Arachidonic acid inhibition of L-type calcium (Ca _V1.3b) channels varies with accessory Ca _Vβ subunits

Abstract

Arachidonic acid (AA) inhibits the activity of several different voltage-gated Ca ²⁺ channels by an unknown mechanism at an unknown site. The Ca ²⁺ channel pore-forming subunit (Ca _Vα ₁) is a candidate for the site of AA inhibition because T-type Ca ²⁺ channels, which do not require accessory subunits for expression, are inhibited by AA. Here, we report the unanticipated role of accessory Ca _Vβ subunits on the inhibition of Ca _V1.3b L-type (L-) current by AA. Whole cell Ba ²⁺ currents were measured from recombinant channels expressed in human embryonic kidney 293 cells at a test potential of −10 mV from a holding potential of −90 mV. A one-minute exposure to 10 µM AA inhibited currents with β _1b, β ₃, or β ₄ 58, 51, or 44%, respectively, but with β _2a only 31%. At a more depolarized holding potential of −60 mV, currents were inhibited to a lesser degree. These data are best explained by a simple model where AA stabilizes Ca _V1.3b in a deep closed-channel conformation, resulting in current inhibition. Consistent with this hypothesis, inhibition by AA occurred in the absence of test pulses, indicating that channels do not need to open to become inhibited. AA had no effect on the voltage dependence of holding potential–dependent inactivation or on recovery from inactivation regardless of Ca _Vβ subunit. Unexpectedly, kinetic analysis revealed evidence for two populations of L-channels that exhibit willing and reluctant gating previously described for Ca _V2 channels. AA preferentially inhibited reluctant gating channels, revealing the accelerated kinetics of willing channels. Additionally, we discovered that the palmitoyl groups of β _2a interfere with inhibition by AA. Our novel findings that the Ca _Vβ subunit alters kinetic changes and magnitude of inhibition by AA suggest that Ca _Vβ expression may regulate how AA modulates Ca ²⁺-dependent processes that rely on L-channels, such as gene expression, enzyme activation, secretion, and membrane excitability.