Morphine and muscle relaxants are receptor-independent G-protein activators and cromolyn is an inhibitor of stimulated G-protein activity

Morphine and muscle relaxants are classical mast cell activators and cromolyn is a mast cell inhibitor. However, the mechanisms underlying the effects of these drugs are obscure. We asked the question whether morphine and muscle relaxants may activate heterotrimeric guanine nucleotide-binding proteins (G-proteins), and whether cromolyn may prevent this activation. Morphine activated G(i)-proteins in HL-60 membranes and purified transducin (TD) at concentrations above 1 mM, but the effects on morphine did not reach saturation up to 10 mM. d-Tubocurarine activated G(i)-proteins and TD in a saturable manner, with EC(50) values of 0.3 mM and 4.2 mM, respectively. Gallamine and succinylcholine were less effective activators of TD than d-tubocurarine. Morphine and d-tubocurarine were about similarly effective activators of G(i)-proteins, whereas d-tubocurarine was a more effective activator of TD than morphine. Cromolyn at 10 mu M and 100 mu M had little effect on TD activity but reduced the stimulatory effect of morphine by 50% and 80%, respectively. Our data suggest the following: (1) Receptor-independent G-protein activation by morphine and muscle relaxants presumably accounts for their mast cell-activating properties. (2) Cromolyn may act by preventing G-protein activation. (3) The variability in responsiveness of mast cells towards morphine and muscle relaxants could be due to differential expression of G-proteins with different sensitivity to activation by these drugs.