Altered Force Development in Response to Calcium and Isopreterenol in Cardiac Muscle from Gravin Knock-Out (AKAP12) Mice

Abstract

Gravin (AKAP12), one of the A-Kinase-Binding-Proteins (AKAPs), serves as a scaffold protein linking β2-adrenergic receptor (β2-AR), phosphatase 2B and several kinases including Protein Kinase A (PKA) and Protein Kinase C (PKC). The presence of gravin facilitates signal transduction of β2-AR and thus affects cardiac excitation-contraction coupling. 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 Ca2+ ([Ca2+]o) was raised from 1 to 10 mmol/L in both gravin-KO and widetype (WT) muscles. Developed force increased in a dose dependent manner as [Ca2+]o was raised. In gravin-KO muscles, lower [Ca2+]o (>3.0 mmol/L) caused higher but insignificant force development as compared with WT muscles. At higher [Ca2+]os, developed forces remained largely flat and significantly lower in gravin-KO muscles as [Ca2+]o was increased up to 10.0 mmol/L. Similarly, developed force increased as doses of isopreterenol (ISO) increased (0.05 nmol/L to 200 nmol/L) in both groups of muscles. However, force response started to blunt in gravin-KO muscle at ISO doses >5 nmol/L and became significantly lower at ISO does >50 nmol/L. These results show that gravin-KO muscles maintain their response to both Ca2+ and ISO, with reduced capacity at higher doses, suggesting that gravin plays an important modulatory role in the argumentation of force by Ca2+ and β2-AR stimulation. Ongoing experiments are focused on changes in intracellular Ca2+ and myofilament Ca2+ responsiveness in gravin-KO mice.