INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR SUBTYPES DIFFERENTIALLY RECOGNIZE REGIOISOMERS OF D-MYO-INOSITOL 1,4,5-TRISPHOSPHATE

The Ins(1,4,5)P-3 regioisomers, Ins(1,4,6)P-3 and Ins(1,3,6)P-3, which can mimic the 1,4,5-arrangement on the inositol ring of Ins(1,4,5)P-3 , were examined for Ca2+ release by using four types of saponin-permea bilized cell possessing various abundances of receptor subtypes, with special reference to the relation of potency to receptor subtype. Ins( 1,4,6)P-3 and Ins(1,3,6)P-3 were weak agonists in rat basophilic leuka emic cells (RBL cells), which possess predominantly subtype II recepto rs, with respective potencies of 1/200 and less than 1/500 that of Ins (1,4,5)P-3 [the EC50 values were 0.2, 45 and more than 100 mu M for In s(1,4,5)P-3, Ins(1,4,6)P-3 and Ins(1,3,6)P-3 respectively]. Similar ra nk order potencies were also evaluated for the displacement of [H-3]In s(1,4,5)P-3 bound to RBL cell membranes by these regioisomers. However , they caused Ca2+ release from GH(3) rat pituitary cells possessing p redominantly subtype I receptors more potently; Ins(1,4,6)P-3 and Ins( 1,3,6)P-3 evoked release at respective concentrations of only one-thir d and one-twentieth that of Ins(1,4,5)P-3 (the EC50 values were 0.4, 1 .2 and 8 mu M for Ins(1,4,5)P-3, Ins(1,4,6)P-3 and Ins(1,3,6)P-3 respe ctively). In COS-1 African green-monkey kidney cells, with the relativ e abundances of 37 % of the subtype II and of 62 % of the subtype III receptor, potencies of 1/40 and approx. 1/200 for Ins(1,4,6)P-3 and In s(1,3,6)P-3 respectively were exhibited relative to Ins(1,4,5)P-3 (the EC50 values were 0.4, 15 and approx. 80 mu M for Ins(1,4,5)P-3, Ins(1 ,4,6)P-3 and Ins(1,3,6)P-3 respectively). In HL-60 human leukaemic cel ls, in spite of the dominant presence of subtype I receptors (71%), si milar respective potencies to those seen with COS-1 cells were exhibit ed (the EC50 values were 0.3, 15 and approx. 100 mu M for Ins(1,4,5)P- 3, Ins(1,4,6)P-3 and Ins(1,3,6)P-3 respectively). These results indica te that these regioisomers are the first ligands that distinguish betw een receptor subtypes; the present observations are of significance fo r the future design of molecules with enhanced selectivity.