Ruthenium red inhibited Ca2+-ATPase and ATP-independent Ca2+ binding with rat heart sarcolemma in a concentration dependent manner; significant effects were evident at 0.25 microM and higher concentrations. The apparent Ka for Ca2+-ATPase was 1.02 +/- 0.02 mM Ca2+ and 1.47 +/- 0.12 mM Ca2+ in the absence and presence of 2.5 microM ruthenium red, respectively; however, no change in the Vmax (41.2 +/- 1.6 mumol Pi/mg/h) was observed. Likewise, the affinity of Ca2+ for both low and high affinity Ca2+ binding sites in sarcolemma was decreased by ruthenium red. Sarcolemmal Na+-dependent Ca2+ uptake, ATP-dependent Ca2+ accumulation, Mg2+-ATPase and Na+,K+-ATPase activities were not affected by ruthenium red. In sarcoplasmic reticulum preparations, ruthenium red (0.25 to 25 microM) enhanced Ca2+ uptake without altering the Ca2+-stimulated ATPase activity. The observed increase in Ca2+ uptake appears to be due to the depressant effect of the dye on Ca2+ release from the sarcoplasmic reticulum. In mitochondrial preparations, ruthenium red (0.025 to 25 microM) showed a marked inhibitory effect on Ca2+ uptake activity whereas the Mg2+-ATPase activity was unaltered. In isolated rat hearts, 0.025 microM ruthenium red produced a slight negative inotropic effect, whereas 0.25 to 2.5 microM ruthenium red elicited a biphasic response both in terms of developed tension and resting tension. High concentrations of ruthenium red (12.5 to 25 microM) resulted in the development of contracture. Electron microscopic studies revealed the presence of ruthenium red in the myoplasm of hearts perfused for 15 to 30 mins with 2.5 to 5 microM dye.(ABSTRACT TRUNCATED AT 250 WORDS)