Parasympathetic involvement in rapid meal-associated conditioned insulin secretion in the rat.

Blood glucose and plasma insulin concentrations were measured in blood sampled via a cardiac catheter in freely moving rats. To obtain a rapid conditioned cephalic phase of insulin secretion, rats were habituated to one of two feeding schedules. Clock-activated opening of doors in front of the food hopper imposed a feeding schedule of either six meals per day or two meals per day. When the doors were opened in both conditions, insulin increased rapidly during the first minute of feeding in the middle of the light phase. However, when presented an empty food hopper immediately after door opening, only rats in the two meal per day condition showed raised insulin levels and not rats in the six meal per day condition. This response was abolished following pharmacological blockade of nicotinic receptors with hexamethonium and muscarinic receptors with atropine. The present study shows that rapid conditioned insulin secretion can be evoked within one minute by a meal-associated stimulus. These results further indicate that this conditioned insulin secretion is vagally mediated and that its occurrence is dependent on the nature of the feeding schedule.