Tl+-ions: comparison on the effects on the slow inward current and contractility of ventricular tissues.

Slow Inward Current, Thallium Intoxication, Voltage Clamp, Available Intracellular Energy To conclude our investigation of thallium effects on cardiac tissues, we studied the slow inward current of sheep cardiac Purkinje fibres exposed to 10“7 to 105 m Tl+ for extended periods of up to 80 min. Our previous results had suggested a possible involvement of the slow inward current during thallium intoxication: a) the modification of contractility staircases observed during thallium exposure, b) action potential recordings o f ventricular muscle, c) changes in spontaneous beating in sino-atrial preparations. The thallium levels chosen were between those yielding strong positive inotropic transients and those producing a marked long­ term decay of contraction force. The slow inward current was measured using a conventional two-microelectrode-technique and the standard voltage clamp protocol for this current system. The experimental work was restricted to the determination of dx , the kinetics of activation o f the slow inward current and of /Si , the current voltage relation of the current system. This was necessary since the effects of thallium were known to be short-lived and therefore frequent repeat runs of the voltage clamp program had to be performed in order to obtain the time courses o f possible transient changes. The results showed that the slow inward current was first increased and then declined at the low concentration of 10"7 M Tl+. At 10“5 m Tl+ the initial increase was smaller, whereas the decay of the slow inward current proceeded to lower values. Comparison with contractility measure­ ments at the same concentrations of thallium showed a distinct parallelism between changes of the slow inward current and myocardial contractility. Despite this apparent relationship, we do not conclude that the contractile events are primarily a result of changes of the slow inward current, since thallium does not seem to specifically alter the parameters of the slow inward current at the membrane level.