The concentrations of ionic and total calcium in plasma of the spontaneously hypertensive rat.

The levels of ionic calcium in whole blood obtained from male spontaneously hypertensive rats (SHR) were slightly decreased compared with normotensive animals during the early stages of blood pressure elevation and in older (28-32 weeks) animals with established hypertension. During these intervals the total calcium concentrations of plasma were not significantly different although the ratios of ionic to total calcium were similar between SHR and control groups. In contrast with male and female SHR showed ionic calcium levels similar to normotensive control rats whereas total calcium concentrations in plasma increased with age and blood pressure elevation. The addition of calcium to plasma in 4- to 5-week-old SHR and normotensive rats revealed a buffering action in terms of the elevation in ionic calcium concentrations. The buffering capacity of plasma from normotensive animals did not change markedly but appeared to be greatly reduced in SHR by 17-25 weeks of age. Similarly, significant elevations in ionic calcium but not total calcium were observed following the injection of calcium in SHR compared with normotensive controls. The minor reductions in ionic calcium levels in blood seen in male SHR and the lowered ionic:total calcium ratios in female SHR may indicate enhanced binding or complexing of exogenous calcium. In view of these findings, the apparent reduction in in vitro and in vivo binding or complexing of exogenous calcium may reflect a decreased availability of sites for association with the calcium ion. The poor correlation between changes in blood pressure and ionic calcium suggested, however, that the disturbance in ionic or total calcium levels of plasma was not a primary causal factor in the development of hypertension.