Plasma immunoreactive N-terminal parathyroid hormone and cyclic AMP and cyclic GMP urinary levels in man after I. M. administration of two different doses of porcine calcitonin

The effects of acute porcine calcitonin (pCT) administration were studied in 11 healthy volunteers with no metabolic disease. Each subject was given, intramuscularly, 1 MRC unit of pCT in glycine vehicle, 160 units of pCT in gelatine vehicle, and a placebo, according to a crossover design. The following parameters were studied: blood calcium, phosphorus and immunoreactive parathyroid hormone (iPTH), urine calcium, phosphorus, cyclic AMP and GMP. Both the pCT preparations produced, at the same time after administration, a hypocalcemic effect (P <0.01) which was not dose related, without any modification of urinary calcium excretion, implying that both doses are able to inhibit completely bone destruction. Despite the blood calcium decrease, no significant modifications in plasma iPTH levels were observed. pCT administration did not modify the urinary excretion of cyclic AMP, while it increased the urinary levels of cyclic GMP, particularly at the higher dose employed. Blood phosphorus decrease and urinary phosphate excretion increase were observed only after the administration of 160 MRC units of pCT. These observations suggest that the effects on urinary cyclic GMP and on blood and urine phosphorus are not mediated by PTH but could be the result of a direct action of calcitonin seen only when high doses are employed. In conclusion, one MRC unit of pCT is sufficient to inhibit bone resorption.