Effect of parathyroid hormone on rat enterocyte Na transport in vitro

In the presence of b.PTH (1.2 I.U./ml) in the incubation medium, the Na efflux rate constant (degree KNa) of isolated rat enterocytes was significantly reduced when compared to control experiments. The mean depression of degree KNa induced by b.PTH was 26% as compared to control (100%). No depressant effect of b.PTH on degree KNa was observed when the isolated enterocytes were incubated with ouabain (4.0mM). Thus, b.PTH appeared to inhibit the ouabain-sensitive Na pump. When incubating the isolated epithelial cells in an EGTA-containing Ca free medium, b.PTH lost its capacity to inhibit degree KNa. Thus, the presence of extracellular Ca appeared necessary for the inhibitory effect of b.PTH. In contrast to its effect on degree KNa, b.PTH induced no change of net Na uptake by isolated enterocytes. Moreover, b.PTH did not induce significant changes in enterocyte cAMP or cGMP concentrations. It was concluded that b.PTH exerted a direct inhibitory effect on the ouabain-sensitive Na efflux rate constant of isolated rat enterocytes. The effect of b.PTH occurred without a measureable activation of the cyclic nucleotide system but needed the presence of Ca in the incubation medium to be operative.