Plasma angiotensin II: interdependence on sodium and calcium homeostasis

Interactions between sodium and calcium metabolism and the renin angiotensin system (RAS) have been studied. In rats drinking highly palatable 0.5% sodium chloride solution for a 6 month period, plasma angiotensin II (p[AII]) levels after 6 months did not differ from control animals drinking water. However, plasma ionized calcium (p[iCa]) levels were significantly reduced compared to controls. In a third group of animals which drank saline, but consumed a calcium supplemented chow (2% calcium by weight vs. 1%), p[AII] was significantly elevated above both other groups. Further experiments were performed to study short term (4 weeks) changes in calcium intake and p[AII] levels. Diets contained high (4%), normal (1%) and low (0.05%) calcium content. All animals drank water. Plasma total calcium (p[tCa]) and p[iCa] concentration were elevated in the 4% calcium group compared with 1% calcium. In the 0.05% calcium group, p[iCa] was significantly reduced compared with the 1% group. Compared with the 1% calcium group, 4% calcium animals showed significant elevation of p[AII] levels. A slight, insignificant elevation was observed in 0.05% calcium rats compared with those consuming 1% calcium. A final experiment studied animals on the same calcium intakes (0.05, 1 and 4%), but consuming 0.5% saline in place of water. No differences in p[iCa], p[tCa] or p[AII] were observed in these experiments. However, consumption of saline lead to the expected reduction in p[AII] levels which was absent after 6 months in the earlier studies, indicating that normal levels of p[AII] in saline drinkers after 6 months was not a measurement error.(ABSTRACT TRUNCATED AT 250 WORDS)