Splanchnic factors enhance the norepinephrine response to oral glucose in aged man

Oral glucose has been shown to increase sympathetic nervous system (SNS) activity more in old than in young subjects. In contrast intravenous glucose during euglycemic hyperinsulinemia increases SNS activity in young but not in old subjects. To evaluate the role of splanchnic factors in this discrepancy, we employed a modification of the glucose clamp technique in 6 young (24-39 years) and 8 old (65-83 years) normal males. Each subject underwent two studies in which insulin was infused at 120 mU/m2 X min for 3 h and either oral glucose (50 gms) or water was given 60 min after initiating insulin. Euglycemia was maintained in all studies. When compared to control drink, oral glucose elevated norepinephrine in old (p less than 0.01), but not in young subjects. The difference between old and young was significant (p less than 0.02). When compared to control drink, oral glucose increased pulse rate and double product in the young, and pulse rate in the old. These results indicate that oral glucose activates the SNS in the elderly via splanchnic mechanisms independent of changes in circulating levels of glucose or insulin.