Choline permeability in cardiac muscle cells of the cat

Effects of chlorpromazine and trifluoperazine on choline metabolism and phosphatidylcholine biosynthesis in cultured chick heart cells under normoxic and anoxic conditions

The effects of chlorpromazine and trifluoperazine on phosphatidylcholine biosynthesis in the heart were investigated in isolated cardiac cells under normoxic and anoxic conditions. The cells were obtained from 7-day-old chick embryos and were maintained in culture. After 96 hr, cells were maintained either in an incubator with oxygen at room air concentration (normoxia) or in an incubator containing 95% nitrogen and 5% CO2 (anoxia). Pulse chase experiments with [methyl-3H]choline were conducted using a 2-hr incubation with choline. Chlorpromazine and trifluoperazine at 10(-5) M produced a significant (P less than 0.05) increase in the incorporation of choline into both phosphocholine and phospholipid. High concentrations of chlorpromazine or trifluoperazine i.e. 10(-4) M, damaged myocardial cells as reflected in a significant (P less than 0.05) reduction in cellular protein and a further reduction in labelled choline in phosphocholine or phospholipid after adjusting for the lower protein concentrations. Anoxia altered choline metabolism but 6 hr of anoxia was the minimum time needed for the effect to be observable. Anoxia, for 24 hr, produced a significant (P less than 0.05) reduction in labelled choline in phosphocholine without a significant change in incorporation of label in phospholipid or cellular protein. Both chlorpromazine and trifluoperazine at 10(-5) M prevented anoxic-induced changes in phosphocholine metabolism. Thus, chlorpromazine and trifluoperazine affect phospholipid biosynthesis in cardiac cells and prevent anoxia-induced changes in phosphatidylcholine biosynthesis.

The effect of calcium ions and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis in chick embryo hearts

The effect of increases in extracellular calcium [Ca]o and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis was assessed in isolated cardiac myocytes. The cells were obtained from 7-day old chick embryos and were maintained in culture. Choline uptake was examined using [methyl 3H] choline. A23187 was found to increase choline uptake through the saturable choline uptake process. Pulse chase experiments using [methyl 3H] choline showed that after a 2 h incubation with choline, about 85% of the label was recovered in phosphocholine with most of the rest in phospholipid and a small amount in CDP-choline and glycerol phosphocholine. Increases in [Ca]o up to 10 mM did not affect the amount of label in phosphocholine or phospholipid, the rate of disappearance of label from phosphocholine, or the rate of appearance of labelled choline in phospholipid. In contrast, A23187, at concentrations up to 10(-4) M, was associated with a significant (p less than 0.05) increase in choline in the phosphocholine and phospholipid pool compared to control cells. The time course of the disappearance of choline from the phosphocholine pool and appearance in phospholipid pool was not significantly different between control cells and those treated with A23187. A23187 increased choline uptake via the specific uptake process. The effect on choline uptake may be attributed to the action of A23187 to facilitate the release of calcium from specific intracellular calcium storage sites rather than a nonspecific increase in [Ca]i that may have resulted from the increase in [Ca]o.

Effects of sodium substitutes on transient inward current and tension in guinea-pig and ferret papillary muscle

We used ouabain-treated guinea-pig and ferret papillary muscles to study transient inward current (Iti), after-contractions, and tonic tension development during voltage-clamp pulses. Li, sucrose and choline were used isosmotically as Na substitutes to evaluate the effect of altering the Na equilibrium potential. We were unable to detect outward Iti at any potential up to +30 mV in normal or Na-depleted solutions. However, reduction of Na had a biphasic effect on Iti, initially increasing it and then reducing it at all clamp potentials from -50 to +20 mV. After-contractions were also initially increased and, in sufficiently Na-depleted solutions, decreased by reduction of extracellular Na. However, the peak in the after-contraction always occurred later than the increase in Iti and frequently coincided with the maximum suppression of the current. Complete suppression of after-contractions was not often achieved and always required more complete Na replacement than Iti suppression. Tonic tension responses were reduced by Na replacement, usually in synchrony with the reduction of Iti. The responses of Iti to Na replacement are consistent with a model of electrogenic Na-Ca exchange over the potential range positive to -50 mV. The responses deviate from the predictions of the model at more negative potentials. The results are consistent with the previous proposal that oscillatory changes in internal free Ca concentration underlie both Iti and after-contractions.

The effect of ouabain, dinitrophenol, and lithium on the pacemaker current in sheep cardiac Purkinje fibers

We studied the effect of ouabain and other inhibitors of the Na-K pump on the pacemaker current (IK2) in sheep cardiac Purkinje fibers. We found that exposure to ouabain (2 X 10(7) M) caused an apparent decrease in the amplitude of the IK2 activation curve. We also found that a similar effect was caused by exposure to dinitrophenol (DNP) or lithium-substituted Tyrode's solution. Our studies showed that after prolonged (more than 60 minutes) exposure to ouabain a characteristic current noise appeared and IK2 was replaced by an oscillatory inward current (IOS). The apparent threshold for IOS was -50 to -60 mV and the peak amplitude of this current increased with increasing levels of depolarization positive to threshold. These results suggest that (1) the apparent decrease in IK2 may be a consequence of ionic redistribution due to Na-K pump inhibition; (2) cardiac glycosides cause abnormal impulse generation by an ionic mechanism separate from IK2 and involving IOS.

Manganese-dependent propagated action potentials and their depression by electrical stimulation in guinea-pig myocardium perfused by sodium-free media

1. Propagated action potentials were recorded in right ventricular papillary muscles from guinea-pig heart while exposed to Na-free, Ca-free and Mg-free solutions containing Mn. 2. When Na was totally replaced by 95 mM-Mn the overshoot was about 45 mV while the resting potential was about -90mV. 3. The overshoot of action potentials was increased by about 20-30 mV per tenfold increase of Mn concentration over the range of 2-50 mM. 4. Similar increases of overshoots with increasing of Mn concentration also occurred in the presence of 0-6 mM-Ca. Increasing of Ca from 5 to 20 mM had little influence on the overshoot but shortened the duration of the Mn-dependent action potential in the presence of 5 mM-MN. 5 Mn-dependent action potentials were not depressed by 3 X 10(5) M tetrodotoxin but by La. 6. These results suggest that Mn passes through the slow inward current channel to generate the action potential seen under the Na-free condition. 7. The overshoot and duration of the Mn-dependent action potential decreased with stimulation. At stimulus frequencies (Hz) of 0-5, 0-2, 0-1, 0-017 and 0-0033 the overshoot of action potential in 5 mM-Mn Tyrode decreased by 0-5-1 mV per an action potential. This depression of the action potential is explained by assuming intracellular accumulation of Mn.

Effects of Na and K ions on the active Na transport in guinea-pig auricles

1. The effect of Na and K ions on active Na transport was studied in guinea-pig auricles by means of flame photometry. 2. The Na influx into preparations rewarmed in Tyrode's solution after cooling was estimated to be about 1.05 mmole/l fibre water - min (l.f.w.-min) or c. 8 pmole/cm2 - s. Intracellular Na ions enhanced the active Na efflux over a wide range of concentrations. A decrease in the extracellular Na concentration ([Na]o) had no major effect on the active Na efflux. 3. Extracellular K ions initiated an active Na efflux from rewarmed auricles with an elevated [Na[i over a narrow range of K concentrations ([K]o). 4. Assuming Michaelis-Menten kinetics the maximal active Na efflux activated by internal Na ions was calculated to be about 4 mmole/l.f.w. - min (30 pmole/cm2 - s). Half maximal Na efflux occurred at about 22 mmole/l.f.w. [Na]i. The maximal K-activated active - min (28 pmole/cm2 - s) and was half maximal at a [K]o of about 0.2 mM. 5. It is tentatively concluded that the maximal active Na efflux from guinea-pig atria is 3--4 times larger than the physiological flux. Under normal conditions active Na efflux in heart is mainly regulated by variations of [Na]i.

Reconstruction of the electrical activity of cardiac Purkinje fibres

1. The electrical activity of Cardiac Purkinje fibres was reconstructed using a mathematical model of the membrane current. The individual components of ionic curent were described by equations which wee based as closely as possible on previous experiments using the voltage clamp technique. 2. Membrane action potentials and pace-maker activity were calculated and compared with time course of underlying changes in two functionally distinct outeard currents, iX1 and iK2. 3. The repolarization of the theoretical action potential is triggered by the onset of iX1, which becomes activated over the plateau range of potentials. iK2 also activates during the plateau but does not play a controlling role in the repolarization. Hwever, iK2 does govern the slow pace-maker depolarization through its subsequent deactivation at negative potentials. 4. The individual phases of the calculated action potential and their 'experimental' modifications were compared with published records. The upstroke is generated by a Hodgkin-Huxley type sodium conductance (gNa), and rises with a maximum rate of 478 V/sec, somewhat less than experimentally observed values ( up to 800 V/sec). The discrepancy is discussed in relation to experimental attempts at measuring gNa. 5. The ole of the transient outward chloride current (called igr) was studied in calculations of the rapid phase of repolarization and 'notch' configuration...

The electrical activity of canine cardiac Purkinje fibers in sodium-free, calcium-rich solutions

Propagated action potentials can be obtained in canine cardiac Purkinje fibers exposed to Na-free solutions containing no inorganic cation other than Ca and K. Essentially similar action potentials are obtained if Na is replaced by tetraethylammonium (TEA), tetramethylammonium (TMA), or choline. In a solution containing 128 mM TEA and 16.2 mM Ca the characteristics of these electrical responses were: maximum diastolic potential, -59 +/- 3.3 mV; overshoot, 20 +/- 6.8 mV; maximum upstroke velocity, 3.7 +/- 2.3 V/s; conduction velocity, 0.1 m/s; and action potential duration, 360 +/- 45 ms. The magnitude of the overshoot varied with log Ca(o) with a slope of about 30 mV/10-fold concentration change. The upstroke velocity was an approximately linear function of Ca(o). The active response was greatly diminished or abolished by Mn and D-600 but was unaffected by tetrodotoxin. These Ca-dependent responses appeared in a region of transmembrane potential (about -50 mV) at which the rapid Na-dependent upstroke is abolished even when Na is present.

Activation of the electrogenic sodium pump in guinea-pig auricles by internal sodium ions

1. The effect of various intracellular Na concentrations ([Na](i)) on the membrane potential after hypothermia was studied in guinea-pig auricles.2. For varying [Na](i), the atria were cooled for 4 hr at 4-6 degrees C in a K-poor solution with different concentrations of NaCl. The auricles were rewarmed in normal Tyrode solution at 35 degrees C.3. Extracellular space (ECS), intracellular Na and K concentrations ([Na](i) and [K](i)) and membrane potential of the atria were measured before and after hypothermia.4. The ECS, measured as inulin space, amounted to 350 ml./kg wet wt. at 35 degrees C and to 300 ml./kg wet wt. at 4-6 degrees C.5. [K](i) decreased during cooling and increased during rewarming the auricles. [Na](i) increased during hypothermia in bathing fluids containing NaCl, but decreased in NaCl- and Na-free solutions. At the beginning of rewarming a net Na transport occurred from cells with high [Na](i), while a net Na uptake took place in atria with low [Na](i).6. At the same time, the membrane potential of auricles with increased [Na](i) hyperpolarized beyond the steady-state value recorded at the end of rewarming, or even beyond the calculated K(+) equilibrium potential (E(K)). Afterwards, the hyperpolarization levelled off, while the E(K) values increased further. The membrane potential of atria with decreased [Na](i) showed no transitory hyperpolarization during rewarming.7. The hyperpolarization beyond the steady-state value of membrane potential in rewarmed auricles was significantly correlated to the active Na efflux.8. From these results it is concluded that the membrane potential of guinea-pig atria after hypothermia is affected by an active, electrogenic Na pump activated by intracellular Na ions.