CGMP modulation of Ca2+ sensitivity in airway smooth muscle

A beta-escin-permeabilized canine tracheal smooth muscle preparation was us ed to test the hypothesis that cGMP decreases Ca2+ sensitivity in airway sm ooth muscle primarily by inhibiting the membrane receptor-coupled mechanism s that regulate Ca2+ sensitivity and not by inhibiting Ca2+/calmodulin acti vation of the contractile proteins. 8-Bromo-cGMP (100 mu M) had no effect o n the free Ca2+ concentration-response curves generated in the absence of m uscarinic receptor stimulation. In the presence of 100 mu M ACh plus 10 mu M GTP, 8-bromo-cGMP (100 mu M) caused a rightward shift of the free Ca2+ co ncentration-response curve, significantly increasing the EC50 for free Ca2 from 0.35 +/- 0.03 to 0.75 +/- 0.06 mu M; this effect of 8-bromo-cGMP was concentration dependent from 1 to 100 mu M. 8-Bromo-cGMP (100 mu M) decreas ed the level of regulatory myosin light chain (rMLC) phosphorylation for a given cytosolic Ca2+ concentration but had no effect on the amount of isome tric force produced for a given level of rMLC phosphorylation. These findin gs suggest that cGMP decreases Ca2+ sensitivity in canine tracheal smooth m uscle primarily by inhibiting the membrane receptor-coupled mechanisms that modulate the relationship between cytosolic Ca2+ concentration and rMLC ph osphorylation.