FIGURE 2.

Biological activities of the alanine-replacement secretin analogues. Top, curves of intracellular cAMP responses in secretin receptor-bearing CHO-SecR cells stimulated by increasing concentrations of the alanine-replacement secretin analogues. Data points represent the means ± S.E.M. of three independent experiments performed in duplicate, normalized relative to the maximal response to secretin. Basal and maximal cAMP levels by secretin were 4.2 ± 1.1 and 197 ± 44 pmol/million cells, respectively. Data are presented in three groups, i.e. amino-terminal (top left panel, positions 1 to 10), mid-region (top middle panel, positions 11 to 19) and carboxyl-terminal (top right panel, positions 21 to 27) based on the positions of incorporating alanine in secretin, with the potencies in each group being illustrated in the order of high to low. Bottom, role of each secretin residue in its biological activity. Shown are the EC₅₀ values of each of the alanine-replacement analogues of secretin and the secretin sequence illustrating the role of each residue in their biological activity. Open circles represent residues whose replacement by alanine resulted in less than 10-fold in biological activity comparing to native secretin. Grey and black circles represent residues whose replacement with alanine resulted in more than 10-fold but less than 100-fold, and more than 100-fold increase in biological activity (dashed lines), respectively, comparing to native secretin. Sec, secretin.