Decreased content of integral membrane calcium-binding protein (IMCAL) in tissues of the spontaneously hypertensive rat.

Prior studies report that plasma membranes of the spontaneously hypertensive rat (Okamoto-Aoki strain) bind less calcium than do the corresponding preparations from Wistar Kyoto controls. The possibility that the differences result from a decrease in the content of integral membrane calcium-binding protein (IMCAL) was explored by the application of immunoassays with polyclonal antisera and a mouse monoclonal antibody. IMCAL binds calcium with relatively high affinity, and its content in many rat tissues is regulated by vitamin D and the level of dietary calcium. Immunoassays of tissue IMCAL demonstrate significant reductions in content in the erythrocyte ghost, intestinal mucosa, kidney, heart, testis, and liver of the spontaneously hypertensive rat as compared to the control strains. The decreases are observed both at 4-5 weeks of age, before the onset of severe hypertension and at 8-9 weeks in the presence of severe hypertension. Moreover, the magnitude of the decrease in erythrocyte IMCAL can account for much of the decrease in the calcium-binding capacity of erythrocyte membranes reported by others. The results are especially significant because an abnormality in the membrane binding of calcium and in the regulation of cytosolic calcium ion concentration could underlie the pathogenesis of the hypertension.