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.