CONTRACTILE BEHAVIOUR OF CARDIAC VENTRICULAR MUSCLE IN STRONTIUM SOLUTION

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Abstract
The contractility of the ventricular muscle of bullfrog in Ca-free Sr Ringer was investigated. The depression of the rate of rise of twitch tension and prolongation of the time to peak tension were observed. The latter change was closely related to the lengthening of the action potential duration. Muscle washout 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 K 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 K 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 predominantly augmented. Noradrenaline [norepinephrine] in 10-5 g/ml exerted a negative inotropic effect on the K contracture of the Ca-perfused muscle but it did not demonstrate any inhibitory effect on the K contracture of the Sr-perfused muscle although a marked prolongation and a positive inotropism of the twitch contraction were observed in the latter condition. EM study revealed that electron-opaque granules, identified as Sr by means of X-ray spectrum microanalysis, are located in the sarcoplasmic reticulum and mitochondria. Apparently Sr replaces Ca in maintaining the electrical and mechanical activities of the heart muscle. Possibly Sr suppresses the inactivation kinetics of the slow ionic channel and exerts some inhibitory effects on myofibrillar ATPase activity. The possibility of the 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 ineffective in Sr uptake.