Abstract
Objective—To determine the effect of histamine on the contractile elements of the respiratory tract in neonatal calves and young adult cattle.
Sample Population—Samples of trachealis muscle, bronchi, and intrapulmonary arteries and veins dissected from the respiratory tracts of healthy bovids (2 to 8 days and 16 to 20 months old).
Procedure—Histamine cumulative concentrationeffect curves (10–8 to 10–3M) were constructed in duplicate smooth muscle samples mounted in organ baths. Contractile responses to histamine were compared with reference contractions elicited by methacholine (10–5M) for airways or KCl (127mM) for vessels.
Results—In young adult cattle, trachealis muscle had a substantial contractile response to histamine (84% of methacholine-induced contraction), whereas bronchi reacted slightly (15 and 20% for large and small bronchi, respectively). Although contractile responses to KCl were comparable in arteries and veins, histamine-induced contractions were greater for intrapulmonary veins than for arteries (202 vs 48% of KCl-induced contraction). In neonatal calves, histamine- induced contraction of veins also exceeded that of arteries (230 vs 54% of KCl-induced contraction); however, unlike in young adult cattle, histamine produced notable contraction of large and small bronchi (48 and 60% of methacholine-induced contraction, respectively).
Conclusions and Clinical Relevance—Compared with intrapulmonary arteries, intrapulmonary veins have greater contractile responses to histamine in neonatal and young adult cattle. Data suggest loss of histamine responsiveness in bronchial smooth muscle as neonatal calves grow to young adults. Venodilation may be useful in treatment of lung edema in cattle. (Am J Vet Res 2003;64:819–822)
