The relative roles of calcium, phosphorus, and parathyroid hormone in glucose- and tolbutamide-mediated insulin release.

Abstract

The relative contributions of Ca++, phosphorus, and parathyroid hormone (PTH) on insulin secretion were evaluated in three groups of dogs. Dogs were studied with glucose infusions (group I) or standard intravenous glucose tolerance tests (IVGTT) (group II) before and after the development of diet-induced hypophosphatemia. Mean serum phosphorus levels for both groups fell from 4.1 to 1.1 mg/100 ml. Animals in group I demonstrated a fall in glucose disappearance rates (Kg) from 5.3+/-0.6% min to 3.5+/-0.5% after induction of hypophosphatemia (P less than 0.001). Mean insulin response was significantly greater in the hypophosphatemic animals than in controls in this group. In group II animals, mean insulin areas obtained during the IVGTT increased from 1,426+/-223 to 2,561+/-141 muU/ml/60 min after induction of hypophosphatemia, and were unaffected by Ca++ or PTH administration. Ca++ administration, but not hypophosphatemia or PTH infusion, increased significantly the mean insulin response to tolbutamide. Secondary hyperparathyroidism was induced by dietary manipulation in four dogs (group III). Mean PTH values increased from 71.4+/-2.1 to 3,012+/-372 pg/ml (P less than 0.001). Mean insulin response to an IVGTT was similar to group III animals, but increased from 1,352+/-128 to 1,894+/-360 muU/ml/60 min after the excessive dietary phosphorus was reduced for 3 mo, and plasma phosphorus fell from 3.2+/-0.1 to 2.8+/-0.3 mg/100 ml. PTH values decreased to 647+/-53 pg/ml. The insulin response to tolbutamide was comparable to that in group II animals, but increased significantly after calcium administration. Immunoreactive insulin disappearance rates were unaffected by hypophosphatemia or diet-induced secondary hyperparathyroidism. These data demonstrate that hypophosphatemia is associated with an augmented glucose-stimulated insulin release, without any effect on tolbutamide-stimulated insulin release. Hypercalcemia produces an augmented tolbutamide-stimulated insulin release with no apparent effect on glucose-stimulated insulin release. Finally, PTH does not appear to be an insulin antagonist and has no apparent effect on either glucose- or tolbutamide-stimulated insulin release in animals with dietary-induced secondary hyperparathyroidism.