Distinct Site and Mechanism of Action

tinct 25-hydroxyvitamin D3-la-hydroxylase (la-hydroxylase) systems was studied in the kidneys of vitamin D-deficient rats; one is localized in the proximal convoluted tubule (PCT), is activated in vitamin D deficiency, and is regulated primarily by parathyroid hormone (PTH) via cyclic AMP; the other is localized in the proximal straight tubule (PST), is latent in vitamin D deficiency, and is selectively stimulated by calcitonin via a cyclic AMP-independent mechanism. The la-hydroxylase activities were measured in the PCT and PST microdissected from the kidney of vitamin D-deficient rats with or without metabolic acidosis of varying duration. The la-hydroxylase activity decreased in the PCT from 0.74±0.07 fmol/mm per h to 0.24±0.02 at day 3 of metabolic acidosis without a further decline at day 7. Neither metabolic acidosis of 16 h duration nor reduction of the incubation medium pH from 7.4 to 7.0 affected the enzyme activity in the PCT. To examine the underlying mechanisrn for the suppression of la-hydroxylase activity, PTH, cyclic AMP, or calcitonin was given to rats with metabolic acidosis of 3 d duration. Although PTH failed to augment the suppressed la-hydroxylase activity in the PCT, cyclic AMP restored it to the level of control rats. The la-hydroxylase activity in the PST remained undetectable in control rats and in acidotic rats with Portions of the study were presented at the Annual Meeting of the American Society of Bone and Mineral Research, 14-17 June 1981, Cincinnati, OH, and the Annual Meeting of the American Society of Nephrology, 22-24 November 1981, Washington, DC, and appeared in abstract form 1981, Calcif. Tissue Int. 33: 337, and 1982, Kidney Int. 21: 135. Address reprint requests to Dr. Kurokawa, Wadsworth Medical Center. Received for publication 11 September 1981 and in revised form 3 March 1982. or without PTH or cyclic AMP treatments. However, calcitonin stimulated the la-hydroxylase activity in the PST equally from undetectable to 0.75±0.09 fmol/ mm per h in control and to 0.78±0.10 in acidotic rats. The data suggests that metabolic acidosis suppresses la-hydroxylase only in the PCT by inhibiting PTHdependent adenylate cyclase, and that cellular events beyond cyclic AMP in the PCT and the events responsive to calcitonin in the PST are unaffected. The results show the definite advantage of using defined single nephron segments to study the hormonal and ionic control of the la-hydroxylase system in the kidney.