Binding affinities of the lactam-constrained [Y10]sec(5-27) analogues. Shown are curves reflecting the ability of increasing concentrations of [Y10]sec(1-27) (1), [Y10]sec(5-27) (2), or lactam-constrained [Y10]sec(5-27) analogues (3-13) to compete for binding of the secretin radioligand, [Y10]rat sec(1-27), to CHO-SecR cell membranes. Values illustrated represent percentages of saturable binding, expressed as the means ± S.E.M. of duplicate values from a minimum of three independent experiments.