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.