Interaction between taurine and angiotensin II: Modulation of calcium transport and myocardial contractile function

Angiotensin II modulates several aspects of cardiac function, including myocardial contractility, heart rate and myocyte growth. Most of these actions are intimately associated with alterations in calcium transport. Since taurine also modulates calcium transport, we examined possible interactions between taurine and angiotensin II at the level of the major cellular extruder of calcium, the Na⁺-Ca²⁺ exchanger. Over a concentration range of 0.5-25 mM, taurine served as an effective inhibitor of angiotensin II-mediated stimulation of the exchanger. An Arrhenius plot of Na⁺-Ca²⁺ exchange activity revealed that angiotensin II (2 nM) increased transporter activity by reducing the activation energy of the transport process. Taurine (25 mM) inhibited the angiotensin II effect by partially preventing the reduction in activation energy. However, neither agent significantly altered the transition temperature, ruling out a change in membrane fluidity or an alteration in the rate limiting step of the transporter as a cause of the observed effects. Since the Na⁺-Ca²⁺ exchanger plays an important role in the handling of [Ca²⁺](i) by the myocardium, the effect of taurine on angiotensin II's modulation of contractile function was also examined. Hearts perfused with buffer containing angiotensin II experienced a slight positive inotropic effect in the absence of taurine but this was converted to a negative inotropic effect in the presence of taurine. The data suggest that taurine inhibits some, but not all of the actions of angiotensin II. The possibility that a phosphorylation event is the site of the angiotensin II-taurine interaction is discussed.