Evidence that somatomedins mediate the effect of hypophosphatemia to increase serum 1,25-dihydroxyvitamin D3 levels in rats

The present studies were undertaken in an effort to determine whether somatomedins (SMs) play a role in the elevation of serum 1,25-dihydroxyvitamin D3 1,25-(OH)2D3] levels during dietary phosphate deprivation. Serum 1,25-(OH)2D3,SM-C, and phosphate levels were measured in rats fed diets containing adequate or very low levels of dietary phosphorus under circumstances known to affect SM levels, including hypophysectomy with and without GH replacement, normal protein vs. low protein diets, and streptozotocin-induced diabetes with and without insulin replacement. In all circumstances, serum 1,25-(OH)2D3 concentrations were directly related to serum SM-C levels. However, the slope for the relationship was increased 2- to 10-fold in animals fed the low phosphorus diets. As observed previously, serum 1,25-(OH)2D3 levels were inversely related to serum phosphate levels, but the slope for this relationship was deceased in the presence of low SM levels and absent in animals with very low SM levels. These results suggest that SM are required for elevation of serum 1,25-(OH)2D3 levels in response to phosphate deprivation.