Quantification of ion transport in perfused rat heart: ¹³³Cs⁺ as an NMR active K⁺ analog

Proper ion balance between intra- and extracellular compartments is necessary for normal physiological function. Conversely, alterations in membrane ion transport occur in numerous pathological states. As a noninvasive, nondestructive spectroscopic technique, nuclear magnetic resonance (NMR) offers a powerful approach to the study of ion balance in intact biological systems. Unfortunately, rare NMR active nuclides that are isotopes of the 100% naturally abundant ²³Na⁺ and ³⁹K⁺ are not available for tracer kinetic studies of Na⁺ and K⁺ transport. However, Cs⁺ is a biologically active analog of K⁺, and the 100% naturally abundant NMR active ¹³³Cs⁺ nuclide can be employed to examine K⁺ transport (Davis, D. G., E. Murphy, and R. E. London. Biochemistry 27:3547-3551, 1988). The distinguishing feature of ¹³³Cs⁺ is that it naturally gives two separate well-resolved NMR resonances for intra- and extracellular ¹³³Cs⁺, permitting study of the time course changes of either of these compartments independent of the other. In this report, the experimental procedures and compartmental modeling formalism are developed that allow quantitative analysis of Cs⁺ membrane transport in the perfused rat heart. Intracellular ¹³³Cs⁺ is shown to be 100% visible by solution-state NMR methods and its influx transport to be markedly inhibited by ouabain, a confirmation of findings previously reported by others. Intracellular ¹³³Cs⁺ spin- lattice and spin-spin relaxation times at 7 T were determined to be 2.1 ± 0.3 (SD) s (n = 8) and 0.065 ± 0.007 (SD) s (n = 8), respectively, for T₁ and T₂. The rate constant for Na⁺-K⁺-ATPase pump dominated intracellular influx was measured to be 0.25 ± 0.07 (SD) min^-1 (n = 27) and that for efflux 0.005 ± 0.001 (SD) min^-1 (n = 14). The rate constant for ¹³³Cs⁺ equilibration in the extracellular space at supraphysiological perfusate flow rate (20 ml/min) was found to be 4.6 ± 0.9 (SD) min^-1 (n = 20). Thus extracellular diffusion limitations do not dominate the ¹³³Cs⁺ transport measurements.