‘Pressure–flow‘‐triggered intracellular Ca 2+ 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 Ca 2+ signalling pathway, generated by ‘pressurized flows’ (PFs) of solutions, resulting in the activation of slowly developing (∼300 ms) Ca 2+ transients lasting ∼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 Ca 2+ releases, suggesting that a small, but replenishable, Ca 2+ pool serves as the source for their activation. PF‐triggered Ca 2+ transients could be activated in Ca 2+ ‐free solutions or in solutions that block voltage‐gated Ca 2+ channels, stretch‐activated channels (SACs), or the Na + –Ca 2+ exchanger (NCX), using Cd 2+ , Gd 3+ , or Ni 2+ , respectively. PF‐triggered Ca 2+ transients were significantly smaller in quiescent than in electrically paced myocytes. Paced Ca 2+ transients activated at the peak of PF‐triggered Ca 2+ transients were not significantly smaller than those produced normally, suggesting functionally separate Ca 2+ pools for paced and PF‐triggered transients. Suppression of nitric oxide (NO) or IP 3 signalling pathways did not alter the PF‐triggered Ca 2+ transients. On the other hand, mitochondrial metabolic uncoupler FCCP, in the presence of oligomycin (to prevent ATP depletion), reversibly suppressed PF‐triggered Ca 2+ transients, as did the mitochondrial Ca 2+ 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 Ca 2+ transients. In rhod‐2 AM‐loaded and permeabilized cells, confocal imaging of mitochondrial Ca 2+ showed a transient increase in Ca 2+ on caffeine exposure and a decrease in mitochondrial Ca 2+ 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 Ca 2+ release process. We conclude that subjecting rat cardiac myocytes to pressurized flow pulses of solutions triggers the release of Ca 2+ from a store that appears to access mitochondrial Ca 2+ .