[Pharmacological study on blood pressure in rats with bone disorders]

To evaluate the relationship between the elevation of blood pressure and altered bone metabolism, the changes of systolic blood pressure in six experimental models for bone disorders were investigated. Rats used were either parathyroidectomized, ovariectomized, fed with a calcium-deficient diet, fed with a vitamin D-deficient diet, treated with HEBP (1-Hydroxyethylidene-1, 1-bisphosphonate) or treated with streptozotocin. Hypertension developed in 5-week-old male rats fed with a calcium-deficient diet for 2 weeks, which evoked hypocalcemia and nutritional hyperparathyroidism. The blood pressure returned to normal when fed with a normal calcium diet. In parathyroidectomized rats receiving a normal calcium diet, the blood pressure did not rise, though the plasma calcium level decreased to an extent similar to the rats fed with the calcium-deficient diet. These findings seem to indicate that hyperparathyroidism, but not hypocalcemia, was involved in the elevation of blood pressure in rats fed with a calcium-deficient diet. Hypertension was not observed in rats fed with a vitamin D-deficient diet or treated with streptozotocin. These rats showed not only an increase in parathyroid hormone (PTH) but also a decrease in 1,25 (OH)2 D3. These results may suggest that the presence of 1,25 (OH)2D3 as well as the enhanced parathyroid function is necessary for the development of hypertension. The elevated blood pressure was reduced by a calcium antagonist, nifedipine, or by calcium supplementation, but not by an inhibitor of angiotensin-converting enzyme, captopril, or by calcitonin. This may indicate that hypertension due to nutritional hyperparathyroidism responds to the calcium antagonist nifedipine and to calcium supplementation, but does not depend on renin or salt. Furthermore, an acute hypotensive effect by human PTH (1-34) was not observed in the hypertension of calcium-deficient rats, suggesting the difference between acute and chronic effects of PTH. The hypertension developed in the present experiment may be a useful model for pharmacological evaluation of antihypertensive drugs, such as calcium and calcium antagonists.