Analyzing equilibrium water exchange between myocardial tissue compartments using dynamical two-dimensional correlation experiments combined with manganese-enhanced relaxography

Water compartments were identified and equilibrium water exchange was studied in excised rat myocardium enriched with intracellular manganese (Mn2+). Standard relaxographic measurements were supplemented with diffusion-T-2 and T-1-T-2 correlation measurements. In nonenriched myocardium, one T, component (800 ms) and three T-2 components (32, 120, and 350 ms) were identified. The correlation measurements re- vealed fast- and slow-diffusing water fractions with mean diffusion coefficients of 1.2 x 10(-5) and 3.0 x 10(-5) cm(2) s(-1). The two shortest T-2 components, which had different diffusivities, both originated from water in intracellular compartments. A component with longer relaxation time (T-1 approximate to 2200 ms; T-2 approximate to 1200 ms), originating from extra-tissue water, was also observed. The presence of this component may lead to erroneous estimations of water exchange rates from multiexponential relaxographic analyses of excised tissues. The tissue T, value is strongly reduced with increasing enrichment of Mn2+, and eventually a second tissue T, component emerges, indicating a shift in the equilibrium water exchange between intra- and extracellular compartments from the fast-exchange limit to the slow-exchange regime. Using a two-site water exchange analysis, the lifetime of intracellular water, T-1, was found to be 475 ms, with a fraction, p(ic), of 0.71.