No abstract

Amino acid-derived isoindolines are synthetic compounds that were created with the idea of investigating their biological actions. The amino acid moiety was included on the grounds that it may help to avoid toxic effects. Recently, the isoindoline MDIMP was shown to inhibit both cardiac excitation-contraction coupling and voltage-dependent calcium channels. Here, we revealed that MDIMP binds preferentially to low-voltage-activated (LVA) channels. Using a holding potential of 290 mV, the following IC 50 values were found (in micromolars): .1000 (Ca V 2.3), 957 (Ca V 1.3), 656 (Ca V 1.2), 219 (Ca V 3.2), and 132 (Ca V 3.1). Moreover, the isoindoline also promoted both accelerated inactivation kinetics of high-voltage-activated Ca 2+ channels and a modest upregulation of Ca V 1.3 and Ca V 2.3. Additional data indicate that although MDIMP binds to the closed state of the channels, it has more preference for the inactivated one. Concerning Ca V 3.1, the compound did not alter the shape of the instantaneous currentvoltage curve, and substituting one or two residues in the selectivity filter drastically increased the IC 50 value, suggesting that MDIMP binds to the extracellular side of the pore. However, an outward current failed in removing the inhibition, which implies an alternative mechanism may be involved. The enantiomer (R)-MDIMP [methyl (R)-2-(1,3-dihydroisoindol-2-yl)-4methylpentanoate], on the other hand, was synthesized and evaluated, but it did not improve the affinity to LVA channels. Implications of these findings are discussed in terms of the possible underlying mechanisms and pharmacological relevance. SIGNIFICANCE STATEMENTWe have studied the regulation of voltage-gated calcium channels by MDIMP, which disrupts excitation-contraction coupling in cardiac myocytes. The latter effect is more potent in atrial than ventricular myocytes, and this could be explained by our results showing that MDIMP preferentially blocks lowvoltage-activated channels. Our data also provide mechanistic insights about the blockade and suggest that MDIMP is a promising member of the family of Ca 2+ channel blockers, with possible application to the inhibition of subthreshold membrane depolarizations.

Using the cut-open oocyte voltage clamp technique, we recorded ionic currents from channel complexes formed by a 1S /b 1a /stac3 with or without g 1 subunit, expressedin Xenopus oocytes. g 1 modified Ca V 1.1 voltage dependence of inactivation by decreasing the effective valence from 5.351.1e 0 (N=4) to 1.8750.2e 0 (N=4) and reducing the non-inactivating component of the ionic current following 10s pulses from $50% to $30%. In addition, we observed that g 1 coexpression did not affect voltage dependence of Ca V 1.1 activation (No g 1 : V half = 37.151.2mV, N=5, and with g 1 : V half = 40.051.5mV, N=4). However, when g 1 and a 2 d-1 were coexpressed, the voltage dependence of the Ca V 1.1 complex (a 1S /b 1a /a 2 d-1/g 1 /stac3) was facilitated. g 1 also strongly modulated the Ca V 1.2 channel isoform (a 1C / b 2b /a 2 d-1) by shifting the voltage dependence of channel inactivation by $20mV toward negative potentials. In conclusion, these studies have revealed the biophysical consequences of g 1 subunit association with two distinct human L-type channel isoforms in heterologous expression systems.