Abstract
The monomolecular film of monooctadecyl phosphate (mOP) spread at an airโwater interface was employed to elucidate the nature of the interaction between peptide hormones and the cell membrane. First, the effect of peptide hormones on the uptake of calcium ion (Ca^++^) by the monolayer was examined from the emission of^45^ Ca radioactivity through the monolayer. Insulin inhibited the initial uptake of Ca^++^ and facilitated the release of adsorbed Ca^++^. These effects were partially or completely lost with insulin analogs. Vasopressin and oxytocin inhibited the adsorption of Ca^++^. Vasopressin facilitated Ca^++^ release, whereas oxytocin did not. Thyrocalcitonin profoundly inhibited Ca^++^ uptake, but facilitated only slightly the release of Ca^++^. Parathyroid hormone had negligible effect. The molecular basis and the biological significance of these results are discussed. Secondly, the effect of peptide hormones on the transport of water through the mOP monolayer was examined. Ca^++^ increased the specific resistance to evaporation of water. In the presence of Ca^++^, insulin and vasopressin lowered the resistance to evaporation (or increased the transโport of water). This effect of the hormones could not be accounted for entirely by the inhibition of Ca^++^ uptake.