Ca2+ Waves within Intact Rat Cardiac Trabeculae with Nonuniform Muscle Contraction

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Background: In the myocardium with nonuniform contraction, Ca2+ waves are initiated not only by spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), but also by dissociation of Ca2+ from the myofilaments. We investigated whether nonuniform contraction affects Ca2+ waves. Methods: Rat trabeculae were regionally exposed to a jet of solution containing 1) 10 mM Ca2+ (JetHCa) for the induction of spontaneous Ca2+ release from the SR, and 2) 5 mM caffeine (JetCaff) or 0.2 mM Ca2+ (JetLowCa) for the regional reduction of muscle contraction. Force, sarcomere length, and [Ca2+]i were measured (24°C, [Ca2+]o=2.0 mM). Results: The JetHCa caused spatially uniform contraction, while the JetCaff and JetLowCa caused nonuniform contraction. The enhancement of force by the 10% stretch increased the propagation velocity (Vprop) of Ca2+ waves by the JetHCa, JetCaff, and JetLowCa. Interestingly, after preincubation with 10 µM colchicine, this enhancement of force did not increase the Vprop by the JetHCa, while it did increase it by the JetCaff and JetLowCa. The reduction of force by 10 µmol/L blebbistatin did not change the Vprop by the JetHCa, while it decreased it by the JetCaff and JetLowCa irrespective of preincubation with colchicine. Conclusions: These results suggest that even with microtubule-mediated modulation, the force developed during muscle contraction modulates the Vprop in the myocardium with nonuniform contraction.