Electrical and mechanical responses in ventricular muscle fibers during barium perfusion

Tension activation and relaxation in frog atrial fibres. Evidence for direct effects of divalent cations (Ca2+, Sr2+, Ba2+) on contractile proteins and Na-Ca exchange

The effect of alkali-earth cations (Ca2+, Sr2+, Ba2+) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca2+, Sr2+ or Ba2+ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba2+ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr2+ exhibited smaller effects. Sr2+ ions could be more efficient than Ba2+ ions in substituting for Ca2+ ions in the Na+-Ca2+ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is the same with regard to transarcolemmal exchange for Na+ ions, presumed uptake by a "second relaxing system", activation of contraction, and inactivation of the slow inward current.

Effects of lysophosphatidylcholine on resting potassium conductance of isolated guinea pig ventricular cells

We studied the effects of lysophosphatidylcholine (LPC), a toxic metabolite of ischemia, on the inward rectifier potassium channel current in isolated guinea pig ventricular cells. LPC (10-50 microM) added to the external solution decreased the resting membrane potential and occasionally induced repetitive action potential discharges, with or without loss of repolarization. In voltage clamp studies, LPC (20 microM) decreased the conductance at the levels of resting potentials (approximately equal to -80 mV) from 26 +/- 8 nS to 16 +/- 3 nS (mean and SD, n = 4) within 10 min. Prolonged application of LPC (greater than 12 min) produced transient inward currents after depolarizing clamp pulses, thereby suggesting that the LPC elevated intracellular Ca2+ concentrations. The effect of LPC on the single inward rectifier K channel current was examined using the patch clamp technique in a cell-attached mode. LPC decreased the single channel conductance, depending on the concentration (5-100 microM). The slope conductance in the presence of 150 mM K+ in the pipette decreased from 45 +/- 7 pS (control) to 32 +/- 17, 20 +/- 19, and 14 +/- 10 pS for 5, 20 and 100 microM LPC, respectively. LPC induced little change with regard to probability of the channel opening. These results suggest that LPC depolarizes membrane by decreasing single channel conductance of the inward rectifier K channel. This reduction partially contributes to the alleged LPC-induced abnormal automaticities and conduction disturbances in the heart.

Barium inhibition of the tonic component of frog atrial contraction

Barium ions are able to substitute for calcium as charge carriers for the slow inward current of cardiac membrane. The effects of such a substitution on the mechanical activity of voltage-clamped frog atrial trabeculae have been investigated using the double mannitol gap device coupled with a mechano-electric transducer. In the presence of barium, the 'tonic' phase of the contraction is suppressed and the remaining contraction is correlated to the influx of barium ions via the slow inward channel. Since a Na-Ca exchange regulates the tonic contraction, the present results suggest that barium ions cannot substitute for calcium in this mechanism.

Calcium-dependent slow action potentials in potassium-depolarized guinea-pig ventricular myocardium enhanced by barium ions

Experiments were performed to investigated the mechanism by which Ba ions facilitate slow response in the myocardium. In partially depolarized guinea-pig papillary muscles, adding 0.2 mmol/l Ba to K-rich solution drastically lowered the stimulus threshold and converted the graded response to an all-or-none activity. This change was accompanied by an increase in the membrane resistance, as determined by cable analysis. When [Ca]0 was altered (0.9--7.2 mmol/l) in the presence of 0.2 mmol/l Ba, the action potentials varied almost in a manner expected for a Ca-electrode, and there were concomitant increases in the twitch tension. The electrical and mechanical activities of muscles treated with 1 mmol/l Ba were also strongly Ca-dependent. At normal [Ca]0, increasing [Ba]0 from 0.2--1 mmol/l only slightly enhanced the action potentials without any appreciable change in the peak twitch tension. When [Ca]0 was lowered to 0.2 mmol/l, elevation of [Ba]0 to over 0.5 mmol/l restored the action potentials, and the resting tension increased. These results suggest that the facilitatory action of the low concentration of Ba on the Ca-dependent slow action potentials is due to a reduction in the membrane shunting conductance and not to a development of any sizeable inward Ba current. However, Ba-dependent action potentials may be generated under conditions of Ca-deficiency. Isoprenaline (0.5--5 x 10(-7) mol/l) induced an automatic activity in the Ba-treated muscles and this phenomenon is probably related to both the drug-induced increase in Ca-permeability and the Ba-induced decrease in shunting conductance. Thus Ba ions appear to be a feasible tool for studies on the myocardial slow channel.

Changes in the intracellular sodium activity of sheep heart Purkinje fibres produced by calcium and other divalent cations

1. The intracellular Na activity of sheep heart Purkinje fibres was recorded with Na+-sensitive glass micro-electrodes. The effects of various external divalent cations on the intracellular Na activity were investigated. 2. Raising the external concentration of divalent cations (Ca, Mg, Mn, Sr or Ba) from 3 to 16 mM resulted in a decrease in the intracellular Na activity of 10-50%. 3. Raising the external concentration of Ca, Sr or Ba could produce a decrease in the intracellular Na activity even when the Na-K pump was inhibited (with strophanthidin, 10(-5) M); but raising the external concentration of Mg or Mn could not. 4. Mn inhibited the decrease in the intracellular Na activity produced by raising external Ca while the Na-K pump was inhibited. 5. Raising external Ca or adding Mn reduced the rate of rise of the intracellular Na activity on inhibition of the Na-K pump. 6. The removal of external K resulted in an increase in the intracellular Na activity. This increase could be stopped and even reversed by raising external Ca. 7. Removal of divalent cations from the external solution produced an increase in the intracellular Na activity. However, replacing external Ca and Mg by another divalent cation, e.g. Mn, did not result in a rise in the intracellular Na activity, except when the Na-K pump was inhibited. 8. The intracellular Na activity decreased by approximately 50% for a tenfold increase in the external Ca concentration. 9. The extent of the decrease in internal Na activity produced by raising external Ca was directly proportional to the internal Na activity before external Ca was raised. 10. We conclude that external Ca influences the intracellular Na activity in two ways: (a) by changing the passive Na influx: the resultant change in the intracellular Na depends on the activity of the Na-K pump; and (b) by a process where internal Na ions are exchanged for external Ca ions.