Cholesterol depletion modulates basal L-type Ca2+current and abolishes its β-adrenergic enhancement in ventricular myocytes

Abstract

Cholesterol is a primary constituent of the plasmalemma, including the lipid rafts/caveolae, where various G protein-coupled receptors colocalize with signaling proteins and channels. By manipulating cholesterol in rabbit and rat ventricular myocytes using methyl-β-cyclodextrin (MβCD), we studied the role of cholesterol in the modulation of L-type Ca2+currents ( ICa,L). MβCD was mainly dialyzed from BAPTA-containing pipette solution during whole cell clamp. In rabbit myocytes dialyzed with 30 mM MβCD for 10 min, a positive shift in membrane potential at half-maximal activation ( V0.5) from −8 to −2 mV developed and was associated with an increase in current density at positive potentials (42% at +20 mV vs. time-matched controls). Isoproterenol (ISO) increased ICa,Lapproximately threefold and caused a negative shift in V0.5in control cells, but it did not increase ICa,Lin MβCD-treated myocytes, nor did it shift V0.5. The effect of MβCD (10 or 30 mM) was concentration dependent: 30 mM MβCD suppressed the ISO-induced increase in ICa,Lmore effectively than 10 mM MβCD. MβCD dialysis also abolished the increase in ICa,Lelicited by forskolin or dibutyryl cAMP, but not that elicited by (−)BAY K 8644. External application of MβCD-cholesterol complex to rat myocytes attenuated the MβCD-mediated inhibition of the ISO-induced increase of ICa,L. Biochemical analysis confirmed that the myocytes′ cholesterol content was diminished by MβCD and increased by MβCD-cholesterol complex. Cholesterol thus appears to contribute to the regulation of basal ICa,Land β-adrenergic cAMP/PKA-mediated increases in ICa,L. We suggest that cholesterol affects the structural coupling between L-type Ca2+channels and adjacent regulatory proteins.