Abstract
Cholesterol is a main structural component in the lipid raft in caveolae that are involved in cAMP-dependent increase of voltage-gated Na+ current in cardiac myocytes. We showed previously that intracellular methyl-beta-cyclodextrin (MβCD) that deprives cholesterol from the membrane accelerated run-down of L-type calcium current (ICaL) and suppressed isoproterenol (ISO)-induced increase of ICaL in isolated rabbit ventricular myocytes. Here we examined further the effects of MβCD on ICaL increase produced by forskolin (Forsk) or cAMP analogue. Whole-cell ICaL were recorded by applying double pulses in the presence of Na+, Cs+ and 1.8 mM Ca2+ in the superfusate and CsCl and BAPTA in the pipette solutions. Pipette solutions that contained no (control) or 30 mM MβCD were dialyzed 10 minutes before the challenge of following test drugs. The ratios of the maximal ICaL amplitude before and after the challenge of the drug in control and in the presence of MβCD were: ISO (1 μM) 2.9±0.1 (n=21) in control and 1.0±0.2 (n=12) in MβCD; Forsk (10 μM) 3.0±0.4 (n=8) in control and 1.3±0.2 (n=8) in MβCD; dibutyl-cAMP (3 mM) 3.1±0.4 (n=9) in control and 1.4±0.2 (n=9) in MβCD. These results indicated unequivocally that cAMP-induced increase of cardiac ICaL was suppressed by intracellular MβCD. It suggests that cholesterol is indispensable for the cAMP-mediated augmentation of cardiac ICaL either in the phosphorylation process or in the normal function of the phosphorylated channels. [Jpn J Physiol 54 Suppl:S106 (2004)]
