Chlorpropamide upregulates antidiuretic hormone receptors and unmasks constitutive receptor signaling

The mechanism by which chlorpropamide (CP) treatment promotes antidiuresis is unknown. CP competitively inhibited antidiuretic hormone (ADH) binding and adenylyl cyclase (AC) stimulation (inhibition constants KiandK′iof 2.8 mM and 250 μM, respectively) in the LLC-PK1cell line. CP (333 μM) increased the apparent Kaof ADH for AC activation (0.31 vs. 0.08 nM) without affecting a maximal response, suggesting competitive antagonism. Because CP lowers “basal” AC activity and the AC activation-ADH receptor occupancy relationship (A-O plots), it is an ADH inverse agonist. Twenty-four-hour CP exposure (100 μM) upregulated the ADH receptors without affecting affinity. This lowered Kaand increased basal AC activity and maximal response (1.86 vs. 1.35 and 14.9 vs. 10.6 fmol cAMP ⋅ min−1⋅ 103cells−1, n= 6, P< 0.05). NaCl, which potentiates ADH stimulation, also increased basal AC activity. This, together with the CP-ADH inverse agonism and increased basal AC activity at higher receptor density, unmasks constitutive receptor signaling. The CP-ADH inverse agonism explains receptor upregulation and predicts the need for residual ADH with functional isoreceptors for CP-mediated antidiuresis. This could be why CP ameliorates partial central diabetes insipidus but not nephrogenic diabetes insipidus.