Influence of induced hypermagnesemia and hypocalcemia on neuromuscular blocking property of oxytetracycline in the horse.

The neurally evoked contractile response of the upper lip muscles of horses anesthetized with halothane was used to evaluate the neuromuscular blocking property of oxytetracycline which was administered intravenously at total dose rate of 21 to 28 mg/kg. This dose rate did not alter the contractile response which had a mean control value (and standard error; SE) of 1.76 plus or minus 0.22 kg. Arterial blood pressure was not affected by these dose rates. Neuromuscular blocking effect of the antibiotic occurred when it was administered during partial neuromuscular blockade which had been induced by infusion of magnesium sulfate (0.12 or 0.16 g/kg). Moderate hypocalcemia induced by infusion of sodium oxalate did not enhance the neuromuscular blocking property of this antibiotic. Since the dose rates of the oxytetracycline preparation used had a small effect on calcium concentration of equine serum (smaller than 0.3 mg/dl), calcium binding does not seem to play a major role in the neuromuscular blocking effect of this antibiotic. It was concluded that the weak neuromuscular blocking effect of oxytetracycline is not a significant etiologic factor in the acute type of adverse reaction to this antibiotic that is occasionally encountered in horses.