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Forearm norepinephrine spillover during standing, hyperinsulinemia, and hypoglycemia
D S Paramore1, C G Fanelli, S D Shah
1Division of Endocrinology, Diabetes, and Metabolism, General Clinical Research Center and the Diabetes Research and Training Center, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
The American Journal of Physiology
|November 14, 1998
Summary
Plasma norepinephrine (NE) is an unreliable indicator of sympathetic activity. Studies show prolonged standing and hyperinsulinemia stimulate sympathetic nerves, while hypoglycemia primarily activates the adrenal medulla.
Area of Science:
- Human physiology
- Autonomic nervous system function
- Endocrinology
Background:
- Plasma norepinephrine (NE) concentrations are a fallible index of sympathetic neural activity due to varied origins (nerves, adrenal medulla) and regional differences.
- Understanding the sympathochromaffin system's response to physiological challenges is crucial for interpreting autonomic function.
Purpose of the Study:
- To investigate the sympathochromaffin system's response to prolonged standing, hyperinsulinemic euglycemia, and hyperinsulinemic hypoglycemia in healthy humans.
- To differentiate between sympathetic neural and adrenomedullary contributions to circulating NE during these conditions.
Main Methods:
- Utilized isotope dilution measurements of systemic NE spillover rate (SNESO) and forearm NE spillover rate (FNESO).
- Assessed autonomic responses during prolonged standing, controlled hyperinsulinemic euglycemia, and hyperinsulinemic hypoglycemia.
- Measured plasma epinephrine levels to assess adrenomedullary activity.
Main Results:
- Prolonged standing caused blood pressure decrements without heart rate increments, with increased SNESO but not FNESO, suggesting non-forearm sympathetic nerve or adrenal medulla activation.
- Hyperinsulinemic euglycemia stimulated sympathetic neural activity (increased FNESO) but not adrenomedullary activity.
- Hyperinsulinemia with hypoglycemia significantly increased adrenomedullary activity (plasma epinephrine), but FNESO did not increase, indicating hypoglycemia per se does not directly stimulate sympathetic neural activity.
Conclusions:
- Circulating NE is a complex indicator; prolonged standing activates non-forearm sympathetic nerves or adrenal medulla.
- Hyperinsulinemia stimulates sympathetic neural outflow to the forearm.
- Hypoglycemia primarily stimulates adrenomedullary epinephrine release, with no direct evidence of concurrent sympathetic neural activation in the forearm.
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