Tilmicosin is a potent antimicrobial with broad-spectrum activity against the bacterial agents involved in the
bovine respiratory disease complex. Recent studies indicate that in addition to being bactericidal,
tilmicosin is capable of modulating
inflammation in the lung. A series of experiments were designed to determine whether
tilmicosin alters alveolar macrophage-
prostaglandin E(2) (
PGE(2)) production induced by Escherichia coli (O55:B5)
lipopolysaccharide (LPS). Twenty-two healthy Holstein bull calves were used to study the effects of LPS-induced
PGE(2) production of alveolar macrophages after in vivo or in vitro treatment with
tilmicosin. In Experiment 1,
tilmicosin was given by
subcutaneous injection (15 mg/kg) twice, 48 hours apart, to four calves; four control calves received no treatment. Twenty-four hours after the second treatment, alveolar macrophages were stimulated with LPS in vitro. In Experiment 2, alveolar macrophages from five untreated calves were harvested and treated in vitro with
tilmicosin, followed by LPS stimulation. In Experiment 3, the ability of in vitro
tilmicosin treatment to alter the expression of LPS-induced
cyclooxygenase-2 (COX-2)
mRNA was evaluated. In Experiments 4 and 5, secretory
phospholipase A(2) activity was examined in untreated calves. Treatment of calves with
tilmicosin resulted in reduced LPS-induced alveolar macrophage
PGE(2) production. Similar reductions in
PGE(2) by LPS-stimulated alveolar macrophages after in vitro
tilmicosin treatment were noted. This in vitro
tilmicosin treatment was not associated with reduction of the expression of LPS-induced COX-2. Alveolar macrophage
phospholipase A(2) activity induced by LPS was significantly reduced by prior
tilmicosin treatment in vitro.
Tilmicosin (in vivo and in vitro) appears to reduce the
PGE(2)
eicosanoid response of LPS-stimulated alveolar macrophages by reducing the in vitro substrate availability without altering in vitro COX-2
mRNA expression.