytoplasmic concentration of Ca²⁺ [Ca²⁺]_i measured by indo-1. A simple physical model is presented. It assumes a biphase intracellular matrix, differing in its potency to restrict hosting fluorescent probe mobility. The first is a mobile nonrestricting phase, made mostly of aqua (aqua zone), while the second is a mobile-restricting phase, allocated mainly at the proximity of the filament sites. Their physicochemical properties such as [Ca²⁺], viscosity, and pH, may differ, thereby influencing the hosting probe fluorescence characteristics differently. These possible influences were examined experimentally. Based on experimental data, the model enables the evaluation, to first order of approximation, of the relative number of fluorescent probes populating the two phases and the time variation viscosity (η_r(t)) of the mobile-restricting filament zones taking place throughout the contraction cycle.