A surface potential change in the membranes of frog skeletal muscle is associated with excitation-contraction coupling.
1. Voltage changes and intramembrane charge movements in the transverse tubule membranes (T-system) of frog fast twitch muscle fibres were compared using the potentiometric dye WW-375 and a Vaseline-gap voltage clamp. As shown previously, the potentiometric dye reports a dynamic surface potential change that occurs on the myoplasmic face of the T-system membranes when the macroscopic potential applied across the surface membrane exceeds the mechanical threshold (about -60 mV). 2. The voltage dependence of the extra surface potential change and charge movement were found to be similar. Both activated with a sigmoid voltage dependence centred around -35 to -40 mV, and saturated at voltages above 0 mV. Both processes inactivated upon sustained depolarization, with a mid-point for inactivation of -40 mV. 3. Pharmacological agents which alter charge movement and excitation-contraction (E-C) coupling altered the non-linear surface potential change in a parallel manner. Perchlorate, which potentiates charge movement and E-C coupling, slowed the activation and deactivation of both charge movement and the non-linear surface potential change at voltages above -40 mV, and shifted the voltage dependence of both processes by 13 14 mV to more negative voltages. Dantrolene, which depresses charge movement and E-C coupling, shifted the voltage dependence of both processes to more positive voltages. Nifedipine, which suppresses charge movement and E-C coupling, reduced the magnitude of both charge movement and the non-linear surface potential change. 4. The non-linear surface potential change remained after the sarcoplasmic reticulum (SR) was depleted of Ca2+, suggesting that it is not a consequence of Ca2+ release. 5. These results suggest that the non-linear surface potential change is closely associated with movements of the voltage sensor (dihydropyridine (DHP) receptor) that control E-C coupling and/or signal transduction across the triadic junction. We propose that the movement of charged intramembrane domains of the DHP receptor which generate charge movement drive a subsequent movement of charged intracellular molecular domains that move within about 1 nm of the T-system membrane to generate a measurable change in surface charge. For example, the postulated mobile surface charges could be on an intracellular domain of the voltage sensor or closely associated protein, or could be a charged molecular domain of a protein that associates/dissociates with T-system membrane or DHP receptor during E-C coupling.
Jong
D S
DS
Department of Molecular and Cellular Physiology, University of Cincinnati, College of Medicine, OH 45267-0576, USA.
Stroffekova
K
K
Heiny
J A
JA
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004396:Coloring Agents; D003620:Dantrolene / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D008958:Models, Molecular; D009119:Muscle Contraction / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D009125:Muscle Relaxants, Central / Q000494:pharmacology; D018482:Muscle, Skeletal / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010472:Perchlorates / Q000494:pharmacology; D011199:Potentiometry; D011892:Rana catesbeiana; D012236:Rhodanine / Q000031:analogs & derivatives; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D017670:Sodium Compounds / Q000494:pharmacology; D013499:Surface Properties
D002121:Calcium Channel Blockers; D004396:Coloring Agents; D009125:Muscle Relaxants, Central; D010472:Perchlorates; D017670:Sodium Compounds; C016902:WW 375; D012236:Rhodanine; C031068:sodium perchlorate; D003620:Dantrolene; D009543:Nifedipine; D002118:Calcium