In vivo monitoring of norepinephrine and its metabolites in skeletal muscle

Although skeletal muscle sympathetic nerve activity plays an important role in the regulation of vascular tone and glucose metabolism, relatively little is known about regional norepinephrine (NE) kinetics in the skeletal muscle. With use of the dialysis technique, we implanted dialysis probes in the adductor muscle of anesthetized rabbits and examined whether dialysate NE and its metabolites were influenced by local administration of pharmacological agents through the dialysis probes. Dialysate dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG) were measured as two major metabolites of NE. The skeletal muscle dialysate NE, DHPG and MHPG were 11.7±1.2, 38.1±3.2, and 266.1±28.7pg/ml, respectively. Basal dialysate NE levels were suppressed by tetrodotoxin (Na⁺ channel blocker, 10μM) (5.1±0.6pg/ml), and augmented by desipramine (NE uptake blocker, 100μM) (25.8±3.2pg/ml). Basal dialysate DHPG levels were suppressed by pargyline (monoamine oxidase blocker, 1mM) (24.3±4.6pg/ml) and augmented by reserpine (vesicle NE transport blocker, 10μM) (75.8±2.7pg/ml). Basal dialysate MHPG levels were not affected by pargyline, reserpine, or desipramine. Addition of tyramine (sympathomimetic amine, 600μM), KCl (100mM), and ouabain (Na⁺-K⁺ ATPase blocker, 100μM) caused brisk increases in dialysate NE levels (200.9±14.2, 90.6±25.7, 285.3±46.8pg/ml, respectively). Furthermore, increases in basal dialysate NE levels were correlated with locally administered desipramine (10, 100μM). Thus, dialysate NE and its metabolite were affected by local administration of pharmacological agents that modified sympathetic nerve endings function in the skeletal muscle. Skeletal muscle microdialysis with local administration of a pharmacological agent provides information about NE release, uptake, vesicle uptake and degradation at skeletal muscle sympathetic nerve endings.