INCREASE IN MILK SECRETION AND MAMMARY BLOOD-FLOW BY INTRAARTERIAL INFUSION OF INSULIN-LIKE GROWTH FACTOR-I INTO THE MAMMARY-GLAND OF THE GOAT

The close-arterial infusion of free insulin-like growth factor-I (IGF-I; 1.1 nmol/min) for 6 h into the pudic artery supplying one mammary gland of lactating goats caused a 25 ± 6% (mean ± S.E.M., n = 6) increase in the rate of milk secretion of that gland. The increase in the rate of milk secretion in the adjacent non-infused gland (14 ± 4%) was not significantly different from that observed during saline infusion (4 ± 5%). Blood flow to the infused gland was increased from 378 ± 26 ml/min 1 h before to 487 ± 56 ml/min approximately 5 h after the start of the infusion of IGF-I, declining to 420 ± 44 ml/min approximately 2 h after the end of the infusion. The total concentration of IGF-I (free and bound) in milk of the infused gland was significantly higher than that of the non-infused gland. The concentrations of IGF-I in carotid arterial plasma samples increased during IGF-I infusion from a mean value of 32 ± 2 nmol/l before to a maximum of 49 ± 3 nmol/l 5 h after the infusion commenced. Circulating concentrations of total IGF-I declined slowly after the infusion with an estimated half-life of 5 h. Infusion of saline alone did not alter mammary blood flow or the concentration of total IGF-I in milk or plasma. The results indicate that the infusion of free IGF-I into the mammary arterial supply enhances milk secretion and mammary blood flow in intact, conscious goats. The more pronounced effect in the infused compared with the non-infused gland suggests that free IGF-I acts directly on the mammary gland. The response in the non-infused gland was attenuated presumably due to association of IGF-I with plasma binding proteins during recirculation.