Altered inactivation of Ca2+ current and Ca2+ release in mouse muscle fibers deficient in the DHP receptor gamma1 subunit.
Functional impacts of the skeletal muscle-specific Ca2+ channel subunit gamma1 have previously been studied using coexpression with the cardiac alpha1C polypeptide in nonmuscle cells and primary-cultured myotubes of gamma1-deficient mice. Data from single adult muscle fibers of gamma-/- mice are not yet available. In the present study, we performed voltage clamp experiments on enzymatically isolated mature muscle fibers of the m. interosseus obtained from gamma+/+ and gamma-/- mice. We measured L-type Ca2+ inward currents and intracellular Ca2+ transients during 100-ms step depolarizations from a holding potential of -80 mV. Ratiometric Ca2+ transients were analyzed with a removal model fit approach to calculate the flux of Ca2+ from the sarcoplasmic reticulum. Ca2+ current density, Ca2+ release flux, and the voltage dependence of activation of both Ca2+ current and Ca2+ release were not significantly different. By varying the holding potential and recording Ca2+ current and Ca2+ release flux induced by 100-ms test depolarizations to +20 mV, we studied quasi-steady-state properties of slow voltage-dependent inactivation. For the Ca2+ current, these experiments showed a right-shifted voltage dependence of inactivation. Importantly, we could demonstrate that a very similar shift occurred also in the inactivation curve of Ca2+ release. Voltages of half maximal inactivation were altered by 16 (current) and 14 mV (release), respectively. Muscle fiber bundles, activated by elevated potassium concentration (120 mM), developed about threefold larger contracture force in gamma-/- compared with gamma+/+. This difference was independent of the presence of extracellular Ca2+ and likely results from the lower sensitivity to voltage-dependent inactivation of Ca2+ release. These results demonstrate a specific alteration of voltage-dependent inactivation of both Ca2+ entry and Ca2+ release by the gamma1 subunit of the dihydropyridine receptor in mature muscle fibers of the mouse.
Ursu
Daniel
D
University of Ulm, Dept. of Applied Physiology, Albert-Einstein-Allee 11, D-89069 Ulm, Germany.
Schuhmeier
Ralph Peter
RP
Freichel
Marc
M
Flockerzi
Veit
V
Melzer
Werner
W
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000172:deficiency / Q000502:physiology*; D002478:Cells, Cultured; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D021122:Protein Subunits / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000502:physiology; D013314:Stress, Mechanical; D013329:Structure-Activity Relationship
D020746:Calcium Channels, L-Type; D021122:Protein Subunits; D011188:Potassium; D002118:Calcium