Ca2+, pH and the regulation of cardiac myofilament force and ATPase activity

When the (pH_i) surrounding myofilaments of striated muscle is reduced there is an inhibition of both the actin-myosin reaction as well as the Ca²⁺-sensitivity of the myofilaments. Although the mechanism for the effect of acidic pH on Ca²⁺-sensitivity has been controversial, we have evidence for the hypothesis that acidic pH reduces the affinity of troponin C (TNC) for Ca²⁺. This effect of acidic pH depends not only on a direct effect of protons on Ca²⁺-binding to TNC, but also upon neighboring thin filament proteins, especially TNI, the inhibitory component of the TN complex. Using flourescent probes that report Ca²⁺-binding to the regulatory sites of skeletal and cardiac TNC, we have shown, for example, that acidic pH directly decreases the Ca²⁺-affinity of TNC, but only by a relatively small amount. However, with TNC in whole TN or in the TNI-TNC complex, there is about a 2-fold enhancement of the effects of acidic pH on Ca²⁺-binding to TNC. Acidic pH decreases the affinity of skeletal TNI for skeletal TNC, and also influences the micro-environment of a probe postioned at Cys-133 of TNI, a region of interaction with TNC. Other evidence that the effects of acidic pH on Ca²⁺-TNC activation of myofilaments are influenced by TNI comes from studies with developing hearts. In contrast, to the case with the adult preparations, Ca²⁺-activation of detergent extracted fibers prepared from dog or rat hearts in the peri-natal period are weakly affected by a drop in pH from 7.0 to 6.5. This difference in the effect of acidic (pH_i) appears to be due to a difference in the isoform population of TNI, and not to differences in isotype population or amount of TNC.