Citation/Abstract

Cardiac myocytes, in the intact heart, are exposed to shear/fluid forces during each cardiac cycle. Here we describe a novel Ca ²⁺ signaling pathway, generated by "pressurized flows" (∼300 ms) intracellular 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 (SAC), 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 signaling 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 tempo1, 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⁺ .