Taurine Effects on Action Potentials and Ionic Currents in Chick Myocardial Cells. In: Iwata H, Lombardini JB, Segawa T, editors. Taurine and the Heart. Boston: Springer US; 1989. pp. 1-19. [DOI: 10.1007/978-1-4613-1647-3_1]

Inhibition by taurine of the inwardly rectifying K+ current in guinea pig ventricular cardiomyocytes

Effects of taurine on the inwardly rectifying K+ current (IK1) in isolated guinea pig ventricular cardiomyocytes were examined using patch voltage-clamp methods. All experiments were performed at 36 degrees C. Taurine (10-20 mM) increased the action potential duration, but failed to affect the resting potential. Holding potential was maintained at -30 mV. The current was activated with an inwardly going rectification, and was completely blocked by Ba2+ (2 mM). Taurine inhibited IK1 at - 120 mV by 28.3+/-1.1% (n=6, P < 0.05) at 10 mM and by 36.0+/-2.1% (n=6, P < 0.01) at 20 mM. The reversal potential was shifted in the hyperpolarizing direction by 3.7+/-0.6 mV (n=6) at 20 mM. In inside-out patch-clamp experiments, the amplitude of unitary channels was -2.7+/-0.3 pA (n=21) at -90 mV. Symmetrical high-K+ (150 mM) solutions in both bath and pipette were used. The channel conductance was 32+/-2 pS (n=9). Taurine did not affect channel conductance, but markedly decreased the open probability at - 120 mV of channel by 21.5+/-2.4% (n=8, P < 0.01) at 10 mM, and by 56.7+/-3.8% (n=8, P < 0.001) at 20 mM. These responses were almost reversible. These results suggest that taurine directly modulates the open probability of the inwardly rectifying K+ current, resulting in regulation of the functions of heart cells.

Modulation of Ca2+ and Na+ transport by taurine in heart and vascular smooth muscle

Using the whole-cell voltage clamp technique, taurine was found to affect different types of various ionic currents including T and L-type Ca2+ currents, slow Na+ and fast Na+ currents as well as the delayed outward K+ current. Also, in normal situations, taurine had no effect on the Na(+)-Ca2+ exchange current. The effect of taurine on the different types of ionic currents appears to depend on [Ca2+]o and [Ca2+]i and may also vary according to the tissue or cell type studied. Using standard Ca2+ imaging techniques, short-term exposure (10 to 20 min) of single heart cells and aortic vascular smooth muscle cells was found to increase total intracellular free Ca2+ in a dose-dependent manner. However, using 3-dimensional Ca2+ and Na+ imaging techniques, long-term exposure of heart and vascular smooth muscle cells to taurine was found to decrease both nuclear and cytosolic Ca2+ without significantly changing either nuclear or cytosolic Na+ levels. Long-term exposure to taurine was found to prevent cytosolic and nuclear increases of Ca2+ induced by permanent depolarization of heart cells with high [K+]o. This preventive effect of taurine on nuclear Ca2+ overload was associated with an increase of both cytosolic and nuclear free Na+. Thus, the effect of long-term exposure to taurine on intranuclear Ca2+ overload in heart cells seems to be mediated via stimulation of sarcolemma and nuclear Ca2+ outflow through the Na(+)-Ca2+ exchanger.

Cardioprotective actions of taurine against intracellular and extracellular calcium-induced effects

The effects of taurine were modulated by the [Ca2+]i or/and [Ca2+]o levels, consistent with recent reports (10, 28, 31). Taurine may directly and indirectly regulate the [Ca2+]i level by modulating Ca2+ channels (dependent on [Ca2+]i/[Ca2+]o) and Na+ channel (via Na(+)-Ca2+ exchange). Thus, taurine antagonizes Ca2+ ([Ca2+]o or [Ca2+]i)-induced cardiac functions. The data for the effects of taurine on the ionic currents and action potentials (automaticity) are summarized in Tables 1 and 2. These results indicate that taurine exerts potent cardioprotective actions under the conditions induced by low Ca2+ level as well as by calcium overload. In conclusion, the effects of taurine are complex, there being a number of actions on cardiac muscle which may show the possible therapeutic use of this sulfur amino acid.

Taurine effects on ionic currents in myocardial cells

The effects of taurine on the slow and fast Na+ currents and slow and fast Ca2+ currents in cultured single ventricular cells from young (3-day-old) and old (10, 17 day) chick embryos were studied using the whole-cell voltage clamp technique. In single 3-day cells that showed only a TTX-insensitive fast (transient) Na+ current (INa(f)), taurine (5 mM) rapidly increased the amplitude of this current. In single cells that showed only a typical slow (sustained) Na+ current (INa(s)), taurine (5 mM) induced a fast transient component. A slow Ca2+ current (ICa(s)) was also present in the 3-day-old embryonic chick cells, and taurine inhibited this current and activated a fast transient component (ICa(f)). Taurine had similar actions in 10-day-old embryonic heart cells. Thus, in embryonic chick heart cells, taurine stimulates the TTX-insensitive fast transient Na+ current and blocks the slow component. Taurine also activates a fast (transient) component of the Ca2+ current (ICa(f)). The activation of the TTX-insensitive INa(f) may increase Ca2+ influx via Na(+)-Ca2+ exchange. This may explain, in part, the positive inotropic effect of taurine in heart muscle, with relatively little effect on the Ca(2+)-dependent slow APs. Taurine (10 or 20 mM) added to the outside markedly inhibited TTX-sensitive INa, but slightly stimulated INa when added internally. ICa(s) and IK both were stimulated by external taurine at pCa 10 but inhibited at pCa 7. Taurine also inhibited IK in aortic VSM cells. In contrast, taurine induced or stimulated ICa(f). Elevation of [Ca]i was induced by taurine. The elevation may result from the enhancement of ICa(f) and possibly of Na(+)-Ca2+ exchange, resulting in a positive inotropic effect. It has been shown that in neurons, taurine increases the Cl current, resulting in hyperpolarization (Taber et al., 1986; Figure 12). Thus, taurine effects are complex, there being a number of actions on the membrane currents of cardiac cells, vascular smooth muscle cells, and neurons.