Molecular Determinants of Cardiac Ca2+ Channel Pharmacology

Abstract

Cardiac L-type Ca2؉ channels are multisubunit complexes composed of ␣ 1C , ␣ 2 ␦, and ␤ 2 subunits. We tested the roles of these subunits in forming a functional complex by characterizing the effects of subunit composition on dihydropyridine binding, its allosteric regulation, and the ability of dihydropyridines to inhibit channel activity.

Extracellular Interaction of the Voltage-dependent Ca2+ Channel α2δ and α1 Subunits

“…In analogous studies on the voltage-dependent Na ϩ channel, multiple domains within the carboxylterminal half of the skeletal muscle Na ϩ channel ␣ subunit were shown to be required for functional response of the coexpressed Na ϩ channel ␤ 1 subunit on inactivation kinetics (27). Association of the ␣ 2 ␦ subunit with the carboxyl-terminal half of the ␣ 1 subunit is consistent with the significant effects that we and others have measured of the ␣ 2 ␦ subunit on dihydropyridine binding affinity (28). The membrane spanning segments IIIS6 and IVS6 of the ␣ 1S subunit have recently been shown to contain amino acids critical for dihydropyridine binding (29), although the S5-S6 extracellular linkers of the III and IV repeats also confer dihydropyridine sensitivity (30).…”

Transmembrane Auxiliary Subunits of Voltage-dependent Ion Channels

“…In contrast, transient coexpression of the Ca 2ϩ channel ␣ 2 ␦ subunit in COS.M6 cells resulted in a significant increase in the affinity of the cardiac ␣ 1 subunit for dihydropyridines, suggesting that the ␣ 2 ␦ subunit causes a conformational change in the structure of the ␣ 1C subunit (30). In addition to being required for high affinity dihydropyridine binding, the ␣ 2 ␦ subunit was also shown to be essential for reconstitution of allosteric regulation by Ca 2ϩ , diltiazem, and (Ϫ)-D600 (30).…”

“…In addition to being required for high affinity dihydropyridine binding, the ␣ 2 ␦ subunit was also shown to be essential for reconstitution of allosteric regulation by Ca 2ϩ , diltiazem, and (Ϫ)-D600 (30). Involvement in the allosteric regulation by Ca 2ϩ ions is particularly interesting, as it suggests that either the ␣ 2 ␦ subunit modifies Ca 2ϩ binding sites on the ␣ 1 subunit or binds Ca 2ϩ ions itself possibly through sialic acid residues or extracellular negatively charged amino acids.…”

A potential site of functional modulation by protein kinase A in the cardiac Ca2+ channel α1C subunit

“…The minimal composition of the cardiac L-type channel is probably (XlC + a2/6 + ~2 [10][11][12][13]. The main, cqc, subunit of the channel purified from adult cardiac tissue is a poor substrate for PKA [14,15].…”