Effects of extracellular calcium ions, verapamil, and lanthanum on active and passive properties of canine cardiac purkinje fibers.

The effects of alteration of extracellular calcium ion concentration ([Ca++]o) were studied in isolated false tendons using microelectrode techniques. Several determinants of cellular excitability and conduction velocity were affected by extracellular calcium. Increasing [Ca++]o from 2 to 8 mM resulted in: (1) a progressive decrease in interelectrode conduction velocity (2) a 7-mV shift of the maximum upstroke velocity-membrane potential relation toward less negative potential, (3) an increase in rheobasic current, (4) a 14-mV shift of the voltage threshold for all-or-none depolarization to less negative potentials, (5) a 52% increase in internal longitudinal resistance per unit length, and (6) a 27% decrease in the capacitance filled by the foot of the action potential from 4.90 to 3.56 microF/cm2. Blockade of the slow inward current by Mn++ or verapamil did not alter the [Ca++]o-induced effects on the maximum upstroke velocity-membrane potential relation. Cable properties were determined during alteration of [Ca++]o in the presence of verapamil (3 X 10(-6) and 1 X 10(-5) M) or in the presence of La+++ (0.2 mM). Verapamil increased membrane resistance X unit length but did not affect internal longitudinal resistance per unit length. La+++ had no effects on either membrane resistance X unit length or internal longitudinal resistance per unit length. Verapamil did not block the increase in ri induced by elevation of [Ca++]o. However, no change in ri occurred during an increase of [Ca++]o when La+++ was present. The results suggest that [Ca++]o-induced changes in internal longitudinal resistance may occur by the influx of calcium ions through the Na+/Ca++ exchange mechanism.