Force development and intracellular Ca²⁺ in intact cardiac muscles from gravin mutant mice

Gravin (AKAP12) is an A-kinase-anchoring-protein that scaffolds protein kinase A (PKA), β₂-adrenergic receptor (β₂-AR), protein phosphatase 2B and protein kinase C. Gravin facilitates β₂-AR-dependent signal transduction through PKA to modulate cardiac excitation-contraction coupling and its removal positively affects cardiac contraction. Trabeculae from the right ventricles of gravin mutant (gravin-t/t) mice were employed for force determination. Simultaneously, corresponding intracellular Ca²⁺ transient ([Ca²⁺]_i) were measured. Twitch force (T_f)-interval relationship, [Ca²⁺]_i-interval relationship, and the rate of decay of post-extrasysolic potentiation (R_f) were also obtained. Western blot analysis were performed to correlate sarcomeric protein expression with alterations in calcium cycling between the WT and gravin-t/t hearts. Gravin-t/t muscles had similar developed force compared to WT muscles despite having lower [Ca²⁺]_i at any given external Ca²⁺ concentration ([Ca²⁺]_o). The time to peak force and peak [Ca²⁺]_i were slower and the time to 75% relaxation was significantly prolonged in gravin-t/t muscles. Both T_f-interval and [Ca²⁺]_i-interval relations were depressed in gravin-t/t muscles. R_f, however, did not change. Furthermore, Western blot analysis revealed decreased ryanodine receptor (RyR2) phosphorylation in gravin-t/t hearts. Gravin-t/t cardiac muscle exhibits increased force development in responsiveness to Ca²⁺. The Ca²⁺ cycling across the SR appears to be unaltered in gravin-t/t muscle. Our study suggests that gravin is an important component of cardiac contraction regulation via increasing myofilament sensitivity to calcium. Further elucidation of the mechanism can provide insights to role of gravin if any in the pathophysiology of impaired contractility.