SUMMARYInorganic orthophosphate (P1) transport, using 32P-labelled orthophosphate as tracer, by lactating rat mammary tissue has been examined using both tissue explants and the intact perfused gland. Pi uptake was predominantly via a Na+-dependent pathway. Li', however, unlike choline, was able to partially substitute for Na+. In addition, Pi release from tissue explants preloaded with 32Pi was stimulated by reversing the Na+ gradient. Thus transferring mammary explants from a buffer containing Na+ to one which was Na+ free (choline replacement) doubled the P1 efflux rate constant. The uptake of Pi by tissue explants was saturable with respect to external Pi, having apparent Km and Vmax values of 1 13 mm and 3 36 mmol (kg cell water)-' (15 min)-', respectively.The stimulation of Pi uptake by tissue explants by external Na+ was also saturable; the K. for Na+ was 9-7 mm. These results, taken together, suggest that the Na+-dependent pathway is a Na'-P, cotransport mechanism. The transport of Pi by the perfused lactating rat mammary gland was examined using a rapid, paired-tracer dilution technique. P1 uptake by the perfused gland was found to be Na+ dependent and displayed saturable kinetics. The results suggest that the Na+-P, cotransporter is situated at the basolateral aspect of the secretory cells. The release of Pi from preloaded tissue explants was trans-accelerated by external Pi but not by Cl-or s042-. However, external Pi stimulated Pi efflux with low affinity.

SUMMARYDegradation of newly synthesized casein in rat mammary gland perfused in situ was measured by a pulse-chase method using [3H]proline. Casein degradation during secretion was observed in the absence of prolactin, but not with prolactin present. Partial inhibition by chloroquine showed that hormone-dependent degradation occurred intracellularly by a lysosomal mechanism. The study indicates that this post-translational mechanism is a physiological regulator of net casein secretion.

Mammary tissue of lactating rats has been perfused in situ to measure the effects of insulin and of previous dietary history on utilization of glucose and its incorporation into lipid. This experimental model allows the direct effects of insulin on mammary tissue to be characterized without influence from the secondary consequences of insulin administration that may accompany treatment of the intact animal with this hormone. In mammary tissue from rats starved for 24 h, glucose utilization was stimulated by insulin treatment. The threshold for insulin response was between 0.01 and 0.02 mU ml‐1, and no further stimulation took place between 0.02 and 0.04 mU ml‐1 insulin. The maximum response was reached within 50 min of the onset of insulin challenge and remained at a plateau value regardless of whether insulin was continuously present or was withdrawn after 15 min. Incorporation of glucose into lipid contributed to this glucose uptake. When no insulin was present in the perfusate, the rate of this process was around 3‐fold greater in perfused mammary tissue from fed lactating rats than from those starved for 24 h. Insulin accelerated lipogenesis from glucose approximately 3‐ and 2‐fold, respectively, in these two experimental groups. As for glucose uptake, the role of insulin in the regulation of mammary lipogenesis appeared to be to ‘prime’ the tissue and commit it to a response that was subsequently insulin independent.