Strom-Spannungsbeziehungen von Purkinje-Fasern bei verschiedenen extracellul�ren Calcium-Konzentrationen und unter Adrenalineinwirkung

Abstract

Voltage-current curves were obtained for short Purkinje fibres of the sheep heart. The bathing solution was completely Na-free and its Ca-concentration was varied (0, 1.8, 7.2 mM). To hold the membrane at a potential difference near 0 mV less current was necessary from an external source if the Ca-concentration was high (Fig. 2). When a square pulse of depolarizing current was applied in Ca-free solution the electrotonic potentials approached their steady-state values with a time-course consistent with capacitative filling. In the presence of Ca-ions the potential-time course showed two steps (Fig. 1). The second step had a “threshold” in the range 30–40 mV (inside negative), with an accelerated rise and an overshoot before reaching a steady potential level. Adrenaline increased the amplitude of the second step if the extracellular Ca-concentration was normal (1.8 mM) or high (7.2 mM), but had no effect on electrotonic potentials if added to a Ca-free solution. A transitory inward Ca current as a consequence of strong depolarizations is thought to provide the most probable explanation for the experimental findings. A decrease of K conductance by Ca ions cannot be excluded, but it seems improbable on the ground of various arguments. Adrenaline is thought to increase Ca influx through a depolarized membrane.