'Pressure-flow'-triggered intracellular Ca2+ transients in rat cardiac myocytes:: possible mechanisms and role of mitochondria

Cardiac myocytes, in the intact heart, are exposed to shear/fluid forces during each cardiac cycle. Here we describe a novel Ca2+ signalling pathway, generated by 'pressurized flows' (PFs) of solutions, resulting in the activation of slowly developing (similar to 300 ms) Ca2+ transients lasting similar to 1700 ms at room temperature. Though subsequent PFs (applied some 10-30 s later) produced much smaller or undetectable responses, such transients could be reactivated following caffeine- or KCl-induced Ca2+ releases, suggesting that a small, but replenishable, Ca2+ pool serves as the source for their activation. PF-triggered Ca2+ transients could be activated in Ca2+-free solutions or in solutions that block voltage-gated Ca2+ channels, stretch-activated channels (SACs), or the Na+-Ca2+ exchanger (NCX), using Cd2+, Gd3+, or Ni2+, respectively. PF-triggered Ca2+ transients were significantly smaller in quiescent than in electrically paced myocytes. Paced Ca2+ transients activated at the peak of PF-triggered Ca2+ transients were not significantly smaller than those produced normally, suggesting functionally separate Ca2+ pools for paced and PF-triggered transients. Suppression of nitric oxide (NO) or IP3 signalling pathways did not alter the PF-triggered Ca2+ transients. On the other hand, mitochondrial metabolic uncoupler FCCP, in the presence of oligomycin (to prevent ATP depletion), reversibly suppressed PF-triggered Ca2+ transients, as did the mitochondrial Ca2+ uniporter (mCU) blocker, Ru360. Reducing agent DTT and reactive oxygen species (ROS) scavenger tempol, as well as mitochondrial NCX (mNCX) blocker CGP-37157, inhibited PF-triggered Ca2+ transients. In rhod-2 AM-loaded and permeabilized cells, confocal imaging of mitochondrial Ca2+ showed a transient increase in Ca2+ on caffeine exposure and a decrease in mitochondrial Ca2+ on application of PF pulses of solution. These signals were strongly suppressed by either Na+-free or CGP-37157-containing solutions, implicating mNCX in mediating the Ca2+ release process. We conclude that subjecting rat cardiac myocytes to pressurized flow pulses of solutions triggers the release of Ca2+ from a store that appears to access mitochondrial Ca2+.