Systemic ionophore modification of metabolism in cattle

Vita.In order to establish an understanding of systemic carboxylic ionophore-induced metabolism modifications in cattle, an experiment was designed with two ionophores, monensin and lysocellin infused i.v. in steers (n = 6, 200 [plus or minus] 10 kg) fed a differential level of intake of a 65% corn, 25% cottonseed hulls, and 10% molasses ration; resulting in animals in either a positive or negative energy balance. The experiment was performed using a cross over design. An amount of 7.5 mg monensin and 5 mg lysocellin (5% of daily dose), was dissolved in 1 ml absolute ethanol and infused i.v. at 3 h post feeding for 3 d. Plasma metabolites and hormones were measured from 30 min prior to infusion to 180 min post-infusion. Level of intake is reflected in different metabolite and hormonal profiles, with the exception of blood glucose. Animals at both intake levels maintained glucose homeostasis. Both ionophores elicit a hormonal response (insulin, glucagon and cortisol) with a different magnitude within each intake level, which is direct and short-lived (7.5 to 15 min). Hormonal changes accommodate a shift in the molar insulin/glucagon (l/G) ratio which in term allows for subsequent hormonal responses reacting to the primary insult. Changes concentrations of free fatty acids and to a lesser extent in plasma urea nitrogen are the result, allowing for increased glucose output. Monensin appeared to be active through a short-lived inhibition of insulin secretion, while lysocellin stimulated both insulin and glucagon secretion, both resulting in changing l/G ratios. Direct systemic effects of ionophores, providing the animal is in a general anabolic status, contribute to the enhanced energy status, possibly directly sparing protein, implying that direct systemic effects contribute to a large extent in the total ionophore induced response. Catabolic conditions as indicated by a negative energy balance, differently utilize the ionophore elicited hormonal response. In this case, regulation appears to be geared to alleviation of the prevailing nutritional condition. Different ionophores impact different cellular processes in apparent agreement with their specific ion affinity. Further investigation is needed in order to establish specific cellular mechanisms affected by the ionophores