IDENTIFICATION OF A SINGLE (FP) RECEPTOR-ASSOCIATED WITH PROSTANOID-INDUCED CA2+ SIGNALS IN SWISS 3T3 CELLS

Thus far, the prostanoid FP-receptor has been characterized only on the basis of agonist studies. It is currently classified as a receptor having particular sensitivity to prostaglandin F-2 alpha (PGF(2 alpha)) but with the ability to recognize prostaglandins D-2 and E(2) (PGD(2) and PGE(2)). We have re-examined this concept by studying second messenger Ca2+ signals to PGF(2 alpha), PGD(2) and PGE(2), and performing radioligand binding studies in Swiss 3T3 cells. The same rank order of potency was obtained for both the Ca2+ transient signal and competition for PGF(2 alpha) binding sites. The potency rank order, PGF(2 alpha) > PGD(2) > PGE(2), was identical to that obtained from functional studies in isolated tissues, such as the cat iris. Additional support for the concept that PGF(2 alpha) PGD(2), and PGE(2) interact with a single receptor to elicit a Ca2+ signal was provided by successive addition studies. Thus, cells pretreated with a supramaximal concentration of PGF(2 alpha) exhibited little or no response to subsequent administration of PGD(2) or PGE(2). Likewise, cells pretreated with a large concentration of PGD(2) or PGE(2) exhibited minimal responsiveness to successive addition of the corresponding alternative prostaglandins. Pretreatment with a maximally effective concentration of PGF(2 alpha) PGD(2), or PGE(2) rendered the cells refractory to the FP-receptor selective agonist fluprostenol, which further supports the hypothesis that Ca2+ transient signals in response to prostanoids in Swiss 3T3 cells are mediated by the FP-receptor. The Ca2+ transient responses to PGF(2 alpha), PGD(2), and PGE(2) also exhibited a similar modest reduction when extracellular Ca2+ was removed. Finally, the DP-receptor antagonist BW A868C did not black the Ca2+ transient response to PGD(2), indicating an absence of DP-receptor involvement. Moreover, Ca2+ responses to the thromboxane A(2) mimetic U-46619 were unaffected by the TP-antagonist BM 13505, which indicates no involvement of the TP-receptor. These results support the contention that the FP-receptor has particular sensitivity to PGF(2 alpha) but will also recognize PGD(2) and PGE(2).