The stimulatory effect of vasoactive intestinal peptide (VIP) on the intracellular calcium concentration ([Ca2+]i) has been investigated in Chinese hamster ovary cells stably transfected with the reporter gene aequorin, and expressing human VPAC1, VPAC2, chimaeric VPAC1/VPAC2 or mutated receptors. The VIP-induced increase in [Ca2+]i was linearly correlated with receptor density, and was higher in cells expressing VPAC1 receptors than in cells expressing a similar density of VPAC2 receptors. The study was performed to establish the receptor sequence responsible for this difference. VPAC1/VPAC2 chimaeric receptors were first used for broad positioning: those receptors having the third intracellular loop (IC3) of the VPAC1 or the VPAC2 receptor behaved, in this respect, phenotypically like VPAC1 and VPAC2 receptors respectively. Replacement in the VPAC2 receptor of the sequence comprising residues 315–318 (VGGN) within IC3 by its VPAC1 receptor counterpart (residues 328–331; IRKS) and the introduction of VGGN instead of IRKS into VPAC1 was sufficient to mimic VPAC1 and VPAC2 receptor characteristics respectively. Thus a small sequence in the IC3 domain of the VPAC1 receptor is responsible for the efficient agonist-stimulated increase in [Ca2+]i.

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