Abstract
The contractility of the ventricular muscle of bullfrog in Ca-free Sr Ringer was investigated. The well-known depression of the rate of rise of twitch tension as well as prolongation of the time to peak tension were observed. The latter change was closely related to the lengthening of the action potential duration. The washout of muscle with a divalent cation-free perfusate caused a more rapid decline of the twitch contraction in the muscle preparation preloaded with Sr than in that preloaded with Ca. High potassium solution containing 30 mM caffeine produced a contracture in the muscle preloaded with Sr even after prolonged divalent cation-free perfusion although its magnitude was smaller than in the muscle preloaded with Ca.
The pattern of the potassium contracture of the muscle perfused with Sr was quite different from that perfused with Ca. In the former condition the initial transient component was suppressed while the secondary sustained component was prominently augmented. Noradrenaline in 10-5g/ml exerted a negative inotropic effect on the potassium contracture of the Ca-perfused muscle whereas it did not demonstrate any inhibitory effect on the potassium contracture of the Sr-perfused muscle although a marked prolongation as well as a positive inotropism of the twitch contraction were observed in the latter condition.
An electron microscopic study revealed that electron-opaque granules, identified as Sr by means of X-ray spectrum microanalysis, are located both in the sarcoplasmic reticulum and mitochondria.
From these results it was concluded that Sr replaces Ca in maintaining the electrical and mechanical activities of the heart muscle. It is also suggested that Sr suppresses the inactivation kinetics of the slow ionic channel and exerts some inhibitory effects on myofibrillar ATPase activity. The possibility of contribution of the intracellular binding sites to the regulation of contraction is discussed.Noradrenaline seems to stimulate Ca uptake by the sequestering system but appears to be ineffective in Sr uptake.
