Abnormal parathyroid hormone-related peptide stimulation of renal 25-hydroxyvitamin D-1-hydroxylase in Hyp mice: evidence for a generalized defect of enzyme activity in the proximal convoluted tubule.

Previous investigations have established that hypophosphatemic (Hyp) mice exhibit diminished PTH/cAMP stimulation of 25-hydroxyvitamin D (25[OH]D)-1-hydroxylase activity in the renal proximal convoluted tubule. Whether similar muted enzyme responsiveness occurs secondary to provocation by other hormones/metabolic factors that operate by a different mechanism in the same cell system, however, remains unknown. In order to investigate this possibility, we compared renal 25(OH)D-1-hydroxylase activity of normal and Hyp mice upon stimulation with PTH-related peptide (PTHrP), a factor which may affect enzyme function in the PCT by a PTH-independent mechanism. Administration of 1-34 PTHrP, 3.0 micrograms/day sc, increased enzyme activity in normal mice (4.9 + 0.63 vs. 50.3 +/- 6.2 fmol/mg kidney.min) to a level significantly greater than that achieved in the Hyp mice (6.9 + 0.86 vs. 14.5 +/- 0.91 fmol/mg kidney.min). Moreover, similar to our observations after PTH stimulation, abnormal PTHrP effects did not result from an altered time course of enzyme activation or a shift in the dose response. Thus, the 25(OH)D-1-hydroxylase activity increased linearly to a maximum at 24 h in both animal models with a slope greater in normals than in mutants (P less than 0.05). Further, administration of PTHrP in graded amounts (0-9.0 micrograms/day) elicited a curvilinear response in normals and Hyp mice, but the mutants exhibited significantly less function (54 +/- 8.6%) at all doses tested. Additional studies revealed that the muted effects of PTHrP occurred via a PTH-independent mechanism. In this regard, we observed that simultaneous infusion of maximally effective doses of PTH and PTHrP in normal and Hyp mice resulted in an additive increment of 25(OH)D-1-hydroxylase activity. This observation that PTH and PTHrP influence renal 25(OH)D-1-hydroxylase by apparently different mechanisms indicates that the muted effects of these agents on enzyme activity in the Hyp-mouse results from a generalized defect in the proximal convoluted tubule.