The GSTM2 C-Terminal domain depresses contractility and Ca²⁺ transients in neonatal rat ventricular cardiomyocytes

The cardiac ryanodine receptor (RyR2) is an intracellular ion channel that regulates Ca²⁺ release from the sarcoplasmic reticulum (SR) during excitation-contraction coupling in the heart. The glutathione transferases (GSTs) are a family of phase II detoxification enzymes with additional functions including the selective inhibition of RyR2, with therapeutic implications. The C-terminal half of GSTM2 (GSTM2C) is essential for RyR2 inhibition, and mutations F157A and Y160A within GSTM2C prevent the inhibitory action. Our objective in this investigation was to determine whether GSTM2C can enter cultured rat neonatal ventricular cardiomyocytes and influence contractility. We show that Oregon green-tagged GSTM2C (at 1 μM) is internalized into the myocytes and it reduces spontaneous contraction frequency and myocyte shortening. Field stimulation of myocytes evoked contraction in the same percentage of myocytes treated either with media alone or media plus 15 μM GSTM2C. Myocyte shortening during contraction was significantly reduced by exposure to 15 μMGSTM2C, but not 5 and 10 μM GSTM2C and was unaffected by exposure to 15 μM of themutants Y160A or F157A. The amplitude of the Ca²⁺ transient in the 15 μMGSTM2C - treatedmyocytes was significantly decreased, the rise time was significantly longer and the decay time was significantly shorter than in controlmyocytes. The Ca²⁺ transient was not altered by exposure to Y160A or F157A. The results are consistent with GSTM2C entering themyocytes and inhibiting RyR2, in a manner that indicates a possible therapeutic potential for treatment of arrhythmia in the neonatal heart.