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.