Mammary Blood Flow and Regulation of Substrate Supply for Milk Synthesis

Abstract

In ruminants, mammary supply of substrate varies with rate of mammary blood flow and concentrations of blood substrates. Blood concentrations of most mammary substrates, except acetate and tryptophan, do not vary greatly with feed intake, short term. Fasting reduces mammary blood flow, whereas milking and injection of growth hormone or thyroxine increase flow. It is proposed that the fraction of cardiac output that perfuses the udder of lactating ruminants plays a role in regulation of nutrient partitioning between milk and body tissues. In fed animals this fraction is 15 to 16% of cardiac output, which declines on fasting to 8 to 9% and increases slightly following growth hormone treatment to 17.6%. Following realimentation of fasted cows or goats, mammary blood flow takes several hours to return to normal. Investigation of the mechanism of this response, in terms of the ability of the animal to recognize its nutritional status and partition nutrients accordingly, should prove fruitful to understanding causes of variations of milk production in response to feed quantity and quality. Several substrates show increased mammary arteriovenous difference with increasing blood concentrations. This may reflect differing ratios of blood flow:milk yield. The steep gradient of concentration of substrates across the mammary epithelial cell membrane suggests that a major impediment to substrate supply for milk synthesis is the rate of substrate transport across the membrane.

Changes in mammary uptake and metabolic fate of glucose with once-daily milking and feed restriction in dairy cows

“…The quantity of blood irrigating the udder results from complex regulatory mechanisms, depending both on the partitioning of cardiac flow between different tissues in the body and local regulation which allows a given organ to adjust its arterial nutrient flow to its level of metabolic activity. In lactating cows, 15-16% of the cardiac output is delivered to the udder and whole mammary blood flow averaged 7 L.min −1 in cows producing 16 kg of milk daily [13]. The udder's efficiency to extract glucose from blood plasma is relatively poor; the glucose extraction rate (arteriovenous difference in glucose concentrations divided by the arterial glucose concentration) is on average 26% [14,15] and can fall to 15% in dairy cows [16].…”

“…This harmful effect on milk yield was principally due to a lower nutrient flow into the udder, related to a 40 to 70% reduction in the mammary blood flow [41][42][43][44]. In the cow, the mammary blood flow was shown to be halved after fasting for 24 h, and only returned to normal 10 to 12 h after feeding restarted [13]. Similarly, Lough et al [45] observed a 5.1 to 4.3 L.min −1 reduction in the mammary blood flow during a 70% restriction of feed.…”

“…A lowering of the heart rate was observed in heifers not receiving concentrate [46]. In the cow, cardiac output and that proportion of the cardiac output allocated to the mammary gland fell by 23 and 53%, respectively, in response to fasting for 24 h [13]. Similarly, they fell by 36 and 52%, respectively, in goats fasted for 48 h [42].…”

A serial evaluation of electrocardiographic indices and cardiac arrhythmias during pregnancy, lactation and dry periods in Saanen goats

“…It has been reported that cardiac output and total volume of systemic blood flow increased significantly during lactation and late pregnancy compared to dry or early pregnancy periods and these increases may well be associated with increasing milk yields and foetal development, respectively (Davis & Collier, 1985;Olsson et al, 2001;Pourjafar et al, 2011;Zarifi et al, 2012). Furthermore, lactation and parturition challenge the homeostatic mechanisms of ruminants, especially those controlling the cardiovascular system, and blood flows to the udder and uterus are directly related to milk production and gestational age, respectively (Davis & Collier, 1985;Olsson et al, 2001;Zarifi et al, 2012). However, it has been documented that the heart size is not influenced by milking and pregnancy status in dairy goats (Olsson et al, 2001).…”

Impacto da utilização da somatotropina bovina (bST) sobre a produção de leite e a avaliação genética de bovinos da raça Holandesa

“…O tratamento com somatotropina eleva a síntese de proteínas de vacas em lactação através de uma melhora da eficiência na utilização de aminoácidos (DAVIS & COLLIER, 1985). Durante balanço energético negativo, o bST reduz a oxidação de aminoácidos como fonte de energia nos tecidos, pois isto aumenta a mobilização de lipídios e acentua o metabolismo da glicose, assim, os aminoácidos poderão ser utilizados para o desenvolvimento e crescimento dos demais tecidos e para a síntese de proteínas no leite (GÜLAY & HATIPOGLU, 2005).…”