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

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Abstract
The levels of ionic Ca 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 wk) animals with established hypertension. During these intervals the total Ca concentrations of plasma were not significantly different although the ratios of ionic to total Ca were similar between SHR and control groups. In contrast with males, female SHR showed ionic Ca levels similar to normotensive control rats whereas total Ca concentrations in plasma increased with age and blood pressure elevation. The addition to Ca to plasma in 4-5 wk old SHR and normotensive rats revealed a buffering action in terms of the elevation in ionic Ca concentrations. The buffering capacity of plasma from normotensive animals did not change markedly but appeared to be greatly reduced in SHR by 17-25 wk of age. Significant elevations in ionic Ca but not total Ca were observed following the injection of Ca in SHR compared with normotensive controls. The minor enhanced binding for complexing of endogenous Ca. The apparent reduction in in vitro and in vivo binding or complexing of exogenous Ca may reflect a decreased availability of sites for association with the Ca ion. The poor correlation between changes in blood pressure and ionic Ca suggested that the disturbance in ionic or total Ca levels of plasma was not a primary causal factor in the development of hypertension.