Early Actions of Parathyroid Hormone and 1,25-Dihydroxycholecalciferol on Isolated Epithelial Cells from Rat Intestine: I. Limited Lysosomal Enzyme Release and Calcium Uptake*

Abstract

Epithelial cells isolated from rat intestine were analyzed for their responsiveness in vitro to parathyroid hormone (PTH) and to 1,25-dihydroxycholecalciferol [1,25-(OH)₂D₃]. Criteria included determination of whether the agonists promoted extracellular liberation of lysosomal enzyme activities above control values during incubation in Ringer's solution at 22 C. PTH-augmented release of the representative hydrolase activities, cathepsin B and acid phosphatase, to the particle-free supernatant fraction of the medium was evident within 5 min of hormone treatment and was sustained in statistically significant degree for at least 30 min to greater than 20% above control levels. Basal release rarely exceeded 15% of the total cellular content of these enzymic activities. Lactate and succinate dehydrogenase activities were undetectable in the particle-free supernatant fraction under conditions of maximal hormone effect, indicating integrity of the cells and selectivity of the organellar response. Treatment of corresponding cells with 1,25-(OH)₂D₃ resulted in similar time of onset, magnitude, and duration of response. The most sensitive indicator of limited lysosomal labilization by either hormone was β-N-acetyl-D-glucosaminidase activity, which underwent accentuated extracellular liberation in the presence of as little as 10^-16 M PTH or 10^-11M 1,25-(OH)₂D₃, the latter eliciting a response of greater than 40% above control levels. Parathyroidectomy diminished basal release of the hydrolase activities and sensitized the intestinal cells to the action of PTH vs. preparations from intact or sham-operated animals, as judged by excess liberation of the glycosidase. Nontarget lung cells failed to respond to supramaximal levels of either hormone by the criterion of reduced latency of lysosomal hydrolases. In additional acute experiments with intestinal cells, both PTH and l,25-(OH)₂D₃ promoted enhanced ⁴⁵Ca²⁺ accumulation above control values. Collectively, these data indicate that PTH is capable of provoking direct effects on intestinal cells, similar in onset and extent to those elicited by 1,25-(OH)₂D₃.