Intracellular Ca++ transients and relaxation in mammalian cardiac muscle

Calcium sensitive photoprotein aequorin was micro-injected into the cells of cat and rat ventricular cardiac muscles. Light and tension were recorded from the injected muscle in order to clarify the relation between the intracellular Ca++ transient and relaxation. The time course of the falling phase of the light signal and the time course of the relaxation were measured under the various conditions, i.e., changes of stimulation frequency, [Ca++] o, and the administration of adrenaline and caffeine. High stimulation frequency accelerated the time course of the fall of light and the relaxation of tension. Caffeine prolonged the relaxation of tension and the falling phase of light. The time course of relaxation did not change substantially but the falling phase time course of the light was accelerated by increase of [Ca++] o and adrenaline. These results confirm that a fall in [Ca++] i always precedes relaxation but suggest that the decrease of [Ca++] i is not always the rate limiting step of the relaxation in the mammalian cardiac muscle.