Monitoring of brain potassium with rubidium flame photometry and MRI

Abstract

An animal model was developed to monitor [K+] in the brain using partial K+ replacement with Rb+ and 87Rb MRI. Fifty-one rats were given 0-80 mM of RbCl in the drinking water for up to 90 days. Focal cerebral ischemia was produced in 15 of the animals. Na, K, and Rb content in precision-guided submilligram samples of cortical brain were determined by emission flame photometry. Multinuclear 87Rb/ 23Na/1H MRI was performed on phantoms and rats at 3T using a twisted projection imaging (TPI) scheme for 87Rb/23Na, and custom-built surface or parallel cosine transmit/receive coils. Brain [Rb+] was safely brought up to 17-25 mEq/kg within 2-3 weeks of feeding. The characteristic patterns of [K+] decrease (with a sharp drop at 3-4 hr of ischemia) and [Na+] increase (at a rate of 31%/hr) observed previously in animals without Rb/K substitution were reproduced in ischemic cortex. The Rb/(Rb+K) ratio increased over time in ischemic areas (R = 0.91, P < 0.001), suggesting an additional index of ischemia progression. Preliminary 87Rb MRI gave an estimate of 20-25 mEq Rb/kg brain weight (N = 2). In conclusion, brain Rb+ is detectable by 87Rb MRI and does not significantly interfere with ion dynamics in ischemic brain, which enables 87Rb MRI studies of K+ in ischemia. © 2007 Wiley-Liss, Inc.