Milking characteristics and their relation to adrenergic receptor mRNA expression and ligand binding in the mammary gland of dairy cows.

Stimulation of alpha- and beta-adrenergic receptors in the bovine mammary gland affects milking characteristics such as milk yield and peak flow rate. The aim of this study was to detect possible correlations between milkability, adrenergic receptor binding capacity and receptor expression at the mRNA level. In addition, dose-response relationships of alpha- and beta-adrenergic receptor stimulation were evaluated after application of an alpha- and beta-adrenergic receptor agonist, respectively in different dosages. Density and distribution of adrenergic receptor binding sites in the region around the large mammary ducts were investigated as well as adrenergic receptor mRNA expression. Milk flow of one-quarter was recorded in 10 cows without or with additional alpha- and beta-adrenergic receptor stimulation in three dosages each. After slaughter, mammary tissue was taken from the region around the large mammary ducts in the previously investigated quarters. Protein and RNA were extracted for measuring alpha(1)-, alpha(2)-, and beta(2)-adrenergic receptor binding sites and mRNA expression levels by real-time reverse-transcription polymerase chain reaction (RT-PCR). Peak flow rate without additional adrenergic receptor stimulation was negatively correlated with alpha(2)-adrenergic receptor binding (maximal binding capacity, B(max)) and positively correlated with alpha(2)-adrenergic receptor expression at the mRNA level (crossing point (CP) of the real-time PCR). During alpha-adrenergic receptor stimulation, there was a positive correlation between milkability and alpha(2)-adrenergic receptor mRNA expression, whereas during beta-adrenergic receptor stimulation no correlations were detected. Dose-response relationships were existing during alpha-adrenergic receptor stimulations, but not during beta-adrenergic receptor stimulations at four dosages each including control milking. Significant changes in milk yield and peak flow rate mainly occurred after application of an alpha-adrenergic receptor agonist. In conclusion, high mRNA expression levels or binding capacities of adrenergic receptors do not necessarily lead to according reactions in vivo, concerning milk yield and peak flow rate. To influence milking characteristics, individual reactions of the cow on adrenergic stimulation have to be considered.