Pancreatic polypeptide receptors: affinity, sodium sensitivity and stability of agonist binding

Cloned rat, human and guinea-pig Y4 pancreatic polypeptide (PP) receptors expressed in Chinese hamster ovary (CHO) cells, as well as the rabbit Y4-like PP receptor, show a selective sensitivity to Na+ over K+ ion in PP attachment, but little sensitivity to Na+ in dissociation of bound PP peptides. Agonist binding to Y4 receptors of intact CHO cells also shows much greater sensitivity to Na+ over K+, and a tenacious attachment of the bound agonist. Binding sensitivity to K+ is greatly enhanced upon receptor solubilization. Pancreatic polypeptide sites also show large sensitivity to modulators of Na+ transport such as N5-substituted amilorides and to RFamides, as different from Y1 or Y2 receptors. Thus, PP binding is modulated by cation-induced changes in site environment (with selectivity for Na+) and ultimately results in a blocking attachment. This would support receptor operation in the presence of ion gradients, as well as prolonged agonist-delimited signaling activity (which can include partial antagonism). Also, this could point to an evolutionary adaptation enabling small numbers of PP receptors to perform extensive metabolic tasks in response to low agonist signals.