Altered vitamin D metabolism in the spontaneously hypertensive rat.

To test the possible impairment of vitamin D metabolism in hypertension, we studied the effect of parathyroid hormone (PTH) on the renal production of 1,25-dihydroxyvitamin D [1,25(OH)2D] in spontaneously hypertensive rats (SHR) before (at 4 weeks of age) and after (at 12 weeks of age) the onset of hypertension. Basal serum of 1,25(OH)2D was normal in SHR at both ages. At 4 weeks of age, rise in serum 1,25(OH)2D following PTH injection (50 U subcutaneously every 2 h, four times) was also normal in SHR. By contrast, at 12 weeks of age it was approximately one-third of that in Wistar-Kyoto rats (WKY) in parallel with an attenuated response to PTH of renal production of 1,25(OH)2D. Basal 1,25(OH)2D production by the kidney in SHR was higher than that in WKY at both ages, which was abolished by thyroparathyroidectomy but not by parathyroidectomy. These data demonstrate that altered vitamin D metabolism exists even before the onset of hypertension in SHR.