Characterization and localization of two ion-binding sites within the pore of cardiac L-type calcium channels.
L-type Ca channels from porcine cardiac sarcolemma were incorporated into planar lipid bilayers. We characterized interactions of permeant and blocking ions with the channel's pore by (a) studying the current-voltage relationships for Ca2+ and Na+ when equal concentrations of the ions were present in both internal and external solutions, (b) testing the dose-dependent block of Ba2+ currents through the channels by internally applied cadmium, and (c) examining the dose and voltage dependence of the block of Na+ currents through the channels by internally and externally applied Ca2+. We found that the I-V relationship for Na+ appears symmetrical through the origin when equal concentrations of Na+ are present on both sides of the channel (gamma = 90 pS in 200 mM NaCl). The conductance for outward Ca2+ currents with 100 mM Ca2+ on both sides of the channel is approximately 8 pS, a value identical to that observed for inward currents when 100 mM Ca2+ was present outside only. This provides evidence that ions pass through the channel equally well regardless of the direction of net flux. In addition, we find that internal Cd2+ is as effective as external Cd2+ in blocking Ba2+ currents through the channels, again suggesting identical interactions of ions with each end of the pore. Finally, we find that micromolar Ca2+, either in the internal or in the external solution, blocks Na+ currents through the channels. The affinity for internally applied Ca2+ appears the same as that for externally applied Ca2+. The voltage dependence of the Ca(2+)-block suggests that the sites to which Ca2+ binds are located approximately 15% and approximately 85% of the electric field into the pore. Taken together, these data provide direct experimental evidence for the existence of at least two ion binding sites with high affinity for Ca2+, and support the idea that the sites are symmetrically located within the electric field across L-type Ca channels.
Rosenberg
R L
RL
Department of Pharmacology, University of North Carolina, Chapel Hill 27599.
Chen
X H
XH
D000818:Animals; D001464:Barium / Q000378:metabolism*; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002104:Cadmium / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D006321:Heart / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D012508:Sarcolemma / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D012964:Sodium / Q000378:metabolism*; D013552:Swine
D015220:Calcium Channels; D002104:Cadmium; D001464:Barium; D012964:Sodium