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 (A2C) caused an increase in sarcolemmal fluidity, and an increased [Ca2+]i. Thus, although sarcolemmal cholesterol depletion as well as A2C 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 A2C is incorporated in the sarcolemma.