Abstract
1. The concentration of insulin in the blood of fifteen fasting baboons, anaesthetized with pentobarbitone sodium, was measured by a specific radio-immunoassay method.2. The mean resting concentration of insulin was: inferior vena cava (IVC) 22 muu./ml. and splenic vein (after splenectomy) 140 muu./ml.3. Subdiaphragmatic stimulation of the cut right vagal trunk (at 10 impulses/sec, duration 1.5 msec, for 10 min) produced a mean increase of 50% over resting levels in IVC insulin concentration and an increase in splenic vein insulin of 30% over resting levels.4. The mean resting level of blood glucose in splenic vein blood was 89.4mg%, that in IVC blood was 79.4mg%. The difference is significant (P < 0.005).5. Vagal stimulation did not alter these glucose levels, so that insulin release following vagal stimulation is not secondary to hyperglycaemia.6. The increased insulin secretion following vagal stimulation might be expected to produce hypoglycaemia. Its failure to do so is discussed.