The Effect of Calcium-Naloxone Treatment on Blood Calcium, β-Endorphin, and Acetylcholine in Milk Fever

Milk fever is a postpartum syndrome of cows characterized by acute hypocalcemia, which reduces the release of acetylcholine (ACH), inducing flaccid paralysis and recumbency. Our aim was to evaluate the effect of calcium (Ca2+) combined with naloxone (Nx, an opioid antagonist; Ca2+-Nx) on plasma concentrations of ACH, ß-endorphin (ßE), and Ca2+ just before treatment (T0) and at 15, 30, and 90 min after treatment (T15, T30, and T90, respectively). Thirty cows were divided into 3 groups of 10 cows each. In group A1, cows affected by milk fever were treated (i.v.) with a combination of 0.2 mL/kg of body weight (BW) of Ca2+ borogluconate (20%) and 0.01 mg/kg of BW of Nx hydrochloride dihydrate. In group A2, cows affected by milk fever were treated (i.v.) with 2 mL/kg of BW of Ca2+ borogluconate (20%). In group C, healthy cows were treated (i.v.) with a combination of 0.2 mL/kg of BW of Ca2+ borogluconate (20%) and 0.01 mg/kg of BW of Nx hydrochloride dihydrate. Cows underwent treatments within 24 h of calving. Blood samples were collected at T0 and at T15, T30, and T90 for quantitative determination of ACH, ßE, and Ca2+. The cows in groups A1 and A2 recovered within a mean of 20 ± 10 min, although 4 cows in group A2 underwent a relapse. Blood Ca2+ concentrations in group C increased slightly at T30 and at T90 (T30: 8.8 ± 0.6 mg/dL; T90: 8.7 ± 0.6 mg/dL) after treatment, whereas the response in groups affected by milk fever was similar, even though Ca2+ concentrations showed a sharp increase (A1: 8.9 ± 0.8 mg/dL; A2: 6.0 ± 0.7 mg/dL), particularly at T15 in group A1. Concentrations of ßE showed a similar pattern in groups A1 and C, with an increase at T15 (A1: 8.2 ± 1.0 ng/mL; C: 2.7 ± 0.4 ng/mL) and a subsequent decrease until T90 (A1: 1.4 ± 0.3 ng/mL; C: 1.4 ± 0.4 ng/mL), whereas ßE remained constant throughout in group A2. Concentrations of ACH in group A1 decreased significantly between T0 and T15, T30, and T90 (T0: 7.2 ± 1.1 nmol/L; T15: 4.2 ± 1.2 nmol/L; T30: 2.9 ± 0.8 nmol/L; T90: 3.1 ± 0.3 nmol/L), whereas in group A2, it did not change. In group C, concentrations of ACH decreased at T15 and increased again at T30 (T15: 1.1 ± 0.3 nmol/L; T30: 3.2 ± 0.7 nmol/L). Our results suggest that administration of Ca2+-Nx, which restored the physiological Ca2+ concentrations, might have an effect on nicotinic receptors by restoring the normal neuromuscular transmission at the motor endplate.