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Calcium and Inorganic Phosphate Transport in Embryonic Chick Intestine: Triiodothyronine Enhances the Genomic Action of 1,25-Dihydroxycholecalciferol¹

Department of General and Experimental Pathology, University of Vienna Medical School, A-1090 Vienna, Austria

The influence of triiodothyronine (T₃) on the induction of intestinal calcium and inorganic phosphate (Pi) transport by 1,25-dihydroxycholecalciferol (1,25-(OH)₂D₃) was studied in 48 h cultures of embryonic chick jejunum. While T₃ alone had no effect on calcium uptake by gut segments cultured on d 20 of embryonic development, the thyroid hormone amplified the effect of 1,25-(OH)₂D₃ on calcium transport and effectively shifted the dose-response curve to lower 1,25-(OH)₂D₃ concentrations. T₃ had a dual effect on Pi uptake by cultured jejunum: It induced transport activity even in the absence of the steroid hormone, and, in addition, synergistically raised l,25-(OH)₂D₃-related Pi uptake. In d 17 embryonic small intestine, which does not respond to 1,25-(OH)₂D₃ by a significant increase in Pi transport, T₃ permitted the induction of Pi transport by the sterol. In general, the thyroid hormone enhanced the responsiveness of cultured embryonic intestine toward 1,25-(OH)₂D₃ by two orders of magnitude, resulting in facilitated induction of calcium and Pi transport by the sterol and, in particular, modulated the stage-specific expression of 1,25-(OH)₂D₃ action on intestinal Pi transport.

KEY WORDS: • calcium transport • phosphate transport • 1,25-dihydroxycholecalciferol • triiodothyronine • embryonic chick small intestine • organ culture

¹ These investigations were supported by Grant No. 61/85 of the Anton Dreher-Gedächtnisschenkung.

² Present address: Department of Physiology, New York State College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

Manuscript received 26 February 1988. Revision accepted 25 August 1988.

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