Blood pH and Brain Uptake of 14C-Morphine

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Abstract
14C-Morphine was injected i.v. in control awake rats or in rats subjected to metabolic alkalosis or acidosis. Radioactivity was determined 10 min later within each of 7 brain regions, after correction was made for intravascular tracer. In each region, parenchymal radioactivity was correlated positively and significantly (P < 0.05) with arterial blood pH. Brain radioactivity was 2-fold to 3-fold greater in alkalotic rats (mean pH = 7.62) than in acidotic rats (mean pH = 7.16). The results are consistent with the pH-partition hypothesis for drug entry into the brain and indicate that morphine uptake can be increased by elevating the fraction of lipid-soluble uncharged morphine base in blood, by means of alkalosis. The observations may account for an exaggerated morphine-induced analgesia in alkalotic patients.