The influence of parathyroid hormone on the bidirectional transport of calcium in rat liver was assessed in vitro. An increase of 41% in the fractional coefficient for calcium influx was observed six hours after the administration of 200 USP units of parathyroid extract (PTE, Lilly), and increases of 50% and 56% were noted after two and three days of treatment with PTE. No increase was observed following one day of treatment, indicating the presence of a biphasic response to the hormone. The fractional coefficient for calcium efflux was not influenced by parathyroid hormone at any of the time intervals studied. Autoradiographic studies utilizing a cryostatic technique which prevented the translocation of calcium confirmed the intracellular locus of the transported radiocalcium. Competitive Chelex resin binding studies of liver supernates from parathyroid hormone treated animals revealed increased cytoplasmic binding of calcium which paralleled the changes in the fractional influx coefficients induced by the hormone. These studies confirm the ability of tissues other than bone, kidney, and gastrointestinal tract to respond to parathyroid hormone with a stimulation of calcium influx and support the thesis that a major action of parathyroid hormone is the maintenance of the supply of calcium to intracellular loci critical to cellular activity.
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