The relation between membrane potential, membrane currents and activation of contraction in ventricular myocardial fibres

TLDR: Membrane currents and contractile responses have been measured in ventricular myocardial preparations under voltage clamp conditions under Voltage clamp conditions in mice with E.coli A.1.

Abstract: 1. Membrane currents and contractile responses have been measured in ventricular myocardial preparations under voltage clamp conditions. 2. In Tyrode solution, steady-state contraction was obtained only after 5-8 depolarizations to a given potential level. The threshold of steady-state tension was identical to the potential where the calcium inward current, ICa, was activated (about −35 mV). Both thresholds were shifted in the same direction along the voltage axis and by the same amount by changing [Ca]o or [Na]o. Maximum tension was obtained at inside positive potentials. 3. The time courses of steady-state tension and of activation of ICa were different by more than one order of magnitude in Tyrode solution. But in order to achieve any appreciable steady-state tension, ICa had to flow during several identical depolarizations. Tension decreased again at potentials above ECa. This suggests that calcium inward current must flow in order to fill intracellular calcium stores from which calcium can be released by an unknown mechanism. 4. The ability of a fibre bundle to contract again after a preceding twitch is greatly dependent on the membrane potential between two equal depolarizations. In Tyrode solutions with 1·8 and 7·2 m M-CaCl2 half restoration of this ability occurred at −45 ± 3 mV (± S.E. of mean) and −23 ± 4 mV, respectively. 5. In sodium-free bathing solutions, steady-state tension was attained upon the first depolarization provided ICa was activated. Furthermore, at different potentials, the time courses of activation of tension and of activation of ICa were identical, i.e. tension reached its maximum when ICa was fully activated. This suggests that in sodium-free solutions the flow of calcium ions into the fibre directly activates contraction.