Regulation of Rabbit Erythrocyte Ca2+-Pump Sensitivity to Calmodulin in Experimental Hyperlipidemia

Intracellular free calcium activity is in part determined by a calmodulin-regulated plasma membrane Ca(2+)-pump. Since changes in Ca2+ permeability have been implicated in atherosclerotic plaque formation, we initiated a lipid hyperalimentation protocol during which we measured various erythrocyte calcium flux parameters and early atheroma development. Adolescent New Zealand White rabbits were fed a diet with 0.5% cholesterol and 2.5% lard over a 3-month period. Plasma cholesterol and triacylglycerols increased on average 18.7- and 13.9-fold respectively, while erythrocyte membrane cholesterol content decreased 18% and total phospholipids by 54%. After 3 months of lipid hyperalimentation, 22% of the aortic arch was covered with large, early-stage, raised atheroma. Basal and calmodulin-activated (Ca2+ + Mg2+)-ATPase activities in erythrocyte membranes increased by 31% and 123%, respectively at 2 months, with a concomitant increase in calmodulin affinity (Km) from 15.6 to 4.2 nM. These differences were transient on account of changes in the control animals which exhibited a slowly developing sensitivity to calmodulin during maturation. Basal Ca2+ transport and passive Ca2+ permeability increased about 7-fold during the hyperlipidemic phase. This suggests that overt hyperlipidemia, leading to atherosclerotic plaque development, alters plasma membrane Ca2+ regulatory mechanisms including passive Ca2+ permeability. The changes in enzymatic function, membrane composition, and Ca2+ permeability seen in this red cell model system may be a reflection of early changes in cells that are directly involved in the development of atherosclerotic plaques.