Potentiation effect of insulin on M3 muscarinic receptor agonist induced airway smooth muscle contraction

Abstract

Background: Obesity increases incidence and severity of asthma, but underlying mechanisms are unclear. We have shown hyperinsulinemia, which is common in obese individuals, potentiates parasympathetic nerve mediated bronchoconstriction. Parasympathetic nerves release acetylcholine, binding to M3 muscarinic receptors on airway smooth muscle to cause bronchoconstriction. Here we tested if insulin potentiates M3 receptor agonist-induced airway smooth muscle contraction. Methods: Obesity prone and resistant rats were fed a low-fat diet for 5 weeks. Some rats were treated with insulin 16 hours before physiological measurements. Methacholine(MCh)-induced tracheal smooth muscle contractions were measured before and after incubation with insulin. Rat tracheal smooth muscle cells were loaded with calcium indicator Fluo4 and MCh-induced changes in intracellular calcium were measured in the absence or presence of insulin. M2 and M3 receptor mRNA expression was quantified from tracheal smooth muscle cells treated with or without insulin. Results: Insulin pretreatment significantly increased vagally induced bronchoconstriction, regardless of whether rats were prone to be obese or not. Insulin significantly potentiated MCh-induced tracheal smooth muscle contraction and increase of intracellular calcium, but did not change M2 or M3 receptor mRNA expression in cultured smooth muscle cells. Conclusions: Insulin potentiates M3 agonist induced airway smooth muscle contraction via increasing intracellular calcium responses. These data may explain why obese individuals with hyperinsulinemia are more prone to airway hyperreactivity and give insights into future targets for asthma treatment.