- A single residue (arg46) located within the N-terminus of the V1a vasopressin receptor is critical for binding vasopressin but not peptide or nonpeptide antagonists. Hawtin, S.R., Wesley, V.J., Parslow, R.A., Simms, J., Miles, A., McEwan, K., Wheatley, M. Mol. Endocrinol. (2002)