1. The effects of synthetic [Asu1,7]-eel calcitonin (0.5 MRC unit/kg body weight intravenously for 30 min) on circulating levels of human calcitonin, calcium and gastrin were investigated in five patients with medullary carcinoma of the thyroid.

2. Blood samples were obtained before and 15, 30, 60, 90, 120 and 180 min after commencement of infusion of [Asu1,7]-eel calcitonin. Plasma levels of human calcitonin were measured by radioimmunoassay. Cross-reactivity with [Asu1,7]-eel calcitonin in this assay was negligible.

3. On infusion of [Asu1,7]-eel calcitonin, the mean plasma level of human calcitonin decreased significantly to 71.0 ± 8.7% of the basal level (mean ± sem, P < 0.05) after 30 min and 68.4 ± 25.4% of the basal level (P < 0.05) after 60 min. The serum calcium level also decreased significantly, but lagged behind the decrease of human calcitonin, being 95.1 ± 0.7% of the basal level (P < 0.01) at 120 min and 94.8 ± 0.6% of the basal level (P < 0.02) at 180 min. The mean plasma gastrin level did not change significantly on infusion of [Asu1,7]-eel calcitonin.

4. In pooled data for all times, the percentage change in human calcitonin was not significantly correlated with either the percentage change in calcium (r = −0.25, P > 0.1) or the percentage change in gastrin (r = −0.3 8, P > 0.1).

5. These data suggested that [Asu1,7]-eel calcitonin suppressed calcitonin secretion in patients with medullary carcinoma of the thyroid. The mechanism of its effect is unknown.

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