The effect of raised intramammary pressure on mammary function in the goat in relation to the cessation of lactation.

1. The effects of raising intramammary pressure on mammary function have been studied in conscious goats. 2. When intramammary pressure was raised to levels which normally occur following cessation of milking (by infusing isosmotic sucrose into the lumen of the gland) the rate of milk secretion fell within 6 hr, but in short-term studies at these pressures, there was no reduction in mammary blood flow (in fact there was a significant increase) and no change in oxygen consumption or glucose uptake. 3. At pressures more than twofold higher than those which occur under physiological conditions, there was a decrease in mammary blood flow within 5 min; in addition the arterio-venous difference for oxygen, but not for glucose, fell. The changes were reversed when pressure was lowered. Similar results were obtained in autotransplanted (denervated) glands. 4. Intramammary pressure-volume curves were determined 3 days before cessation of milking. By determining the rate of secretion after cessation it was evident that the rate of secretion fell as the pressure-volume curve steepened. 5. A positive correlation was found between empty gland volume and functional capacity of the gland (volume of liquid needed to raise intramammary pressure to 30 mmHg at the tip of the teat) in goats of the Saanen breed. 6. No apparent effect on blood-milk potential difference was obtained at pressures within the range observed after cessation of milking. At higher pressures, the potential difference fell, eventually to zero. 7. It is concluded that in the goat the arrest of milk secretion following cessation of milking is caused by mammary distension but is not due to a primary effect of mammary distension on blood flow, and that the loss of integrity of the mammary epithelium, which also occurs, is not due simply to mechanical rupture. 8. From present and previous studies, a scheme is proposed to account for the control of mammary function after the cessation of milking in late lactation in the goat.