In Xenopus oocytes expressing the rat mu receptor and the G protein-gated, inwardly rectifying K+ channel (known as KGA or GIRK1), application of [D-Ala2,MePhe4,Glyol5]enkephalin) a mu opioid agonist, evoked a dose-dependent increase in K+ conductance. With sustained agonist exposure, the amplitude of the response decayed with a t1/2 of 8 +/- 2 min. In oocytes coexpressing the mu and 5HT1A receptors with GIRK1, stimulation of either receptor resulted in heterologous desensitization of the subsequent response to the other. Injection of guanosine 5'-O-(thiotriphosphate) (1 mM) increased the basal GIRK1 activity and the total response to the application of agonist, but did not affect the rate of desensitization. Basal channel activity in the absence of agonist also desensitized at the same rate when the oocytes were exposed to high K+ (96 mM) solution. The above results indicate that the desensitization of the response occurred at a site downstream of the receptor, possibly at the channel. The rate of desensitization was not significantly altered by any of the following treatments: removal of external Ca2+, preloading the oocytes with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-tetra-(acetoxymethyl)-ester (0.5-1 mM), elevation of cAMP levels, treatment with phorbol esters (1 microM), staurosporine (0.5 microM), okadaic acid (1 microM), or cytochalasin B (0.5 microM). These results suggest that desensitization may not involve a calcium or phosphorylation-dependent mechanism.