Evidence for multiple rat VPAC(1) receptor states with different affinities for agonists

We compare the binding properties of [I-125-VIP] and [I-125]-Ro 25 1553 to VPAC(1) receptors, expressed in stably transfected CHO cells. [I-125]-VIP l abelled two VPAC1 receptor states, while [I-125]-Ro 25 1553 labelled select ively a limited number of high-affinity receptors. This high-affinity state probably corresponds to an agonist-receptor-G, ternary complex as its prop erties (guanyl nucleotides, EC50 values and maximal effect) were affected b y cholera toxin pre-treatment. Both high- and low-affinity receptors partic ipated in the adenylate cyclase activation. This suggested that agonists ac tivate not only low-affinity uncoupled receptors by facilitating the ternar y complex formation, but also activated the high-affinity ternary complex b y accelerating the GTP binding to emptied, receptor-bound G proteins. (C) 1 999 Elsevier Science Inc.