Control of phosphate transport in liver.

It has becn suggested that the phosphorylation potential, i.c. the energy required for ATP synthesis under the prevailing conditions and calculated from the (ATP)/@DP)( P,) ratio, is an important factor in the regulation of energy metabolism [ 11. Measurements of cytosolic concentrations of ATP, ADP and PI by n.m.r. spectroscopy support this view 121 and in order to assist with the stabilization of the phosphorylation potential, it is likely that the cytosolic PI level is maintained within fairly narrow bounds. The role of P, as a substrate and/or allosteric effector for many key metabolic reactions provides further reason to believe that cytosolic P, is subject to control 131. In searching for putative cellular control systems the liver seems to be a good choice of tissue because o f its general importance in metabolism. Hepatocytes isolated from adult rat liver [4] are being used for studies of PI transport and for investigating the effect of ambient PI availability on aspects of liver metabolism and P, transport. Hepatocytes prepared from the livers of fed animals accumulate PI linearly with time for at least 5 min. The rate of uptake, determined by a method that was developed for proximal tubule cells [ 51, is strongly dependent on Na'. The Na'-dependent uptake does not saturate at high P, concentrations and at 2 mM-P, in the bathing medium, the rate is approx. 3.5 nmol/min per mg of protein. In the absence o f Na+, the rate is less than one-tenth of this figure. Incubation of the hepatocytes for 10 min with insulin stimulates the rate of uptake by 50-100"/0 over control values (Table I ) . The relatively low uptake in the absence of Na + is not stimulated by insulin however. Since our method measures the accumulation of P,, the increased uptake could arise from an acceleration of P, influx and/or a diminution in the rate of P, efflux. The Na+ requirement persuades us that influx rather than efflux is affected by insulin. This hormone also stimulates the Na',K'-ATPasc, however 161, so that it is possible that the action of the hormone on PI uptake is secondary to an effect on the Na+ gradient. Table 1. Effect of IO-' M-insulin on the rute of Nu' -dependent I', uptake by hepurocytes offed ruts