Plasma levels of catechols during reflexive changes in sympathetic nerve activity

Relationships between changes in levels of catechols and directly recorded sympathetic nerve activity were examined using simultaneous measurements of renal sympathetic nerve activity and arterial and renal venous concentrations of norepinephrine (NE), dihydroxyphenylalanine (dopa), and dihyroxyphenylglycol (DHPG) during reflexive alterations in renal sympathetic nerve activity in anesthetized, adrenal-demedullated rats. Nitroprusside infusion increased renal sympathetic nerve activity by 90%, arterial levels of dopa by 96%, NE by 326%, and DHPG by 141%. Phenylephrine infusion increased arterial DHPG levels by 81% and decreased renal sympathetic nerve activity by 37% and NE levels by 26%; arterial dopa levels were unchanged. Ganglionic blockade by chlorisondamine (with concomitant phenylephrine infusion to maintain MAP) decreased renal sympathetic nerve activity by 65% and NE concentrations by 37%; arterial dopa concentrations were unchanged, and DHPG concentrations increased by 60%. Proportionate responses of arterial levels of NE were strongly related to proportionate changes in renal sympathetic nerve activity. Clearance of DHPG from arterial plasma was prolonged by phenylephrine-induced hypertension and by nitroprusside-induced hypotension. The results suggest that changes in arterial NE levels reflect changes in sympathetic activity; changes in dopa levels reflect changes in catecholamine biosynthesis; and changes in DHPG levels depend on reuptake of released NE and on hemodynamic factors affecting DHPG clearance.