Effect of Chlordecone (Kepone®) on Calcium Transport Mechanisms in Rat Heart Sarcoplasmic Reticulum*

Previous studies from our laboratory have indicated that chlordecone (Kepone®, CD), an organochlorine insecticide, inhibited cardiac sodium pump activity and catecholamine uptake suggesting that CD may interfere with cardiac function. Sarcoplasmic reticulum (SR) calcium pump has an important role in myocardial contraction and relaxation, besides Na + transport. Since CD interferes with cardiac Na + ion translocases, we have studied CD effects on cardiac SR calcium pump activity. Experiments were carried out both in vitro and in vivo. SR was isolated from heart ventricles of male Sprague‐Dawley rats. Cardiac SR Ca 2+ ‐ATPase, 45 Ca‐uptake and cAMP as well as calmodulin (CaM) dependent protein phosphorylation were measured. Ca 2+ ‐ATPase was differentiated into low affinity and high affinity forms by measuring the activity using 50 and 0.7 μM free Ca 2+ respectively. CD in vitro inhibited 45 Ca‐uptake by SR in a concentration dependent manner with an IC50 value of 7 μM and SR 45 Ca‐uptake was totally inhibited at 20–30 μM CD. In agreement with this, both high affinity and low affinity Ca 2+ ‐ATPases, which are involved in Ca 2+ transport across membranes, were also inhibited by CD in a concentration dependent manner with IC50 values of 0.7 and 3.2 μM respectively. Both Ca 2+ ‐ATPase and 45 Ca‐uptake by cardiac SR were significantly lower in rats treated with CD (25, 50 or 75 mg/kg) when compared to control rats. cAMP as well as CaM significantly elevated the 32 P‐binding to SR proteins in vitro to about 70–80%. In the presence of CD, this 32 P‐binding was reduced, however, not concentration dependent. In agreement with in vitro studies, 32 P‐bound to proteins was significantly lowered in rats treated with CD. SDS‐polyacrylamide gel electrophoresis of the cardiac SR revealed the presence of at least 30 comassie blue‐stainable bands with mobilities corresponding to molecular weights ranging from 9 to 120 kDa using 15% acrylamide gels. Autoradiographs from samples incubated in the presence of cAMP or CaM indicated 32 P‐incorporation in 7 bands. Of these, bands corresponding to about 24 kDa and adjacent lower molecular weights decreased in their intensity by CD in vitro as well as in vivo. These results indicate that CD treatment may be reducing SR calcium transport mechanisms by altering phosphorylation of a number of proteins including phospholamban in rat cardiac SR.