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.
Berbey
Céline
C
Physiologie Intégrative Cellulaire et Moléculaire, Université Lyon 1, Centre National de la Recherche Scientifique Unité Mixte de Recherche, 5123 Villeurbanne, France.
Allard
Bruno
B
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