Interactions Between Sodium and Calcium Intake and Blood Pressure Responses to Norepinephrine and Parathyroid Hormone

Summary

The present study has examined the vascular responses to infusion of norepinephrine (NE) and parathyroid hormone (PTH 1-34) in animals subjected for a six month period to one of four dietary regimens. Some animals received normal calcium chow (1.0% Ca by weight) and drank water (subgroup A), others consumed the same chow, but water was replaced by 0.5% saline (subgroup B), a third group consumed chow which had 2.0% Ca content and also drank 0.5% saline (subgroup C) and a fourth group consumed the 2.0% Ca chow, but drank water (subgroup D). No differences were found in the pressor response to NE across subgroup A, B, and C, while pressor response to NE in subgroup D was markedly reduced. Depressor responses to PTH were not significantly different across any of the four groups. The ability of changes in calcium homeostasis to affect blood pressure responses to NE and PTH were evaluated in animals consuming reduced dietary calcium (0.1%) for two and four weeks and compared with animals on normal calcium intake (1.0%). This dietary treatment resulted in only mild effects on calcium balance; after four weeks no significant difference in plasma total calcium concentration was observed, but plasma PTH levels were increased in animals on the low Ca diet. No effects on the blood pressure response to NE or PTH infusion were observed after 2 weeks of dietary treatment. At four weeks, NE responses remained unchanged, while responses to PTH were blunted in animals on 0.1% Ca chow. The present results indicate that long term elevation of dietary sodium intake does not result in enhancement of pressor responses to NE in the rat. Dietary calcium supplementation for six months produced a blunting of the pressor response to NE which was absent if the Ca supplementation was accompanied by saline consumption Perturbations in calcium homeostasis accompanying long term saline consumption did not appear to effect vascular responses to PTH, though four weeks of dietary calcium restriction reduced PTH depressor effects. This level of calcium deficiency did not alter pressor responses to NE.

Key-Words

Sodium - Calcium - Hypertension - Norepinephrine - Parathyroid Hormone - Blood Pressure