Parathyroidectomy abolishes the increase of renal 25-hydroxyvitamin D-1 alpha-hydroxylase in lactating rats

Serum ionized calcium (Ca), but not inorganic phosphorus or immunoreactive parathyroid hormone, negatively correlates with renal 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) and serum 1,25-dihydroxyvitamin D in intact lactating rats. The present study tested the hypothesis that the presumed stimulation of renal 1 alpha-hydroxylase by hypocalcemia requires the presence of intact parathyroid glands. Lactating and nonlactating rats were surgically parathyroidectomized (PTX) or sham-operated (sham) at 9-10 days of lactation. Later (24 h) the rats were bled, nephrectomized, and killed. In lactating PTX rats, serum ionized Ca decreased to 50% of the level of sham rats, and serum 1,25-dihydroxyvitamin D fell to 37 +/- 5.0 pg/ml compared with 82 +/- 13.0 pg/ml for sham lactating rats but was still 2.5 times the value for nonlactating PTX rats (15 +/- 0.8 pg/ml). In contrast to the still elevated serum 1,25-dihydroxyvitamin D concentration in lactating PTX rats, renal 1 alpha-hydroxylase was suppressed to the same low level as in nonlactating PTX rats, suggesting the existence of extrarenal synthesis of 1,25-dihydroxyvitamin D in lactation. A curvilinear relationship was revealed between serum ionized Ca and renal 1 alpha-hydroxylase in sham lactating and nonlactating rats (r2 = 0.71, P < 0.0001). However, in PTX rats, decreasing ionized Ca did not lead to any increase in 1 alpha-hydroxylase above the low baseline values seen at ionized Ca concentrations between 1.3 and 1.5 mM. We therefore conclude that intact parathyroid glands are required for hypocalcemia to activate renal 1 alpha-hydroxylase in female rats.