Glucose increases rat plasma norepinephrine levels by direct action on the brain

Abstract

The hypothesis that glucose can selectively activate the sympathetic nervous system (SNS) by direct action on the brain was tested using plasma norepinephrine (NE) and epinephrine (Epi) responses to intracarotid and intravenous glucose injections as indexes of SNS and adrenal medullary responses, respectively. Intracarotid glucose bolus injections (0.1 g/kg) transiently raised plasma glucose (22%) and insulin (98%) levels at 2 min and increased plasma NE, but not Epi, levels from 2 to 60 min. Areas under the NE curve were 700% higher than equiosmolar doses of mannitol. An intravenous glucose bolus (1 g/kg) gave quantitatively similar but delayed (30 min) NE responses to the 0.1 g/kg intracarotid dose but raised plasma glucose 500% and insulin 1,700% above baseline at 2 min postinjection. Slow intracarotid glucose infusions for 60 min at 4 mg.kg-1.min-1 raised plasma NE levels from 30 to 60 min with 250% higher areas under the NE curve than the intracarotid and intravenous bolus doses but without a change in plasma glucose, insulin, or Epi levels. Infusions at 6 mg.kg-1.min-1 transiently raised plasma NE levels at 30 min without altering glucose, insulin, or Epi levels. These results suggest that glucose alone can produce a selective, delayed SNS activation by a direct action on the brain.