Abstract P079: Cardiac Contractility in Response to Isoproterenol from Gravin Knockout (AKAP12) Mice

Abstract

Gravin (AKAP12), one of the A-Kinase-Binding-Proteins (AKAPs), serves as a scaffold protein linking β 2 -adrenergic receptor (β 2 -AR), phosphatase 2B and several kinases including Protein Kinase A (PKA) and Protein Kinase C (PKC). The presence of gravin facilitates signal transduction of β 2 -AR and thus affects cardiac excitation-contraction coupling. We previously showed decreased cardiac hypertrophy, increased contractility and decreased arrhythmias in gravin-KO mice, following chronic β-AR stimulation. This response was associated with enhanced [Ca 2+ ] i homeostasis. Here, we test whether cardiac contraction is also affected in gravin knock-out (gravin-KO) mice. Trabeculae or small papillary muscles from the right ventricles were mounted between a force transducer and a motor arm, and superfused with K-H solution (pH 7.4) at room temperature. Developed force increased as external Ca 2+ ([Ca 2+ ] o ) was raised from 1 to 10 mM in both gravin-KO and wide-type (WT) muscles. Developed force and intracellular Ca 2+ transient (Ca 2+ i ) increased in a dose dependent manner as [Ca 2+ ] o was raised. Gravin-KO muscles had lower Ca 2+ i at any given [Ca 2+ ] o , but had similar force at [Ca 2+ ] o >4.0 mM; at higher [Ca 2+ ] o s, developed forces was significantly lower. Isoproterenol (ISO, 0.05-200nM) increased force and Ca 2+ i in both groups of muscles. However, force and Ca 2+ i started to blunt in gravin-KO muscle at higher ISO doses and became significantly lower at ISO does >50 nM. Both force and Ca 2+ i were lower at a given rest interval in gravin-KO muscles with no changes in recirculation fraction (RF). These results show that gravin-KO muscles maintain their response to both Ca 2+ and ISO with reduced capacity at higher doses, and to rest potentiations with lower magnitudes, but with no changes in recirculation fraction of Ca 2+ through the sarcoplasmic reticulum (SR). These findings suggest: (1) gravin-KO muscles may likely have smaller Ca 2+ content in the SR; (2) gravin plays an important modulatory role in the argumentation of force by Ca 2+ and β 2 -AR stimulation; (3) at physiological [Ca 2+ ] o , gravin-KO increases myofilament Ca 2+ responsiveness. Ongoing experiments are focused on changes myofilament Ca 2+ responsiveness in gravin-KO mice.