Received for publication September 5, 2003.
Revised October 28, 2003.
Accepted for publication November 4, 2003.

Insulin regulates neuronal M₂ muscarinic receptor function in the ileum of diabetic rats

Abstract

Acetylcholine release from cholinergic nerves in the gastrointestinal tract is limited by neuronal M₂ muscarinic receptors. In diabetic animals, M₂ muscarinic receptor function in the ileum is increased leading to decreased acetylcholine release and smooth muscle contraction in response to nerve stimulation. The mechanisms responsible for increased M₂ muscarinic receptor function are unknown but may contribute to the gastrointestinal dysmotility that occurs frequently in diabetics. In this study, we investigated whether insulin modulates M₂ muscarinic receptor function in the gastrointestinal tract of diabetic rats. M₂ muscarinic receptor function was tested by measuring the ability of an agonist, pilocarpine, to inhibit and an antagonist, methoctramine, to potentiate electrical field stimulation (EFS)-induced contraction of ileum in vitro. Insulin administration (0.2, 0.6, 2U s.c. daily for seven days) reversed the diabetes-induced increase in M₂ muscarinic receptor function and restored normal contractions to EFS. Insulin had no effect on the function of post-junctional M₃ muscarinic receptors, determined by measuring contractile responses to acetylcholine. These data suggest that insulin tonically inhibits neuronal M₂ muscarinic receptors. Thus, loss of insulin removes this inhibition and increases M₂ muscarinic receptor function leading to decreased acetylcholine release and contraction to EFS. In non-diabetic rats, there was a trend that higher insulin doses (0.6, 2U) increased M₂ muscarinic receptor function suggesting a bell-shaped concentration-response relationship for insulin. In conclusion, lack of insulin or excess insulin increases M₂ muscarinic receptor function in rat ileum. This mechanism may contribute to decreased acetylcholine release in the gastrointestinal tract of diabetics, resulting in dysmotility.

Key words: Acetylcholine, Diabetes, Dysmotility, Parasympathetic nerves, Small intestine, Smooth muscle