THE EFFECT OF SARCOLEMMAL CHOLESTEROL CONTENT ON INTRACELLULAR CALCIUM-ION CONCENTRATION IN CULTURED CARDIOMYOCYTES

In this study the relationship between sarcolemmal free cholesterol co ntent and intracellular calcium ion concentration ([Ca2+](i)) was expl ored. In cultured neonatal rat cardiomyocytes the cellular free choles terol content was modulated by treatment with liposomes. Using cholest erol-rich or cholesterol-free liposomes, sarcolemmal free cholesterol content was raised or diminished, respectively. An increased sarcolemm al free cholesterol content resulted in a decreased sarcolemmal fluidi ty, whereas cholesterol depletion resulted in an increase in sarcolemm al fluidity. Cholesterol enrichment was associated with an increased [ Ca2+](i), while cholesterol depletion resulted in a decreased [Ca2+](i ). The membrane mobilizing agent 2-(2-methoxyethoxy)ethyl 8-(cis-2-c-o ctylcyclopropyl)-octanoate (A(2)C) caused an increase in sarcolemmal f luidity, and an increased [Ca2+](i). Thus, although sarcolemmal choles terol depletion as well as A(2)C treatment increased sarcolemmal fluid ity, their effects on [Ca2+](i) are opposite. These results indicate t hat the effect of sarcolemmal free cholesterol content on [Ca2+](i) is not mediated by sarcolemmal fluidity. The mechanisms responsible for the observed results are: (i) activated Ca2+ channels when the sarcole mma is enriched with cholesterol, (ii) most likely a stimulated Ca2+-A TPase activity when the sarcolemma is depleted of cholesterol, and (ii i) inhibited Na+/Ca2+ exchanger activity when A(2)C is incorporated in the sarcolemma. (C) 1994 Academic Press, Inc.