ATTENUATION OF AGONIST-INDUCED DESENSITIZATION OF THE RAT SUBSTANCE-PRECEPTOR BY MICROINJECTION OF INOSITOL PENTAKISPHOSPHATE AND HEXAKISPHOSPHATE IN XENOPUS-LAEVIS OOCYTES

Recently, inositol hexakisphosphate (phytic acid) was shown to bind to photoreceptor arrestin and block its interaction with rhodopsin. Such an interaction might predict that inositol polyphosphates could alter G protein-coupled receptor desensitization. To investigate the possib le roles of higher inositol polyphosphates on receptor desensitization , we have expressed the rat substance P receptor in Xenopus laevis ooc ytes. The functional expression of substance P receptor was monitored by voltage-clamp recording of substance P-induced Ca2+-dependent Cl- c urrents. When control oocytes were stimulated with substance P (30 nM) , after 10 min of washing the second responses to substance P were app roximately 15% of the first responses. Cytosolic injection of inositol pentakisphosphate (100 mu M) Or inositol hexakisphosphate (100 mu M) inhibited the reduction of the second substance P-induced current resp onses, maintaining the second responses to 57-58% of the initial respo nses. The protective effects of inositol pentakisphosphate and inosito l hexakisphosphate against agonist-induced desensitization were concen tration and time dependent and structurally specific, in that inositol hexasulfate and inositol tris- and tetrakisphosphate isomers were ina ctive. Microinjection of inositol hexakisphosphate did not (a) change the potency of substance P or the sensitivity of the expressed substan ce P receptor to substance P, (b) inhibit 12-O-tetradecanoylphorbol-13 -acetate-induced loss of substance P-induced current responses, or (c) alter the currents elicited by microinjection of inositol-1,4,5-trisp hosphate. These results suggest that inositol pentakisphosphate and in ositol hexakisphosphate have specific inhibitory effects on the agonis t-induced loss of responsiveness of the rat substance P receptor. More over, these protective effects of inositol hexakisphosphate against de sensitization were also observed with the endogenous lysophosphatidic acid/phosphatidic acid receptor, indicating that this mechanism is not specific to ectopic receptors. These results suggest that inositol pe ntakisphosphate and inositol hexakisphosphate may be novel pharmacolog ical tools for the Study of agonist-induced desensitization.