Altered kinetics of contraction of mouse atrial myocytes expressing ventricular myosin regulatory light chain

To investigate the role of myosin regulatory light chain isoforms as a dete rminant of the kinetics of cardiac contraction, unloaded shortening velocit y was determined by the slack-test method in skinned wild-type murine atria l cells and transgenic cells expressing ventricular regulatory light chain (MLC2v). Transgenic mice were generated using a 4.5-kb fragment of the muri ne a-myosin heavy chain promoter to drive high levels of MLC2v expression i n the atrium. Velocity of unloaded shortening was determined at 15 degrees C in maximally activating Ca2+ solution (pCa 4.5) containing (in mmol/l) 7 EGTA, 1 free Mg2+, 4 MgATP, 14.5 creatine phosphate, and 20 imidazole (ioni c strength 180 mmol/l, pH 7.0). Compared with the wild type (n = 10), the u nloaded shortening velocity of MLC2v-expressing transgenic murine atrial ce lls (n = 10) was significantly greater (3.88 +/- 1.19 vs. 2.51 +/- 1.08 mus cle lengths/s, P < 0.05). These results provide evidence that myosin light chain 2 regulates cross-bridge cycling rate. The faster rate of cycling in the presence of MLC2v suggests that the MLC2v isoform may contribute to the greater power-generating capabilities of the ventricle compared with the a trium.