High selectivity of calcium channels in single dialysed heart cells of the guinea-pig.
Membrane currents and action potentials were recorded in single ventricular cells obtained from guinea-pig hearts by enzymatic dissociation. Ca2+ channel currents carried by Ba2+ or Ca2+ were recorded with a suction pipette (5-10 microns diameter) for voltage clamp and internal dialysis. Currents through Na+, K+ and non-selective monovalent cation channels were suppressed by suitable holding potentials and external and internal solutions. The dialysis method allowed exchange within minutes of alkali metal cations (e.g. Cs+) and small molecules (e.g. quaternary derivatives of lidocaine and verapamil). Nevertheless, Ca2+ channels remained functional for considerable periods, typically 20 min and sometimes more than 1 h. With Ba2+ outside and Cs+ inside, current flow through Ca2+ channels changed from inward to outward at strongly positive levels beyond a clear-cut reversal potential Erev. Several methods for defining Erev were in close agreement: (1) zero-crossing of leak-subtracted peak current, (2) inversion of time-dependent current changes during channel activation or inactivation, (3) inversion of drug-sensitive current as defined by channel blockers such as Cd2+ or D-600. Erev varied with external Ba2+ or internal Cs+. Erev increased by 29 mV per 10-fold increase in Ba2+. Interpreted with constant-field theory, Erev values correspond to PBa/PCs of approximately 1360. With 5 mM-Ca2+ outside and 151 mM-Cs+ inside, Ca2+ channel current reversed near + 75 mV, corresponding to PCa/PCs approximately 6000. Earlier measurements of Erev (Lee & Tsien, 1982) suggest that PCa/PK greater than 1000. At strongly positive membrane potentials where channel activation is maximal, the Ca2+ channel current-voltage relationship is strongly non-linear, with conductance increasing on either side of an inflexion point near Erev. Activation of inward or outward currents through Ca2+ channels follows a sigmoid time course, as expected if activation were a multi-step process.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001464:Barium / Q000378:metabolism; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002586:Cesium / Q000378:metabolism; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008012:Lidocaine / Q000031:analogs & derivatives / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism
D007473:Ion Channels; D002104:Cadmium; D002586:Cesium; C012647:QX-314; D001464:Barium; D008012:Lidocaine; D012964:Sodium; D002118:Calcium