Rate of active tension development from rigor in skinned atrial and ventricular cardiac fibres from swine following photolytic release of ATP from caged ATP

We investigated the rate of tension development (kappa td) after photolytical release of ATP from P3-1-(2-nitrophenyl)-ethyladenosine-5'-triphosphate ('caged ATP') of atrial and ventricular fibre bundles from pig. Contraction was initiated from high-tension (HT) and low-tension (LT) rigor at maximal Ca2+ activation (pCa 4.5). The kappa td of atrial fibre bundles was 6.8 s-1 from LT and 6.9 s-1 from HT rigor. Rate of tension development of ventricular fibre bundles was significantly lower (P < 0.001) being 1.06 s-1 and 0.94 s-1 from LT and HT rigor, respectively. The kappa td of skinned ventricular fibre bundles incubated in a high K+, low Ca2+ (cardioplegic) solution prior to the skinning procedure decreased significantly (P < 0.05) to 0.73 s-1 and 0.63 s-1 from LT and HT rigor, respectively, whereas that of skinned atrial fibre bundles remained at 7.1 s-1 and 6.9 s-1 from LT and HT rigor, respectively. Phosphorylation levels of the myosin light chain 2 isoform in the atrial fibre bundles (ALC-2) was 15.6 +/- 2.7%. The corresponding values for the two ventricular isoforms, VLC-2 and VLC-2*, were 31.2 +/- 0.4% and 25.1 +/- 2.1%, respectively. Phosphorylation levels of fibre bundles incubated in cardioplegic solution prior to skinning were 11.6%, 18.9%, and 15.4% of the ALC-2, VLC-2 and VLC-2*, respectively. The results show that the rate of tension development is more than seven-fold higher in the atrial compared with ventricular fibre bundles. These results correlate with the differences in ATPase activity of the contractile proteins in solution and, most likely, reflect differences in the myosin isoform composition. In ventricular fibre bundles the increased levels of light chain phosphorylation were associated with increased rate of contraction.