Abstract

Using 1-deamino-[8-D-arginine]-vasopressin (dDAVP) as a reference substance because of its practically pure antidiuretic activity with very little smooth muscle agonism, substitutions for 4-glutamine were made in the following order: serine, glycine, alanine, α-aminobutyric acid, valine, isoleucine, and leucine. These substances were assayed for antidiuretic activity in trained, unanesthetized, water-loaded rats and for antagonism to the pressor action of arginine-vasopressin and angiotensin II amide in ganglion blocked, urethane-anesthetized rats. Some of the 4-substituted analogues were also tested in normal, consenting human volunteers in water diuresis. Circular dichroic spectra of the series were measured in 0.02 M phosphate buffer, pH 7.1, and in hexafluoroacetone. In the order given, antidiuretic activity decreased from the natural glutamine at position 4 through serine to practically nil at glycine, increased again to a peak at valine, and then decreased through isoleucine to leucine. Antagonistic action to arginine-vasopressin on vascular smooth muscle became evident first with 4-α-aminobutyric acid substitution and increased gradually through valine and isoleucine to leucine. There was no evidence of competition of any of these molecules with dDAVP or arginine-vasopressin at. the antidiuretic receptor, or with angiotensin II amide at the vascular smooth muscle receptor. This activity spectrum appeared to be related to both bulk and hydrophobicity of the side chain at position 4. Analysis of the circular dichroic spectra showed that while the basic conformation was not qualitatively altered by substitution at position 4 (with the possible exception of 1-deamino-[4-α-aminobutyric acid,8-D-arginine]-vasopressin) there were quantitative differences in the separate bands. There was a suggestive correlation between antidiuretic activity and the amplitude of the positive band at 225 nm.