Phosphate transport via Na+-P-i cotransport and anion exchange in lactating rat mammary tissue

Inorganic orthophosphate (P-i) transport, using P-32-labelled orthophosphate as tracer, by lactating rat mammary tissue has been examined using both tissue explants and the intact perfused gland. P-i uptake was predominantly via a Na+-dependent pathway. Li+, however, unlike choline, was able to partially substitute for Na+ In addition, P-i release from tissue explants preloaded with (32)p(i) 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 P-i efflux rate constant. The uptake of P-i by tissue explants was saturable with respect to external P-i, having apparent K-m and V-max values of 1.13 mM and 3.36 mmol (kg cell water)(-1) (15 min)(-1), respectively. The stimulation of P-i uptake by tissue explants by external Na+ was also saturable; the K-m for Na+ was 9.7 mM. These results, taken together, suggest that the Na+-dependent pathway is a Na+-P-i, cotransport mechanism. The transport of P-i by the perfused lactating rat mammary gland was examined using a rapid, paired tracer dilution technique. P-i uptake by the perfused gland was Found to be Na+ dependent and displayed saturable kinetics. The results suggest that the Na+-P-i, cotransporter is situated at the basolateral aspect of the secretory cells. The release of P-i from preloaded tissue explants was trans-accelerated by external P-i but not by Cl- or SO42-. However, external P-i stimulated P-i efflux with low affinity.