Alterations in Serum Parathyroid Hormone and Electrolyte Concentrations and Urinary Excretion of Electrolytes in Horses with Induced Endotoxemia

Hypocalcemia and hypomagnesemia are common in horses with sepsis and endotoxemia. We hypothesize that endotoxemia triggers a systemic inflammatory response that results in hypocalcemia and hypomagnesemia. The goal of this study was to determine the effect of endotoxin (lipopolysaccharide [LPS]) administration to healthy horses on serum parathyroid hormone (PTH), ionized calcium (Ca2+) and total calcium (tCa), ionized magnesium (Mg2+) and total magnesium (tMg), phosphate (Pi), potassium (K+), sodium (Na+), chloride (Cl-), and insulin concentrations, and on the urinary excretion of these electrolytes. Twelve mares were infused with Escherichia coli LPS (30 ng/kg/h IV) for 1 hour. Six mares were infused with saline (controls). In LPS-infused horses, heart rate increased significantly from (mean ± SD) 40.0 ± 1.3 to 70.0 ± 9.0 beats/min, respiratory rate from 12.7 ± 1.0 to 21.1 ± 3.0 breaths/min, body temperature from 37.4 ± 0.3 to 38.9 ± 0.6°C, and tumor necrosis factor-α concentrations from 6.6 ± 3.5 to 507 ± 260 pg/mL (P < .05). White blood cell count decreased significantly from 7,570 ± 600 to 1,960 ± 560 cells/μL. Serum concentrations of Ca2+ decreased from 6.5 ± 0.3 to 6.0 ± 0.3 mg/dL, of Mg2+ from 0.53 ± 0.06 to 0.43 ± 0.04 mM, of tMg from 0.78 ± 0.05 to 0.62 ± 0.08 mM, of K+ from 4.3 ± 0.4 to 3.0 ± 0.5 mEq/L, and of Pi from 3.4 ± 0.5 to 1.7 ± 0.5 mg/dL (all P < .05). PTH increased significantly from 1.3 ± 0.4 to 6.0 ± 5.2 pM; however, in some horses (n = 2), PTH did not increase despite hypocalcemia. Insulin increased significantly from 9.4 ± 3.6 to 50.5 ± 9.6 μIU/mL (n = 3). Urinary fractional excretion of Ca2+ decreased significantly from 4.7 ± 1.4 to 1.7 ± 1.2%, of Mg2+ from 36.6 ± 6.5 to 11.7 ± 7.3%, and of K+ from 37.9 ± 11.3 to 17.7 ± 6.2%. Fractional excretion of Pi increased from 0.02 ± 0.02 to 0.14 ± 0.07% and of Na+ from 0.26 ± 0.13% to 1.2 ± 0.5%. No changes were found in serum tCa, Na+, and Cl- concentrations. In conclusion, endotoxemia in horses resulted in electrolyte abnormalities that included hypocalcemia, hypomagnesemia, hypokalemia, hypophosphatemia, and increased serum PTH and insulin concentrations.