LACTATION IN THE RABBIT - MAMMARY BLOOD-FLOW AND CARDIAC-OUTPUT

In anesthetized rabbits, cardiac output (CO) and its distribution to the mammary glands, heart, liver and kidneys were determined in established lactation (11-13 days), late lactation (26-27 days) and in virgins. During lactation, the volume of circulating blood, CO, mammary blood flow and mammary weight were significantly greater than in virgins. There were no significant differences in CO and % CO received by the mammary glands between established and late lactation, and no significant decrease in mammary blood flow in late lactation. The weights of the liver and kidneys were significantly increased in lactation but there were no significant differences in liver, heart (coronary) and kidney blood flow. The rate of growth of the young was positively and significantly correlated with % CO received by the mammary glands and mammary weight, but not with CO. Strong correlation was observed between the % CO received by the mammary glands and mammary weight. There were no significant differences in CO, mammary % CO and mammary blood flow in animals in established lactation 2-3 h and 24 h after suckling (i.e., shortly after and just before suckling). By 48 h after the last suckling mammary blood flow and % CO, but not CO, were significantly decreased. Possible factors causing these changes are discussed. The results are discussed in relation to the change in milk composition that occurs in late lactation in this species and to the role and effects of prolactin. Events occurring during lactation may have different sensitivities to prolactin.