Effect of vitamin D status on the activity of carbonic anhydrase in chicken epiphysis and kidney

Chickens were raised for 6 weeks from the date of hatch under red light on a vitamin D-free diet; controls were given an oral vitamin D supplement. Vitamin D-deficient animals showed decreased total serum calcium concentration and decreased DNA content in epiphysis and kidney homogenates. In calcifying epiphysis, total carbonic anhydrase (CA) activity was decreased, but activity per microgram DNA was slightly increased and specific activity was double that of the controls. Polyacrylamide gel isoelectric focusing after preparation of the enzyme showed a picture similar to that seen after parathyroid hormone (PTH) administration in chicks; therefore, this could be considered a secondary hyperparathyroidism. The CA activation was not seen in the kidney which can be explained by induction of an endogenous inhibitor protein of the cyclic AMP-dependent protein kinase exclusively in the kidney in vitamin D deficiency. In an additional experiment, chickens were raised for 3 weeks from the date of hatch under red light on a vitamin D-free diet. Daily oral substitution by different vitamin D metabolites (1,25(OH)2D3, 25OHD3, 24,25(OH)2D3) over 7 days led to CA activation compared with controls probably by restoring protein kinase activity in the kidney. Our results show that CA activity is inversely correlated with serum calcium concentrations which is in agreement with a regulatory mechanism recently proposed by us.