Role of G proteins in agonist-induced Ca2+ sensitization of tracheal smooth muscle

Increased sensitivity to intracellular Ca2+ concentration ([Ca2+]) is an important mechanism for agonist-induced contraction of airway smooth muscle, but the signal transduction pathways involved are uncertain. We studied Ca2+ sensitization with acetylcholine (ACh) and endothelin (ET)-1 in porcine tracheal smooth muscle by measuring contractions at a constant [Ca2+] in strips permeabilized with alpha-toxin or beta-escin. The peptide inhibitor G protein antagonist 2A (GP Ant-SA), which has selectivity for G(q) over G(i), inhibited contractile responses to ET-I,ACh, and guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), but the proportional inhibition of ACh responses was less than that of ET-1. Pretreatment with pertussis toxin reduced ACh contractions but had no effect on those of ET-1 or GTP gamma S. Clostridium botulinum C3 exoenzyme, which inactivates Rho family monomeric G proteins, caused similar reductions in contractile responses to ACh, ET-1, and GTP gamma S. Farnesyltransferase inhibition, which inhibits Ras G proteins, reduced responses to ET-1. We conclude that the heterotrimeric G proteins G(q) and G(i) both contribute to Ca2+ sensitization by ACh, whereas ET-1 responses involve G(q) but not G(i). Both G(q) and G(i) pathways likely involve Rho family small G proteins. A Ras-mediated pathway also contributes to Ca2+ sensitization by ET-1 in airway smooth muscle.