Na⁺ and Ca⁺ ions enter mammalian cardiac muscle cells during each cycle of membrane depolarization, triggering the release , through Ca⁺ channels, of larger amounts of Ca⁺ from internal stores in the sarcoplasmic reticulum (SR).  The resulting increase in intracellular Ca⁺ interacts with troponin C and hence is responsible for activating the cross-bridge interactions between actin filaments and myosin cross-bridges that results in sarcomere shortening.   The electrochemical gradient for Na⁺ across the sarcolemma is maintained by active (i.e., ATP-consuming) transport of Na⁺ out of the cell by the sarcolemmal Na⁺, K-ATPase.  Na is actively extruded by Na⁺, K-ATPase, while the bulk of cytosolic Ca⁺ is pumped back into the SR by a Ca⁺-ATPase, where it is bound by the protein calsequestrin, and the remainder is removed from the cell by either a plasma membrane Ca⁺-ATPase  or a high capacity Na⁺-Ca⁺ cation exchange protein.  This sarcolemmal membrane protein exchanges 3 Na⁺ ions for every Ca⁺ ion, using the electrochemical potential of Na⁺ to drive Ca⁺ extrusion.   Note that the direction of cation transport may reverse briefly during depolarization, when the electrical gradient across the sarcolemma is transiently reversed.  Beta-Adrenergic agonists and phosphodiesterase inhibitors, by increasing intracellular cyclic AMP levels, activate protein kinase A, which enhances the contractile state by phosphorylating target proteins, including phosphoamban and the alpha subunit of the L-type Ca⁺ channel.