Dual actions of calmodulin on the Ca²⁺-dependent regulation of cardiac L-type Ca²⁺ channel

Abstract

Our previous study shows that calmodulin (CaM) Ca²⁺-dependently induces channel activity in the inside-out patch mode. Ca²⁺ channel activity induced by CaM increases with the increment of Ca²⁺ concentration ([Ca²⁺])in the range of 0∼300nM, but rapidly disappears at [Ca²⁺]>500nM. In the present study, we used CaM mutants, in which four Ca²⁺-binding sites were differently mutated, to investigate Ca²⁺-dependent actions of CaM on the L-type Ca²⁺channel in guinea pig ventricular myocytes. We found that increment of [Ca²⁺] from 80nM to 1μM had no effect on Ca²⁺ activity induced by CaM₁₂₃₄. However, Increment of [Ca²⁺] from <10nM to 1μM significantly increased channel activity induced by CaM₁₂. On the other hand, channel activity induced by CaM₃₄ disappeared when [Ca²⁺] was increased to 1μM. Furthermore, channel activity induced by CaM₁ and CaM ₂ was not inactivated by 1μM [Ca²⁺]. These results indicate that the Ca²⁺-dependent regulation of Ca²⁺ channel activity is mediated by CaM. CaM bifurcates the Ca²⁺-dependent facilitation and inactivation through its N and C lobes: N lobe (containing 1st and 2nd Ca²⁺ binding sites) is responsible for Ca²⁺-dependent inactivation, while C lobe (containing 3th and 5th Ca²⁺ binding sites) for Ca²⁺-dependent facilitation. [Jpn J Physiol 55 Suppl:S131 (2005)]