Receptor-dependent G protein-mediated Ca2+ sensitization in canine airway smooth muscle

To determine the mechanisms of receptor-dependent Ca2+ sensitization in airway smooth muscle, canine tracheal smooth muscle (CTSM) was permeabilized with alpha-toxin or beta-escin. Although the effects of 5-hydroxytryptamine (100 mu M), histamine (100 mu M), and the thromboxane A(2) analogue U-46619 (100 mu M) were negligible, carbachol (100 mu M) and endothelin-1 (ET-1, 1 mu M) evoked additional contractions of 47.0 +/- 5.90% and 25.0 +/- 5.37% (n = 6) at pCa 6.7 with GTP (3 mu M) (normalized to the maximum contraction at pCa 4.5) in alpha-toxin-permeabilized CTSM. GDP-beta-S (1 mM) reversed the carbachol and ET-1 responses completely, GTP-gamma-S (30 mu M) and 4 beta-phorbol 12,13-dibutyrate (PDBu, 3 mu M) increased the Ca2+ sensitivity (median effective pCa) of contraction by 1.8- and 4.4-fold, respectively (n = 4-11, P < 0.05), The effects of saturating concentrations of GTP-gamma-S and PDBu were additive, A synthetic peptide (T-2) corresponding to the actin-binding site of calponin caused a dose-dependent contraction of beta-escin permeabilized CTSM, with the peak effect (25 +/- 4%, n = 4) at 1200 mu M. PDBu (3 mu M) caused contraction of the T-2 peptide-treated CTSM. In conclusion, Ca2+ sensitization of CTSM depends on receptor type and is mediated by G proteins and protein kinase C whose effects are additive, with a partial contribution by calponin.