G. Klaus et al., INTERMITTENT AND CONTINUOUS EXPOSURE TO 1,25(OH)2D3 HAVE DIFFERENT EFFECTS ON GROWTH-PLATE CHONDROCYTES IN-VITRO, Kidney international, 44(4), 1993, pp. 708-715
Intermittent 1,25(OH)2D3 administration is widely used to suppress par
athyroid glands in secondary (renal) hyperparathyroidism. It is unknow
n whether the effects of continuous and intermittent 1,25(OH)2D3 diffe
r on vitamin D target organs other than parathyroids. Using primary cu
ltures of rat chondrocytes (tibia) we compared the effects of continuo
us versus intermittent exposure to physiologic concentrations of 1alph
a25(OH)2D3 on proliferation (radiothymidine incorporation), cell count
, protein synthesis ([H-3]-leucine incorporation), alkaline phosphatas
e activity (as a marker of differentiation) and la,25(OH)2D3 receptor
(VDR) regulation. Cells were synchronized and then exposed for variabl
e periods to a medium containing 10% delipidated FCS and 10(-8) M) to
10(-12) M 1alpha,25(OH)2D3 (or 1beta,25(OH)2D3 as specifity control).
Intermittent (8 hr exposure every 48 hr) as well as continuous (sham w
ashing) administration of 1alpha,25(OH)2D3 had a biphasic effect on pr
oliferation, that is, stimulation at low (10(-12) M) and inhibition at
high (10(-8) M) concentrations. At 10(-12) M intermittent 1alpha,25(O
H)2D3 yielded higher cell counts than continuous 1,25(OH)2D3. This was
seen in the log phase, which was day 3 (continuous 141 +/- 2.3% of so
lvent control; intermittent 185 +/- 2.0%) and in the plateau phase of
growth, which was day 6 (128 +/- 2.6 vs. 169 +/- 2.7% of solvent contr
ol). Dependence on extracellular Ca is suggested by the effects of var
ying nominal Ca concentrations in the medium and of Ca channel blocker
s. Even two hours of exposure to 1alpha,25(OH)2D3 (10(-12) M) Yielded
maximal activation of AP during postincubation. VDR more than doubled
after 24 hours following brief (8 hr) or continuous exposure to 10(-12
) M 1alpha,25(OH)2D3 in the absence, but not in the presence of cycloh
eximide (5 mug/ml). Subsequently VDR declined with continuous 1alpha,2
5(OH)2D3, but not with intermittent 1alpha,25(OH)2D3. After 48 hours K
(d) was unchanged, but N(max) was significantly lower with continuous
(2807 bound molecules/cell) than intermittent (5987 molecules/cell) 1a
lpha,25(OH)2D3. We conclude that intermittent exposure to 1,25(OH)2D3
in primary chondrocyte cultures is more effective in (i) stimulating c
ell proliferation and (ii) sustaining up-regulated VDR.