Increased calcium absorption in prehypertensive spontaneously hypertensive rat. Role of serum 1,25-dihydroxyvitamin D₃ levels and intestinal brush border membrane fluidity

Changes in Ca absorption have been described in the spontaneously hypertensive rat (SHR) compared with Wistar-Kyoto (WKy) rats. In 3.5-wk-old SHR and age-matched WKy controls, we measured direct arterial blood pressure, Ca absorption, and serum 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] levels and small intestine brush border membrane (BBM) fluidity and lipid composition. The two objectives were (a) to define the nature of the absorptive changes before detectable hypertension and (b) to evaluate the potential mechanism(s). We found that even at this normotensive stage (106 ± 4 vs. 107 ± 2 torr for the female and 109 ± 3 vs. 104 ± 3 torr for the male), the SHR (a) absorbed more Ca (1.46 ± 0.06 vs. 1.14 ± 0.08 mmol/d and 1.53 ± 0.06 vs. 1.28 ± 0.06 mmol/d, respectively) and retained more Ca, (b) had higher serum 1,25(OH)₂D₃ levels (340 ± 36 vs. 160 ± 18 pg/ml and 230 ± 25 vs. 150 ± 16 pg/ml, respectively), and (c) possessed BBM with increased fluidity and with reduced fatty acyl saturation index owing to decreased stearic (32.2 ± 2.6% vs. 38.2 ± 0.9%) but increased linoleic acids (12.2 ± 2.0% vs. 7.6 ± 1.6%). These results demonstrate increased Ca absorption in prehypertensive SHR associated with increased serum 1,25(OH₂)D₃ levels, increased intestinal BBM fluidity, and reduced saturation index, which singly or in combination could produce the changes in intestinal Ca transport.