Partial replacement of K + with RB + in the rat brain in vivo does not change the tissue potassium dynamics after focal cerebral ischemia and is detectable by 87 Rb MRI

Introduction

An abrupt change in brain tissue potassium concentration, [K⁺]_br, has been suggested as an index of progression of ischemic damage ¹ . MRI has a potential to study Na⁺ homeostasis in brain ² . To monitor K⁺ by MRI, partial Rb/K replacement has been proposed ³ based on the properties of Rb⁺ as a congener for K⁺ in tissues and its much better MR-sensitivity. It is not known, however, to what extent such substitution would influence ion balance in ischemic brain.

Methods

Five normally-fed male Sprague-Dawley rats (240–300 g) were given 30–60 mM RbCl in the drinking water for 12–19 days. Focal cerebral ischemia was induced by MCA transection (MCAT) ⁴ , and was maintained during 2.5 (n=3) or 5 (n=2) hours. Samples of cortical brain of 1-mm diameter were punched from the ischemic core and contralateral areas ⁵ using the change in surface reflectivity of ischemic cortex as a guide ⁶ . Na, K and Rb content in the brain were determined by emission flame photometry at 589, 766 and 791 nm for Na, K and Rb, respectively, and corrected for the mutual interference effects.

Results

The total of [K]_br+[Rb]_br perfectly fits our previous data on the time course of [K⁺]_br after focal ischemia featuring a sharp drop at 3.5 hours in animals without Rb/K substitution ¹ . [K]_br and [Rb]_br were pooled together, because the degree of Rb/K replacement (i.e., the Rb/(K+Rb) ratio) varied between 0.12 and 0.20 for different animals. As expected, [Na⁺]_br concentration increased with time after ischemia onset. The Rb/(K+Rb) ratio increased over time in ischemic areas (normalized to the contralateral brain): 1.057±0.007 at 2.5 hours and 1.15±0.01 after 5 hours post MCAT (p<0.05). This suggests that the Rb⁺ efflux from the injured tissue is slower than the K⁺ efflux.

Conclusion

Rb⁺ loading does not significantly change the pattern of [K⁺]_br decrease in the ischemic brain over time. Therefore, the animals with partial Rb/K substitution are suitable for ⁸⁷Rb MRI studies of K⁺ homeostasis in ischemia (See Figure 1).

Figure 1

Concentrations of monovalent cations (average±SEM over all animals in the corresponding time groups) in the rat brain cortex after MCAT.