Taurine Effects on Ion Channels of Cardiac Muscle. In: Noble D, Earm YE, editors. Ionic Channels and Effect of Taurine on the Heart. Boston: Springer US; 1993. pp. 93-118. [DOI: 10.1007/978-1-4615-3074-9_5]

Taurine modulates I(Kr) but I(Ks) in guinea-pig ventricular cardiomyocytes

1. Effects of taurine on the delayed rectifier K+ current (I(K)) in isolated guinea-pig ventricular cardiomyocytes were examined at different intracellular Ca2+ concentration ([Ca2+]i), using whole-cell voltage and current clamp techniques. Experiments were performed at 36 degrees C. 2. Addition of taurine (10-20 mM) decreased the action potential duration (APD) at pCa 8, but increased the APD at pCa 6. Taurine (20 mM) enhanced I(K) at 70 mV by 22.4 +/- 3.1% (n = 6, P < 0.01) at pCa 8, whereas taurine inhibited the I(K) by 27.1 +/- 2.7% (n = 6, P < 0.01) at pCa 6. These responses behaved in a concentration-dependent manner. 3. The I(K) is composed of the rapid and slow components (I(Kr) and I(Ks)). When [Ca2+]i was pCa 6, taurine at 20 mM reduced the tail current of I(Kr) at 70 mV by 16.5 +/- 2.7% (n = 5, P < 0.05) and that of I(Ks) at 70 mV by 27.1 +/- 2.8% (n = 6, P < 0.01). In contrast, at pCa 8, the tail currents of I(Kr) and I(Ks) at 70 mV were enhanced by 13.4 +/- 3.2% (n = 7, P < 0.05) and by 22.4 +/- 3.1% (n = 7, P < 0.01), respectively. The voltages of half-maximum activation (V1/2) for I(Kr) and I(Ks) were not modified by taurine. 4. Addition of E-4031 (5 microM) to taurine had a complete blockade of the tail current of I(Kr), but not I(Ks). The remained tail current (I(Ks)) in the presence of E-4031 (5 microM) was not affected by taurine (20 mM), but was blocked by 293B (30 microM). 5. These results indicate that taurine modulates I(Kr) but not I(Ks), depending on [Ca2+]i, resulting in regulation of the APD.

Cardiac actions of taurine as a modulator of the ion channels

During ischemia, hypoxia and cardiac failure, the heart undergoes several adverse changes, including a reduction in taurine (2-aminoethanesulfonic acid). Oral administration of taurine under these disease conditions would be expected to act like a mild cardiac glycoside. Taurine would exert improvement in the accumulation of [Na]i and the loss of alpha-amino acids. Nonetheless, when intracellular taurine content is raised, there would be the benefit of increased Ca2+ release from the sarcoplasmic reticulum and increased Ca2+ sensitivity of the contractile proteins, as well as possible changes in the action potential associated with the actions of taurine on ion channels. In fact, intracellular application of taurine produces the opposite actions to extracellularly administration of the amino acid. From our previous experiments, the electrophysiological actions of taurine on cardiac muscle cells include the following. (a) Prolongation of action potential duration (APD) at high [Ca]i and shortening of APD at low [Ca]i. In multicellular preparations, however, taurine did not always prevent [Ca]o-induced effects. (b) Stimulation of spontaneous activity at low intracellular and extracellular Ca2+ concentrations ([Ca]i and [Ca]o), and vice versa. (c) Inhibition of the L-type Ca2+ current (ICa(L)) at high [Ca]i, and vice versa. (d) Enhancement of the T-type Ca2+ current (ICa(T)). (e) Inhibition of fast Na+ current (INa). (f) Enhancement of TTX-insensitive slow Na+ current. (g) Inhibition of delayed rectifier K+ current (IKrec) at high [Ca]i, and vice versa. (h) Enhancement of the transient outward current (Ito). (i) Inhibition of the ATP-sensitive K+ current (IK(ATP)). Since taurine acts on so many ion channels and transporters, it is clearly non-specific. Although it is very difficult to understand the diversity of taurine's actions, it is possible that taurine can exert its potent cardioprotective actions under the conditions of low [Ca]i, as well as Ca2+ overload. Thus, although taurine-induced modulation of ion channels located on the cardiac cell membrane is complex, the multiple effects may combine to yield useful therapeutic results.

Regulation of the slow Ca++ channels of myocardial cells

Contraction of the heart is regulated by a number of mechanisms, such as neurotransmitters, hormones, autacoids, pH, intracellular ATP, and Ca++ ions. These actions are mediated, at least in part, by actions on the sarcolemmal slow (L-type) Ca++ channels, exerted directly or indirectly. The major mechanisms for the regulation of the slow Ca++ channels of myocardial cells includes the following. cAMP/PK-A phosphorylation stimulates the slow Ca++ channel activity, whereas cGMP/PK-G phosphorylation inhibits. DAG/PK-C phosphorylation and tyrosine kinase phosphorylation are suggested to stimulate the slow Ca++ channel activity. Intracellular application of Gs alpha protein increases the slow Ca++ currents (ICa(L)). Lowering of intracellular ATP inhibits ICa(L). Acidosis and increase in [Ca]i inhibits ICa(L). A number of changes in the Ca++ channels also occur during development and aging. Thus, it appears that the slow Ca++ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of extrinsic and intrinsic factors, and thereby control can be exercised over the force of contraction of the heart.

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.