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

In this study the relationship between sarcolemmal free cholesterol content and intracellular calcium ion concentration ([Ca2+](i)) was explored. In cultured neonatal rat cardiomyocytes the cellular free cholesterol content was modulated by treatment with liposomes. Using cholesterol-rich or cholesterol-free liposomes, sarcolemmal free cholesterol content was raised or diminished, respectively. An increased sarcolemmal free cholesterol content resulted in a decreased sarcolemmal fluidity, whereas cholesterol depletion resulted in an increase in sarcolemmal 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-octylcyclopropyl)-octanoate (A(2)C) caused an increase in sarcolemmal fluidity, and an increased [Ca2+](i). Thus, although sarcolemmal cholesterol depletion as well as A(2)C treatment increased sarcolemmal fluidity, their effects on [Ca2+](i) are opposite. These results indicate that 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 sarcolemma is enriched with cholesterol, (ii) most likely a stimulated Ca2+-ATPase activity when the sarcolemma is depleted of cholesterol, and (iii) inhibited Na+/Ca2+ exchanger activity when A(2)C is incorporated in the sarcolemma. (C) 1994 Academic Press, Inc.