Regulation of functional muscarinic receptor expression in tracheal smooth muscle cells

Recent studies have shown that cultured tracheal smooth muscle cells (TSMCs) do not respond to muscarinic agonists with a significant increase in intracellular Ca2+ concentration. This may be due to a downregulation of muscarinic receptors (mAChRs) in TSMCs. We report here that the individual component of growth factors or hormones at the concentration used is not sufficient to stimulate growth of TSMCs in the primary culture with 1% fetal bovine serum (FBS). In the presence of 1% FBS, TSMCs withdraw from the cell cycle and express high levels of cell surface mAChRs. Furthermore, insulin-like growth factor I (IGF-I) and insulin (Ins), alone or in combination, could stimulate the expression of mAChRs on the cultured TSMCs in 1% FBS without changing the affinity of receptors. Heparin could inhibit these stimulatory effects on mAChR expression. The pharmacological response of functional mAChRs, determined as accumulation of inositol phosphates induced by carbachol, is greater in the medium containing IGF-I and Ins than those cultured in 1% FBS. This action may be partially mediated through a cholera toxin-sensitive protein. The results conclude that IGF-I and Ins are necessary for TSMCs to express functional mAChRs.