The Skeletal L-type Ca2+ Current Is a Major Contributor to Excitation-coupled Ca2+ entry

The term excitation-coupled Ca(2+) entry (ECCE) designates the entry of extracellular Ca(2+) into skeletal muscle cells, which occurs in response to prolonged depolarization or pulse trains and depends on the presence of both the 1,4-dihydropyridine receptor (DHPR) in the plasma membrane and the type 1 ryanodine receptor in the sarcoplasmic reticulum (SR) membrane. The ECCE pathway is blocked by pharmacological agents that also block store-operated Ca(2+) entry, is inhibited by dantrolene, is relatively insensitive to the DHP antagonist nifedipine (1 microM), and is permeable to Mn(2+). Here, we have examined the effects of these agents on the L-type Ca(2+) current conducted via the DHPR. We found that the nonspecific cation channel antagonists (2-APB, SKF 96356, La(3+), and Gd(3+)) and dantrolene all inhibited the L-type Ca(2+) current. In addition, complete (>97%) block of the L-type current required concentrations of nifedipine >10 microM. Like ECCE, the L-type Ca(2+) channel displays permeability to Mn(2+) in the absence of external Ca(2+) and produces a Ca(2+) current that persists during prolonged ( approximately 10-second) depolarization. This current appears to contribute to the Ca(2+) transient observed during prolonged KCl depolarization of intact myotubes because (1) the transients in normal myotubes decayed more rapidly in the absence of external Ca(2+); (2) the transients in dysgenic myotubes expressing SkEIIIK (a DHPR alpha(1S) pore mutant thought to conduct only monovalent cations) had a time course like that of normal myotubes in Ca(2+)-free solution and were unaffected by Ca(2+) removal; and (3) after block of SR Ca(2+) release by 200 microM ryanodine, normal myotubes still displayed a large Ca(2+) transient, whereas no transient was detectable in SkEIIIK-expressing dysgenic myotubes. Collectively, these results indicate that the skeletal muscle L-type channel is a major contributor to the Ca(2+) entry attributed to ECCE.