A high phosphate diet lowers blood pressure in spontaneously hypertensive rats

Abstract

Plasma phosphate values are significantly lower in spontaneously hypertensive rats (SHR) than in normotensive Wistar-Kyoto rats (WKY). In this study, we increased plasma phosphate in SHR by a high dietary phosphate intake and followed the effects on blood pressure. Fifteen male WKY and 15 male SHR were housed from 4 weeks of age up to 26 weeks. Al 4 weeks of age all SHR manifested a hypophosphatemia compared with age-matched WKY (F= 62, p<0.0003). At 5 weeks of age, the rats were divided into three diet groups: a control group, a group receiving 1.41% (wt/vol) KCI in drinking water, and a group receiving 2% (wt/vol) K2HPO4, KH2PO4 in drinking water. In the control (F=16.2, p<0.02) and KCI groups, (F=36.3, p<0.03), hypophosphatemia persisted throughout the study. The phosphate-supplemented diet normalized plasma phosphate level in SHR but did not change plasma phosphate level in WKY. As a consequence, no difference in plasma phosphate level between WKY and SHR was present in the group receiving additional phosphate from that time on (F= 1.2, p>0.41). The phosphate-supplemented diet significantly decreased systolic blood pressure in both strains. In phosphate-supplemented SHR, a significant decline in systolic blood pressure was observed from 20 weeks of age on (at 20 weeks of age: 222 ± 3 mm Hg for control SHR vs 198 ± 5 mm Hg for phosphate-supplemented SHR; p< 0.0003). In phosphate-supplemented WKY, a slight but significant decrease in systolic blood pressure was observed from 24 weeks of age on (at 25 weeks of age: 133 ± 1 mm Hg for control WKY vs 125 ± 1 mm Hg for phosphate-supplemented WKY; p<0.0004). The KCI diet had no significant effect on systolic blood pressure in either strain. Plasma electrolyte (sodium, potassium, calcium, and magnesium) levels were not influenced by the two diets throughout the study. We conclude that a high dietary phosphate intake, with concomitant normalization of hypophosphatemia, results in a decrease in blood pressure in SHR. The phosphate-induced hypotensive effect was not apparent until 15 weeks after initiation of the diet. The mechanism behind this effect remains to he elucidated. © Williams and Wilkins 1987. All Rights Reserved.