INTERMITTENT AND CONTINUOUS EXPOSURE TO 1,25(OH)2D3 HAVE DIFFERENT EFFECTS ON GROWTH-PLATE CHONDROCYTES IN-VITRO

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.