CapZ and actin capping dynamics increase in myocytes after a bout of exercise and abates in hours after stimulation...

The time course of the response and recovery after acute activity seen in exercise is not well understood. The goal of this work is to address how proteins of the thin filament (actin and its capping protein CapZ) are changed by 1 h of mechanical stimulation and return to baseline over time. Neonatal rat ventricular myocytes in culture were subjected to cyclic 10% strain at 1 Hz for 1 h to mimic increased mechanical loading during exercise. CapZ and actin dynamics were analyzed by fluorescence recovery after photobleaching (FRAP) using CapZβ1-GFP, actin-GFP, or actin-RFP. After cyclic strain, CapZ dynamics increased above resting controls and abated 2-3 h after cessation of the cyclic strain. Similarly, actin dynamics initially increased and abated 1.5-2 h after the end of stimulation. Neurohormonal hypertrophic stimulation by phenylephrine or norepinephrine treatments also elevated actin dynamics but required a much longer time of treatment (24-48 h) to be detectable. The actin capping mechanism was explored by use of expression of CapZβ1 with a COOH-terminal deletion (CapZβ1ΔC). Increased dynamics of actin seen with CapZβ1ΔC was similar to the response to cyclic strain. Thus it is possible that mechanical stimulation alters the dynamics for CapZ capping of the actin filament through the CapZβ1 COOH terminus, known as the β tentacle, thereby remodeling sarcomeres in cardiac myocytes. This adaptive mechanism, which is probably regulating thin-filament addition, declines a few hours after the end of a bout of exercise.