Effect of parathyroid hormone on 1,25-dihydroxyvitamin D formation in type I pseudohypoparathyroidism

It has been presumed that the stimulating effect of parathyroidhormone (PTH) on 1-hydroxylation of 25OHD in the kidney is mediated through cAMP. To explore this assumption parathyroid extract (PTE) was infused to a patient with type I pseudohypopara-thyroidism and control subjects. In the controls the infusion resulted in a prompt and marked increase in both plasma cAMP and urinary excretion of cAMP, whereas there was only a negligible increase in the patient. Following the PTE infusion plasma 1,25(OH)2D showed a distinct increase after 2 hours both in the controls and the patient, with a further increase after 4 hours. The finding in this patient with pseudohypoparathyroidism type I seems to negate the presumption that PTH regulation of the 25OHD-1-hydroxylase is mediated by cAMP.Measurements of plasma 1,25(OH)2D during PTE infusion test might be a useful adjunct in the work up of various disorders of parathyroid-vitamin D-calcium metabolism.