Characteristics of gastric and pancreatic responses to vagal stimulation with varied frequencies: evidence for different fiber calibers?

Abstract

In order to explore the possibility that different-sized vagal motor fibers innervate different abdominal targets as well as to identify efficient stimulation parameters for future experimentation, response characteristics to various stimulus frequencies were determined for 6 visceral responses caused by vagal nerve stimulations. Gastric acid secretion, plasma levels of insulin, glucagon and glucose, as well as heart rate and mean arterial blood pressure were monitored in anesthetized rats subjected to 1–16 Hz square wave electrical stimulation of either the left or right cut cervical vagus. The frequency-response curves evidenced distinctly different profiles for the gastric, pancreatic, and cardiovascular responses respectively: acid secretion was half-maximal at < 1 Hz, insulin and glucagon responses were half-maximal at ∼ 3 Hz, and cardiovascular responses were shifted still more to the right (half maximal frequency ∼ 15 Hz). These results suggest but do not prove that the gastric parietal cells may be innervated by small C-fiber caliber axons and the pancreatic islets by axons in the large C-fiber or small B-fiber range. Alternatively, these findings could reflect differences in neuro-effector couplings of the two organs. Furthermore, the present results provide an experimental technique that makes it feasible to select a frequency that will maximize one of the vagally mediated responses or minimize the activation of a second response.