1. When goats were kept in continuous milk production for 2-4 yr the rate of milk secretion oscillated in a sinusoidal fashion with a mean wavelength of 49.7 +/- 1 weeks, being highest in summer and lowest in winter.2. Milk [Cl] tended to be higher in summer and [protein] highest in winter but there were no seasonal changes in [fat], [lactose], [Na] or [K].3. There were no seasonal oscillations in body weight but the udders were larger in summer than in winter although this did not solely account for the changes in milk yield.4. Although the autumnal fall in yield and the rise in the spring coincided with the breeding season, seasonal variations also occurred in two ovariectomized animals.5. Seasonal fluctuations were independent of diet, occurring equally in animals that had never eaten grass but were fed continuously on dry feed and hay ad libitum.6. Annual oscillations in yield and udder size but not [protein] and [Cl] were also exhibited in two goats kept in a constant temperature chamber at 14 +/- 2 degrees C, fed every 2 hr and subjected continuously to 16 hr light and 8 hr dark. These observations have so far persisted for 2 yr in one goat and for 3 yr in the other. However, the wave-length was reduced to 45 +/- 1 weeks. Oestrus was suppressed for 2 years.

Animals↗

PubMed1973

The time course of cardiovascular changes in lactation in the rat.

1. Cardiovascular changes in lactating rats have been traced from the first day post-partum to the end of the third week of lactation. The pattern of changes showed three phases.2. Between days 1 and 5 of lactation there were sharp rises in both cardiac output and in the blood flow/g tissue for most organs, but little change in the distribution of the cardiac output.3. Between days 5 and 15 of lactation cardiac output remained steady. The blood flow to tissues actively involved in the body's response to lactation (mammary glands, liver, gastrointestinal tract) also remained at high steady levels, but the blood flow to other tissues declined due to a redistribution of the cardiac output away from them and towards the growing mammary glands and splanchnic organs.4. Between days 15 and 22 of lactation there were further rises in both cardiac output and in the blood flow/g tissue for most organs.5. It is suggested that the increases in organ blood flows that occurred in the first few days after parturition (days 1-5) and at the end of lactation (days 15-22) were largely dependent on increases in cardiac output and may represent the maternal response to rapidly rising demands from the young at these times.

Animals↗

PubMed1973

The effects of engorgement with milk and of suckling on mammary blood flow in the rat.

1. Mammary blood flow was estimated in rats by measuring the cardiac output and the proportion of it received by the mammary glands.2. When the young were removed on the 10th day of lactation the mammary glands began to fill with milk and mammary blood flow fell from 78 ml./min. 100 g tissue to 45 ml./min. 100 g within 8 hr and decreased further to 34 ml./min. 100 g within 8 hr and decreased further to 34 ml./min. 100 g in the next 16 hr. These changes were associated with both a fall in cardiac output and a fall in the proportion of the cardiac output taken by the mammary glands.3. When the young were allowed to continue suckling, but milk removal was prevented by sealing the teat ducts with adhesive, more milk collected in the mammary glands within 8 hr and mammary blood flow was unchanged (74 ml./min. 100 g).4. In rats which had been separated from their young for 24 hr, milk was removed from the engorged glands by allowing the pups to suckle again. Mammary blood flow did not rise immediately following the removal of milk but only after 4 hr of suckling, and was associated largely with an increase in cardiac output.5. Upon resumption of suckling mammary blood flow was the same in emptied glands, and in full glands with the teats sealed.6. When the young were removed from 15-day lactating rats mammary blood flow after 24 hr was directly related to the volume of milk in the glands.7. It is concluded that the accumulation of milk in the mammary gland does not mechanically restrict the flow of blood through the tissue and that, in the rat, mammary blood flow and milk secretion are strongly dependent on a continually applied suckling stimulus.

Animals↗

PubMed1973