Cardiac functions and taurine's actions at different extracellular calcium concentrations in forced swimming stress-loaded rats

Modulation of the sinus rate and contractile force by taurine at different extracellular Ca2+ concentrations ([Ca2+](o)) was examined using rat right atria loaded with forced swimming stress. Serum concentration of corticosterone profoundly increased in stress-loaded rats as compared with native rats. The taurine level in serum also increased in stress-loaded rats, but was not changed in the different heart tissues and aorta. Heat-shock protein (HSP72) was detectable in cardiac muscles and in the lumen of cardiac blood vessels of stress-loaded rats using a monoclonal antibody. Increasing [Ca2+](o) (from 0.9 to 3.6 mM) enhanced the sinus rate and contractile force in a [Ca2+](o)-dependent fashion in native rats, but not in stress-loaded rats. Taurine (1-20 mM) caused a negative chronotropic and inotropic effect in a concentration-dependent manner. At 1.8 mM [Ca2+](o), the negative chronotropic effect of taurine (10-20 mM) was attenuated in stress-loaded rats as compared with native rats. These results indicate that swimming stress causes a release of taurine into the serum and reduces the sensitivity to [Ca2+](o). Taurine administration might, in part, exhibit the protective actions on acute stress-induced responses.