‘Pressure-flow’-triggered intracellular calcium 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+ signaling pathway, generated by “pressurized flows” (∼300 ms) intracellular Ca2+ 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 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 (SAC), or the Na+/Ca2+ exchanger (NCX), using Cd2+, Gd3+, or Ni 2+, 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 Ca 2+ 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 signaling 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 tempo1, 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+.