13C saturation transfer effect of carbon dioxide-bicarbonate exchange catalyzed by carbonic anhydrase in vivo.

Carbonic anhydrase catalyzes reversible hydration of carbon dioxide and dehydration of bicarbonate. In this article we report that the rapid exchange catalyzed by carbonic anhydrase causes a large magnetization (saturation) transfer effect on the 13C signal of bicarbonate at 160.7 ppm in vivo when the resonance of the undetectable carbon dioxide at 125.0 ppm is irradiated with RF pulses. In isoflurane-anesthetized adult rat brain the unidirectional, pseudo first-order rate constant of this exchange in the dehydration direction was determined to be 0.47 +/- 0.05 sec(-1) following intravenous infusion of uniformly 13C-labeled glucose for labeling bicarbonate. Intralateral ventricular administration of the highly specific carbonic anhydrase inhibitor acetazolamide, which is a drug used for treating glaucoma and epilepsy, was also shown to significantly attenuate the observed 13C magnetization transfer effect of the carbon dioxide-bicarbonate exchange in the rat brain.