The control of tonic tension by membrane potential and intracellular sodium activity in the sheep cardiac Purkinje fibre

Do calcium-activated potassium channels exist in the heart?

Changes of intracellular Ca concentration in cardiac muscle have significant effects on transmembrane currents and many of these effects can be accounted for by postulating the existence of Ca-activated K channels in the heart. However, the evidence that such channels exist is equivocal. This is partly because of technical problems, for example the difficulty of identifying an individual ionic current amongst the many currents that exist in the heart. An additional problem, however, is posed by the fact that other currents may also be modulated by Ca ions. It is important therefore to distinguish between these currents and those caused by Ca-activated, K-specific channels. In this review we consider the evidence for Ca activated currents in the heart and, in particular, we discuss whether or not these currents are carried exclusively by K ions.

Cellular calcium fluctuations in mammalian heart: direct evidence from noise analysis of aequorin signals in Purkinje fibers

Indirect evidence suggests that fluctuations, or oscillations, in the intracellular free calcium concentration [( Ca2+]i) can occur spontaneously in intact cardiac preparations, but such [Ca2+]i fluctuations have never been demonstrated directly. We used the bioluminescent Ca2+-sensitive protein aequorin to detect fluctuations in the [Ca2+]i in canine cardiac Purkinje fibers. Noise analysis of the aequorin luminescence reveals prominent peaks of power density at frequencies of 1-4 Hz; these peaks become larger and shift to higher frequencies as the [Ca2+]i increases. Caffeine and ryanodine abolish the [Ca2+]i fluctuations, suggesting that Ca2+ release and uptake by the sarcoplasmic reticulum generate these events. When [Ca2+]i fluctuations are present, less tension is produced at any given level of mean aequorin luminescence. Thus, [Ca2+]i fluctuations will undermine attempts to relate [Ca2+]i and force in intact myocardium.

The role of intracellular sodium activity in the anti-arrhythmic action of local anaesthetics in sheep Purkinje fibres

The effects of lidocaine have been examined on the arrhythmogenic transient inward current (ITI) in voltage-clamped sheep cardiac Purkinje fibres. Tension and intracellular Na activity (aiNa) were measured simultaneously. The addition of lidocaine (200-300 microM) produced an immediate decrease of inward holding current and a gradual fall of aiNa. The relative magnitudes of the changes of current and aiNa were shown to be consistent with the outward shift of current representing principally a reduction of inward Na current. The Na pump was inhibited by reducing the external Rb concentration in a K-free solution. This produced an after-contraction and transient inward current (ITI) along with a rise of aiNa. The subsequent addition of lidocaine decreased the magnitude of ITI and the after-contraction while decreasing aiNa. Tetrodotoxin (TTX) had qualitatively similar effects to lidocaine on inward holding current, aiNa, ITI and the after-contraction. When aiNa was changed by (i) lidocaine, (ii) TTX or (iii) small changes of external Rb concentration, a hysteresis was seen in the relationship between aiNa and ITI or after-contraction. The hysteresis was similar to that previously found between aiNa and contraction (Eisner, Lederer & Vaughan-Jones, 1981). Despite this hysteresis, neither lidocaine nor TTX affected the relationship between magnitudes of ITI and the after-contraction. It is suggested that the fall of aiNa is a major factor in the reduction of ITI by lidocaine. These results are discussed in relation to the anti-arrhythmic actions of lidocaine.