Orally administered prednisolone decreases plasma arginine vasopressin and serum copeptin concentrations in healthy dogs

Abstract Background The pathophysiology of polyuria and polydipsia secondary to exogenous glucocorticoid excess is incompletely understood. Objective Investigate plasma AVP (pAVP) and serum CoP (sCoP) concentrations in healthy dogs before, during, and after abrupt discontinuation of a long‐term course of orally administered prednisolone. Animals Eight healthy neutered young adult research Beagles. Methods In our prospective longitudinal study, Beagles were treated with a placebo PO q24h for 15 days (baseline), followed by a 35‐day course of prednisolone (2.35‐2.75 mg/kg PO q24h) and then abrupt discontinuation of prednisolone. Serial pAVP and sCoP concentrations, urine specific gravity (USG) and calculated plasma osmolality (pOsm calculated ) were determined during placebo and prednisolone administration, and up to 4 weeks after prednisolone discontinuation. Paired plasma samples for pAVP measurement were obtained in EDTA tubes with (pAVP P800 ) and without (pAVP EDTA ) a proprietary combination of protease, esterase, and dipeptidyl peptidase‐IV inhibitors (BD Biosciences P800). Results Mean pAVP P800 and sCoP concentrations were significantly lower at the end of the prednisolone course (25.8 ± 8.1 pg/mL and 166 pg/mL, range, 131‐223) vs baseline (34.1 ± 5.4 pg/mL and 243 pg/mL, range, 157‐336; P = .02, P = .02, respectively). Correlations between pAVP P800 and sCoP ( r = .77, P = .001) and pAVP P800 and USG ( r = .61, P = .02) were positive, despite no correlation between pAVP P800 and pOsm calculated , sCoP and pOsm calculated , and sCoP and USG. On paired samples, mean pAVP EDTA was significantly lower (5.0 ± 2.5 pg/mL) than mean pAVP P800 (34.1 ± 5.4 pg/mL; P < .0001). Conclusions and Clinical Importance Orally administered prednisolone led to markedly decreased plasma AVP and serum CoP concentrations despite increased calculated plasma osmolality and stable systolic blood pressure.