A G-PROTEIN, NOT CYCLIC-AMP, MEDIATES EFFECTS OF VIP ON THE INWARDLY RECTIFYING K-CELLS( CHANNELS IN ENDOTHELIAL)

Because some endothelial cells contain a high density of functional va soactive intestinal peptide (VIP) receptors, it is possible that in so me cases, relaxation of blood vessels by VIP is mediated by endotheliu m. We showed earlier that VIP inhibited inwardly rectifying K+ current s (I-Kin) in cultured bovine pulmonary artery endothelial cells. Our s tudies now provide both direct and indirect evidence that activation o f these receptors does not occur through an elevation of cAMP level in these cells. Isoproterenol increased cAMP in endothelial cells from 3 0% to 35% over the basal levels. In contrast, VIP did not elevate cAMP in endothelial cells and even decreased it in some instances. In whol e-cell patch-clamp experiments, isoproterenol weakly inhibited the I-K in (about 80% less than VIP). The magnitudes of effects evoked by othe r activators of the cAMP cascade (forskolin, cAMP analogs) on this cur rent were intermediate between those of VIP and isoproterenol. Althoug h cAMP elevation can reduce the I-Kin current in endothelial cells, it is not responsible for the inhibitory effect of VIP on this current. We demonstrated that VIP receptors interact with the I-Kin channels th rough a G protein. Guanosine 5'-(3'-O-thiotriphosphate, a nonhydrolyza ble GTP analog, or cholera toxin inhibited these channels in a manner similar to inhibition by VIP. The activity of the I-Kin channels was p ertussis toxin-insensitive. Furthermore, guanosine-5'-O-(2-thiodiphosp hate) blocked the VIP receptor-mediated effect on the I-Kin. Our resul ts suggest that VIP receptors couple to I-Kin channels through a G pro tein.