14. Mammary blood flow and nutrient uptake

Abstract

Sow milk is the major source of nutrients for suckling piglets and taking into account the large litter sizes of our current sow genotypes, it is imperative to maximize nutrient use by the mammary gland. The amount of nutrients available to mammary tissue is dependent upon the concentrations of nutrients in blood and the rate of its flow to the lactating glands. Nutrient availability to the udder may be estimated by measuring mammary arteriovenous differences, and mammary blood flow can be measured either directly or via indirect calculations. For all these measures, mammary venous and arterial blood samplings are required and catheters must be inserted in mammary veins. Blood flow to the udder is influenced by numerous factors, such as litter size, time since feeding, postural behaviour, vasoactive substances, and ambient temperature; yet, the single most important effector of mammary blood flow is milk removal. Glucose represents 40 to 60% of the total carbon mass taken up by sow mammary tissue and its uptake by the mammary gland appears to be mediated by glucose transporter proteins. Other energetic precursors which are used by the mammary gland are triglycerides, phospholipids, acetate, propionate and lactate. The uptake of amino acids by sow mammary glands has been extensively studied and is affected by diet, stage of lactation, and milk demand. Recent data also showed that amino acid uptake is controlled by transporter proteins that channel amino acids intracellularly, and that these proteins are in turn affected by diet and physiological status. Data on mammary uptake of hormones are contradictory; yet, the endocrine involvement in the regulation of milk synthetic processes is evidenced by the presence of specific hormonal receptors in mammary tissue.