Binding and biological activities of lactam-constrained full length [Y10]sec(1-27) analogues. Left, curves reflecting the ability of increasing concentrations of [Y10]sec(1-27) (1) or c[E16,K20][Y10]sec(1-27) (14) or c[E16,K20]sec(1-27) (15) 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. Right, intracellular cAMP responses in CHO-SecR cells by [Y10]sec(1-27) (1) or c[E16,K20][Y10]sec(1-27) (14) or c[E16,K20]sec(1-27) (15). Data points represent the means ± S.E.M. of three independent experiments performed in duplicate, normalized relative to the maximal responses in these cells to [Y10]sec(1-27) (1).