Electrically silent divalent cation entries in resting and active voltage-controlled muscle fibers.
Ca2+ is known to enter skeletal muscle at rest and during activity. Except for the well-characterized Ca2+ entry through L-type channels, pathways involved in these Ca2+ entries remain elusive in adult muscle. This study investigates Ca2+ influx at rest and during activity using the method of Mn2+ quenching of fura-2 fluorescence on voltage-controlled adult skeletal muscle cells. Resting rate of Mn2+ influx depended on external [Mn2+] and membrane potential. At -80 mV, replacement of Mg2+ by Mn2+ gave rise to an outward current associated with an increase in cell input resistance. Calibration of fura-2 response indicated that Mn2+ influx was too small to be resolved as a macroscopic current. Partial depletion of the sarcoplasmic reticulum induced by a train of action potentials in the presence of cyclopiazonic acid led to a slight increase in resting Mn2+ influx but no change in cell input resistance and membrane potential. Trains of action potentials considerably increased Mn2+ entry through an electrically silent pathway independent of L-type channels, which provided 24% of the global Mn2+ influx at +30 mV under voltage-clamp conditions. Within this context, the nature and the physiological role of the Ca2+ pathways involved during muscle excitation still remain open questions.
Physiologie Intégrative Cellulaire et Moléculaire, Université Lyon 1, Centre National de la Recherche Scientifique Unité Mixte de Recherche, 5123 Villeurbanne, France.
Céline
C
Berbey
Bruno
B
Allard
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002138:Calibration; D002413:Cations, Divalent / Q000378:metabolism*; D004553:Electric Conductivity; D016257:Fura-2 / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D012508:Sarcolemma / Q000187:drug effects / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism
D020746:Calcium Channels, L-Type; D002413:Cations, Divalent; D002104:Cadmium; D008274:Magnesium; D002118:Calcium; D016257:Fura-2