Characterization of functional VIP/PACAP receptors in the human erythroleukemic HEL cell line

The presence of VIP/PACAP receptors was investigated on the human erythroleukemic cell line HEL. Specific binding of [125I]-PACAP or [125I]-VIP on HEL cells or membranes was very low and did not allow to perform competition curves. At 37 degrees C PACAP transiently increased cAMP levels in the presence of the non-specific phosphodiesterase inhibitor IBMX, suggesting rapid desensitization. Kinetic studies revealed that optimal conditions to measure the EC(50) of PACAP(1-27) were 10 min at 20 degrees C. Under those conditions, PACAP-related peptides increased cAMP levels with EC(50) in agreement with the pharmacological profile of the VPAC(1) receptor subtype: PACAP = VIP > [K(15), R(16,) L(27)]VIP(1-7)/GRF(8-27) = [R(16)]ChSn (two VPAC(1) agonists) >> helodermin = secretin. RO 25-1553, a selective activator of VPAC(2) receptor was inactive at 1 microM. Dose-response curves of VPAC(1) agonist molecules (PACAP, VIP, [K(15), R(16), L(27)]VIP(1-7)/GRF(8-27), [R(16)]ChSn) were shifted to the right by the VPAC(1) receptor antagonist [AcHis(1), D-Phe(2), Lys(15), Leu(17)]VIP(3-7)/GRF(8-27), with a K(i) of 3 +/- 1 nM (n = 3). The presence of VPAC(1) receptor mRNA was confirmed by RT-PCR. Preincubation with PACAP or PMA showed that VPAC(1) receptors underwent homologous and heterologous desensitization. This study provides the first evidence for the expression of functional VPAC(1) receptors undergoing rapid desensitization in HEL cells.