During heart failure, force production by the heart decreases. This may be overcome by Ca2+-sensitizing drugs, which increase myofibril Ca2+ sensitivity without necessarily altering intracellular Ca2+ concentration. However, Ca2+ sensitizers slow the relaxation of intact cardiac muscle. We used diazo-2, a caged chelator of Ca2+, to study the effects of the Ca2+ sensitizers caffeine and CGP 48506 on the intrinsic relaxation rate of cardiac myofibrils. Trabeculae from rat right ventricles were skinned by 1% Triton X-100 and were activated in a 10-microL bath. In steady state experiments, CGP 48506 (10 micromol/L) shifted the force-pCa curve leftward by 0.41+/-0.03 pCa units (mean+/-SEM, n=6). An identical shift was induced by caffeine (20 mmol/L). Photolysis of diazo-2 by a flash of light (160 mJ, 310 to 400 nm) caused an immediate decrease in Ca2+-activated force produced by the trabeculae. Relaxation was fitted by a double-exponential decay, and the rate constants were found to be independent of force and preflash Ca2+ concentration. The initial fast rate, corresponding to myofibrillar relaxation, was increased from 17.3+/-2.0 to 30.9+/-3.7 s(-1) (n=4) by caffeine but was unaffected by CGP 48506 (16.6+/-1.7 and 14.4+/-2.3 s(-1) in the absence and presence of drug, respectively; n=5). Thus, myofibril relaxation need not be slowed by Ca2+-sensitizing agents but can even be accelerated. Despite similarities in their effects on myofibril Ca2+ sensitivity, caffeine and CGP 48506 affect the myofibrils at least partly via different mechanisms.