Abstract

In estradiol-dominated rat myometrium, endothelin (ET)-1 caused contraction and increased the accumulation of [3H]inositol phosphates (EC50 = 70 nM), with the sequential generation of inositol trisphosphate, inositol bisphosphate, and inositol monophosphate. There was a coincident early decrease in phosphatidyl-inositol bisphosphate. The ET-1 stimulatory effect was pertussis toxin insensitive, suggesting an activation of phospholipase C via Gq/G11 proteins. ET-1 also inhibited the generation of cAMP induced by forskolin (EC50 = 30 nM). The inhibition was maintained in Ca(2+)-depleted medium and was prevented by pertussis toxin, suggesting G(i)-mediated inhibition of adenylyl cyclase. The rank order of potency for these various ET-1 effects [ET-1 > (Thr2)-sarafotoxin-b > ET-3], as well as the inhibitory effect displayed by BQ123, a specific ETA receptor antagonist, provided evidence for the involvement of the ETA receptor subtype. Exposure to ET-1 (15 min) resulted in concentration-dependent and homologous desensitization (40%) of the inositol phosphate response triggered by ET-1. There was virtually no recovery of ET-1-mediated inositol phosphate responses in the desensitized tissue even after 180 min of incubation. In contrast, the persistent low level of ET-1 activity that was observed in spite of several washings and in the absence of rechallenge with ET-1 was progressively revsersed and totally eliminated by BQ123. The ET-1 inhibitory effect on cAMP was also desensitized, as evidenced by the attenuation of the inhibitory effect of ET-1 after 15 min of ET-1 pretreatment. The data indicate that in rat myometrium the ETA receptor is coupled, via two distinct G proteins, to two main signal transduction cascades, which both undergo rapid desensitization.