Renal distribution and metabolism of [²H₉]choline. A ²H NMR and MRI study

Trimethylamines are required as substrates in the biosynthesis of a number of important molecules in the cell. Herein, we describe the use of choline, deuterated in its 9 methyl positions, as an NMR label for following the distribution and metabolism of methyl groups after intravenous choline infusion. Deuterium (²H) NMR spectroscopy of the rabbit kidney in vivo revealed a linear uptake of infused choline that was directly proportional to the rate of infusion. The sensitivity limit for the spectroscopic studies in vivo was in the order of 100 μM for a 2 min data collection. After the infusion, ²H NMR imaging of the kidney in vivo demonstrated high trimethylamine concentrations in both the cortex and inner medulla but not in the outer medulla. The inner medullary fraction, however, was more labile to diuresis induced by furosemide. Companion high resolution ²H NMR studies of extracts revealed a cortex betaine/choline concentration ratio of 0.69 ± 0.05 (mean ± SEM, n = 3) before furosemide administration. Following furosemide infusion, the cortex betaine/choline concentration ratio was 3 ± 1 (n = 6). Thus, ²H renal images following furosemide treatment can be interpreted as metabolic maps of betaine distribution. In addition, extraction studies revealed high concentrations of labelled choline and betaine in the liver. These data demonstrate that ²H‐labelled choline is an effective marker of choline methyl metabolism in vivo and should provide a unique tool for the investigation of this important substrate.