Use-dependent reduction and facilitation of Ca2+ current in guinea-pig myocytes

1. Action potentials, calcium currents (iCa) and cell contraction have been recorded from single guinea-pig myocytes during periods of stimulation from rest. Voltage clamp was carried out using a single microelectrode. Cell contraction was measured optically. All experiments were performed at 18-22 degrees C. 2. An inverse relationship was observed between cell contraction and action potential duration or iCa. Mixed trains of action potentials and voltage clamp pulses preserved this relationship. Long voltage clamp pulses induced negative 'staircases' of iCa and positive 'staircases' of cell contraction. A facilitation of iCa was observed during repetitive stimulation with clamp pulses of 100 ms duration or less and was accompanied by a decrease in cell contraction. 3. The voltage dependence of inward current staircases was found to depend on Ca2+ entry rather than membrane voltage for long voltage clamp pulses and was not affected by 30 mM-TEA or 50 microM-TTX. Current reduction was greatest at 0 mV (P less than 0.05) when iCa was largest. Changes in cell contraction during pulse trains showed a similar voltage dependence. The time constant of current staircases was only mildly voltage dependent. 4. Interference with normal cellular mechanisms for Ca2+ uptake and release by strontium, 1-5 mM-caffeine and 1 microM-ryanodine increased current staircases and could abolish iCa facilitation with short clamp pulses. 5. Variations in the level of Ca2+-dependent inactivation of iCa can explain many features of the changes in iCa during stimulation after rest. Long clamp pulses (or action potentials) may increase cell Ca2+ loading and inhibit iCa. Short clamp pulses reduce available Ca2+ for cell contraction and this may reflect a lowered myoplasmic Ca2+ level which allows facilitation of iCa.

A computer simulation of the effect of heart rate on ion concentrations in the heart

At steady-state the passive fluxes of Na+ and K+ across the cell membrane of a heart cell are exactly matched by active fluxes of the two ions in the opposite direction via the Na-K pump, and the concentrations of Na+ and K+ both within the cell and in the clefts between cells are steady. An alteration of the heart rate (or the rate of stimulation) disrupts this balance because the passive fluxes are affected, and there are changes in pump activity as well as the Na+ and K+ concentrations. A computer model incorporating a cell separated from the bathing medium by a restricted extracellular cleft was devised to investigate these changes further. The model was able to simulate the changes observed with a variety of stimulation protocols as well as the effect of block of the Na-K pump. It is concluded that the changes in Na+ and K+ balance with heart rate can be explained in terms of the known properties of cardiac tissue incorporated into the model.

The effect of heart rate on the membrane currents of isolated sheep Purkinje fibres

1. The effect of the rate of stimulation on the membrane currents of sheep Purkinje fibres at 37 degrees C has been examined. 2. In the diastolic range of potentials, the pacemaker current if was unchanged at different stimulus rates. 3. On going from 6 to 60 min-1 the effect on the background current was similar to that of a decrease in the bathing K+ concentration, because there was a decrease in outward current in the plateau range of potentials and an increase in outward current in the diastolic range of potentials. At rates above 60 min-1 there was extra outward background current at all potentials. Partial block of the Na+-K+ pump by ouabain reduced these changes in background current. 4. On going from 6 to 60 min-1 there was an increase in the slow inward current isi, but at rates above 60 min-1 there was usually a decrease in isi. The decrease in isi at rates greater than 60 min-1 was in part the result of insufficient time between action potentials for complete recovery of isi from inactivation. 5. The effect of these rate-dependent changes in membrane current on electrical activity has been considered. 6. The increase in the pacemaker potential at high rates is likely to be the consequence of the decrease in membrane conductance at diastolic potentials as a result of the changes in background current. 7. The increase in the maximum diastolic potential at high rates is likely to be the result of the extra outward background current at diastolic potentials. 8. The prolongation of the action potential on going from 6 to 60 min-1 is likely to be the result of the increase of isi, as well as the decrease in outward background current at plateau potentials. 9. The shortening of the action potential above 60 min-1 is likely to be the result of the decrease in isi although the extra outward background current may also contribute.

The arrhythmogenic transient inward current iTI and related contraction in isolated guinea-pig ventricular myocytes

1. The arrhythmogenic transient inward current, iTI, and contractions were recorded in isolated guinea-pig ventricular myocytes, after exposure to strophanthidin or low external K+ (0.5 mM), using a single-microelectrode voltage-clamp technique and an optical measure of contraction. 2. The inward current, iTI, and after-contraction occurred on repolarization after a depolarizing pre-pulse. Longer pre-pulses to more positive potentials increased the size and reduced the latency of iTI. Oscillatory currents and contractions also occurred during pulses to positive potentials. 3. The voltage dependence of iTI was studied by repolarizing to different potentials after a constant depolarizing pulse. Inward currents preceded after-contractions at all potentials. The iTI was maximal at about -50 mV, diminishing in magnitude at more negative and positive potentials. It remained inward at potentials up to +47 mV. The contraction exhibited a similar voltage dependence. The current-voltage relation varied in the same cell with longer exposure to glycosides. Thus, the voltage dependence of iTI reflected not only that of an underlying ionic mechanism but also the effects of potential on intracellular Ca2+ oscillations which trigger iTI. 4. Uniformity of internal Ca2+ transients was achieved by clamping to different potentials at the peak of an inward current. The iTI remained inward at positive potentials. An inward tail current, seen on repolarizing during iTI at the end of a depolarizing pre-pulse, progressively increased at negative potentials. This voltage dependence may be close to that of the Ca2+-activated inward current responsible for iTI. 5. Replacement of Na+ by Li+ initially increased the magnitude of iTI, but further exposure abolished the inward current, while the after-contractions continued to increase. The potential dependence of iTI was not affected by exposure to zero Na+. Replacement of Ca2+ by Sr2+ also abolished iTI and the after-contraction, but the main effect was to slow their occurrence. 6. The voltage dependence of the Ca2+-activated inward current in guinea-pig ventricular myocytes leads us to favour electrogenic Na-Ca exchange current as a major component of iTI, under our experimental conditions.

Inward current related to contraction in guinea-pig ventricular myocytes

1. A component of inward current has been identified in isolated guinea-pig ventricular cells that is closely correlated with the contraction of the cell and not with the rapidly activated calcium current. This is a delayed current most clearly seen as a current 'tail' after 50-200 ms depolarizing pulses. At 22 degrees C the delayed current has a maximum amplitude of approximately 0.5 nA at -40 mV (consistently 10-20% of the peak amplitude of the calcium current) and decays with a half time of approximately 150 ms. 2. Paired-pulse protocols show that at pulse intervals (300-400 ms) at which the calcium current is nearly fully reprimed, the delayed component is very small. It recovers over a time course of several seconds, as does the contraction. Adrenaline speeds the decay of the delayed current (approximately 50%) and similarly accelerates cell relaxation. Adrenaline also shortens the recovery time of both the contraction and the delayed current. 3. During long trains of repetitive pulses, the delayed current amplitude follows that of the contraction 'staircase'. The half-time of the decay of the current 'tail' also matches that of contraction and suggests that both may reflect the time course of the underlying intracellular calcium transient. 4. The half-time of decay of the delayed current is only moderately voltage dependent over the potential range -80 to 0 mV. The amplitude of the delayed current normally reaches a minimum around -20 mV and increases at more negative potentials. 5. The voltage dependence and kinetics of decay of the current show that it should flow and decay largely during the action potential plateau and repolarization rather than during diastole. 6. Diffusion of high concentrations of EGTA into cells abolishes the delayed current and cell contraction. Under these conditions the fast calcium current is increased and its inactivation delayed. 7. When calcium is replaced by strontium, the delayed current amplitude is greatly reduced even though the contraction is larger and slower. 8. The results are consistent with the hypothesis that the delayed inward current is activated by the intracellular calcium transient. It may be carried by the sodium-calcium exchange process and/or by calcium-activated non-specific channels (especially when interal calcium is elevated by reduction of external sodium). 9. In the presence of 1 microM-ryanodine, the calcium current is greatly reduced, whereas the delayed current is not significantly altered.

Changes in the electrical activity of dog cardiac Purkinje fibres at high heart rates

Rate-dependent changes in the electrical activity of dog Purkinje fibres have been studied. At high rates of stimulation the rate of repolarization is greater, the action potential is shorter, the maximum diastolic potential is increased, the pace-maker potential is reduced in amplitude, and on cessation of rapid stimulation there can be a suppression of spontaneous activity. After an increase of the stimulus frequency there is an abrupt shortening of the action potential, which can be attributed to incomplete recovery of the plateau currents; this is followed by a progressive decline in action potential duration over the next several hundred seconds. The factor responsible for the slow changes in duration could also be responsible for the accompanying increase in maximum diastolic potential because this develops along a similar time course. These slow changes in electrical activity have been investigated with the phase-plane technique. They are the result of an increase in the net outward current over a wide range of potentials (approximately -10 to approximately -90 mV) during the repolarization phase of the action potential. In voltage-clamp experiments background current has been observed to be strongly rate dependent: the background current during a test voltage-clamp pulse after a train of action potentials is more outward at higher stimulus frequencies. When the frequency is increased, background current slowly becomes more outward over several hundred seconds, and this change therefore occurs along the appropriate time course to explain the slow alteration in electrical activity under these conditions. The extra outward background current at high rates is relatively independent of membrane potential in the range from -110 to -40 mV (more circumstantial evidence indicates that this range may extend to at least +10 mV); this potential dependence is similar to that of the Na-K-pump current (Eisner & Lederer, 1980). Strophanthidin and ouabain, agents known to block the Na-K pump, alter both the changes in background current and the slow rate-dependent changes in electrical activity. Although after an increase in rate there is a gradual change in background current that can be explained by an increase in electrogenic Na-K-pump activity, the initial effect of switching rate is to produce a change in current that is consistent with an increase of the extracellular K concentration. A transient increase in the K concentration of restricted extracellular clefts has been recorded under these conditions in dog Purkinje strands by Kline & Kupersmith (1982) using K-sensitive microelectrodes. The effect on electrical activity of these changes is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)