The Mode of the Staircase Phenomenon in Relation to Ryanodine Sensitivity in Rat Papillary Muscle, Bullfrog Atrium and Frog Ventricle
Frog ventricular muscle, when repetitively stimulated at an interval of 1 sec, showed a gradual increase in contraction strength; i.e., the positive staircase phenomenon. The tension of rat papillary muscle, on the other hand, was maximum in the first contraction, after which it decreased gradually during repetitive stimulation; the negative staircase phenomenon. The tension of bullfrog atrial muscle was large in the first contraction, but decreased in the next 15 to 20 contractions and then increased in the following ones. The tension was small if the stimulation interval was prolonged in frog ventricular muscle (positive relationship between contraction strength and stimulation interval). This relationship was reversed in rat papillary muscle. The optimum Ca2+ concentrations for maximum tension development at a stimulation interval of 5 sec were 7.2-9.0 mM in frog muscles and 3.6 mM in rat muscles. Ryanodine inhibited the contraction in rat papillary muscle at lower concentration than those in bullfrog atrial muscle, and it had much less effect on frog ventricular muscle. These results indicate that tissue having high sensitivity to ryanodine exhibits the negative staircase phenomenon, a negative relationship between interval and strength, and low dependency on external Ca2+ concentration.