Plasma 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol concentrations are decreased in hind limb unloaded Dahl salt-sensitive female rats

Abstract

Plasma 1,25-dihydroxyvitamin D (1,25-(OH)(2)D) concentration was shown to decrease during bed rest in several studies when baseline plasma 25-hydroxyvitamin D (25-OHD) concentration was sub-optimal. Dahl salt-sensitive female (S) rats, but not Dahl salt-resistant female (R) rats, demonstrated a 50% decrease in plasma 1,25-dihydroxycholecalciferol (1,25-(OH)(2)D(3)) concentration after 28 days of hind limb unloading (HU, disuse model) during low salt intake (0.3%). We tested the vitamin D endocrine system response of female S rats to hind limb unloading during high salt intake (2%, twice that of standard rat chow to mimic salt intake in the USA). Hind limb unloading resulted in lower plasma 25-OHD(3) concentrations in S-HU rats than in R-HU rats (P<0.05) and greater urinary loss of 25-OHD(3) by S-HU rats than by S rats (P<0.05). Plasma 1,25-(OH)(2)D(3) concentration of S-HU rats was half that of S rats, but was unchanged in R-HU rats. The association of low plasma 25-OHD concentration with decrease in plasma 1,25-(OH)(2)D concentration of hind limb unloaded rats and of bed rest participants (published studies) suggests that low vitamin D status might be a risk factor for decrease in plasma vitamin D hormone concentration during long-term immobilization or bed rest.

Fig. 1

Plasma 25-hydroxycholecalciferol and 24,25-dihydroxycholecalciferol concentrations are decreased in hind limb unloaded Dahl salt-sensitive female rats fed 2% salt. Dahl salt-sensitive (S) and salt-resistant (R) female rats were fed a 2% salt diet and hind limb unloaded (HU) for 28 days as described in Section 2.2. Loaded S and R rats served as controls. Plasma (A) 25-hydroxycholecalciferol (25-OHD₃) and (B) 24,25-dihydroxycholecalciferol (24,25-(OH)₂D₃) concentrations at day 28 were measured by radioimmunoassay after pre-purification [9, 12, 24, 41]. Values are means ± SEM, n = 6–8 rats per group. Statistical significance was evaluated by two-way ANOVA. Salt sensitivity effect on plasma 25-OHD₃, P = 0.01, and 24,25-(OH)₂D₃, P < 0.001, concentrations. HU effect on plasma 24,25-(OH)₂D₃, P = 0.012. ^bS-HU vs. R-HU, P < 0.05. ^dS vs. R, P < 0.01.

Fig. 2

Urinary 25-hydroxycholecalciferol and 24,25-dihydroxycholecalciferol are increased by hind limb unloading of Dahl salt-sensitive female rats fed 2% salt. Dahl salt-sensitive (S) and salt-resistant (R) female rats were fed a 2% salt diet and hind limb unloaded (HU) for 28 days as described in Section 2.2. Loaded S and R rats served as controls. Urinary 25-hydroxycholecalciferol (25-OHD₃) and 24,25-dihydroxycholecalciferol (24,25-(OH)₂D₃) at day 21 (non-detectable for R rats) were measured by radioimmunoassay after pre-purification [9, 12, 24, 41]. Values are means ± SEM, n = 5–7 rats per group. Statistical significance was evaluated by the Mann-Whitney rank sum test. ^a S vs. S-HU, P < 0.05.

Fig. 3

Urinary 25-hydroxycholecalciferol binding activity and protein are increased by hind limb unloading of Dahl salt-sensitive and salt-resistant female rats fed 2% salt. Dahl salt-sensitive (S) and salt-resistant (R) female rats were fed a 2% salt diet and hind limb unloaded (HU) for 28 days as described in Section 2.2. Urinary (A) 25-hydroxycholecalciferol binding activity (DBA) and (B) protein at day 21 were measured as described in Section 2.4. Values are means ± SEM, n = 6–8 (protein) and n = 3–7 (binding activity). Statistical significance was evaluated by two-way ANOVA. Salt sensitivity effect on urinary binding activity and protein, P < 0.001. HU effect on urinary protein, P < 0.001. ^aS vs. S-HU, P < 0.05. ^bS-HU vs. R-HU, P < 0.001. ^dS vs. R, P < 0.001.

Fig. 4

Plasma 1,25-dihydroxycholecalciferol concentrations are decreased in hind limb unloaded Dahl salt-sensitive female rats fed 2% salt. Dahl salt-sensitive (S) and salt-resistant (R) rats were fed a 2% salt diet and hind limb unloaded (HU) for 28 days as described in Section 2.2. Plasma 1,25-dihydroxycholecalciferol concentrations at day 28 were measured by radioimmunoassay as described in Section 2.3. Values are means ± SEM, n = 4–6 rats per group. Statistical significance was evaluated by two-way ANOVA. Interaction between salt sensitivity and HU, P = 0.02. ^aS vs. S-HU, P < 0.01. ^dS vs. R, P = 0.02.