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The Histamine H₁ Receptor Is Not Involved in Local Control of Mammary Blood Flow in Dairy Cows

T. G. Madsen^*, D. R. Trout, S. R. L. Cieslar, N. G. Purdie^,, M. O. Nielsen^* and J. P. Cant^,1

^* Department of Basic Animal and Veterinary Sciences, The Faculty of Life Sciences, Copenhagen University, DK-1870 Frederiksberg C, Denmark
Department of Clinical Studies, and
Department of Animal and Poultry Science, University of Guelph, Ontario N1G 2W1, Canada
Schools of Animal Studies and Veterinary Science, University of Queensland, St. Lucia 4072, Brisbane, Australia

¹ Corresponding author: jcant{at}uoguelph.ca

Low concentrations of the essential amino acid histidine in circulation have been shown to increase mammary blood flow and it has been suggested that this effect is mediated by histamine. The hypotheses tested in this experiment were that interstitial histamine concentrations in the mammary gland are related to arterial His concentrations and that mammary blood flow is reduced by extracellular histamine via H₁ receptors. The hypotheses were tested by infusing saline or chlorpheniramine, a blocker of the H₁ histamine receptor, into the arterial supply of the mammary glands of lactating cows infused with 44 g/h of amino acid mixtures with or without His for 10 h. Infusates were administered in a 2 x 2 factorial arrangement within a 4 x 4 Latin square to 4 multiparous Holstein cows in mid lactation. Exclusion of His from the infusate decreased protein content in milk from the infused udder half from 3.98 to 3.77%, and increased arterial -aminonitrogen concentration from 3.2 to 3.4 mM. Neither the decreased arterial His concentration nor the H₁ blocker affected plasma flow to the infused udder half. We conclude that histamine is not involved in the regulation of mammary blood flow. The H₁ blocker decreased milk production in the infused udder half from 4.6 to 3.5 kg without affecting protein, fat, and lactose percentages, suggesting an inhibition of milk ejection. Cows on chlorpheniramine ate less feed during the infusion than saline-infused cows, which resulted in lower arterial concentrations and mammary uptakes of acetate. The efficiency of plasma triacylglycerol uptake across the mammary glands was decreased by chlorpheniramine but net uptake of long-chain fatty acids was not affected. The mechanism by which an amino acid deficiency influences mammary blood flow does not involve histamine signaling through the H₁ receptor and remains unidentified.

Key Words: histidine • histamine • mammary plasma flow • dairy cow