- G protein function in human mononuclear leucocytes was measured through isoproterenol, carbamylcholine and dopamine-enhanced 3H-Gpp(NH)p binding. Dopamine and carbamylcholine exerted their effects through D5 and M2 receptors, respectively. ADP-ribosylation by bacterial toxins indicates that dopamine and isoproterenol affected Gs, while carbamylcholine affected Gi. Quantitative G proteins measures were conducted through immunoblot analyses with specific polyclonal antibodies against G alpha s, and G alpha i subunits. Simultaneous functional and quantitative measures of G proteins showed significant correlations between function and immunoreactivities. Agonist-enhanced guanine nucleotide exchange is thus suggested as a method for measurement of early events in signal transduction beyond receptors in leucocytes, which can potentially serve for detecting alterations in G proteins measures in human disease.