Abstract
Parathyroid hormone (PTH) and calcitonin exert well known effects on the renal tubule which are thought to involve specific hormone receptors and adenyl cyclase. In the intestine, it is not clear whether the action of PTH and calcitonin is only indirect or also direct, and their mechanisms of action are much less well established. In the present study, possibly direct effects of PTH and calcitonin on Na + transport in isolated intestinal epithelial cells of rats were investigated. In the presence of bovine PTH (1.2 I.U./ml) in the incubation medium, the Na + efflux rate constant ( o K Na ) of isolated enterocytes was significantly reduced when compared to that in control experiments with the hormone vehicle only. The mean depression of o K Na induced by bovine PTH was 26% as compared to the control (100%) and to that induced by ouabain (4.0mM) which was 44%. No depressant effect of bovine PTH on o K Na was observed when the isolated enterocytes were incubated with ouabain (4.0 mM). Thus, bovine PTH appeared to inhibit the ouabain-sensitive Na + pump. When incubating the isolated epithelial cells in an EGTA-containing Ca 2+-free medium, bovine PTH lost its capacity to inhibit ( o K Na ). Thus, the presence of extracellular Ca 2+ appeared necessary for the inhibitory effect of bovine PTH. In contrast to its effect on o K Na , bovine PTH induced no change in net Na + uptake by isolated enterocytes. Moreover, no significant effect on enterocyte Na + transport could be demostrated for salmon or porcine calcitonin at two different concentrations in the incubation medium. Neither bovine PTH nor salmon calcitonin induced significant changes in enterocyte cyclic AMP or cyclic GMP concentrations. It was concluded that bovine PTH, but not calcitonin, exerted a direct inhibitory effect on the ouabain-sensitive o K Na of isolated rat enterocytes. The effect of bovine PTH occured without measurable activation of the cyclic nucleotide system but needed the presence of Ca 2+ in the incubation medium to be operative.
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