The sufficient myoplasmic Ca++ to react with the contractile proteins is necessary to induce contraction of a cardiac muscle. These Ca++ for the production of muscle contraction are supplied from the three recognized Ca++ sources; internal Ca++ release via the sarcoplasmic reticulum(SR), Ca++ influx through a gated Ca-channel in the membrane as a Isi, and Ca++ transport by the mechanism of Na/ca exchange. However, it is still controversial which Ca++ sources act as a main contributor for myoplasmic Ca++, Therefore, this study was undertaken in order to examine the Ca++ sources for the contraction of frog ventricle. There is evidence that the SR is sparse in frog ventricular fibers, and that T-tubules are absent. Isolated ventricular strips of frog, Rana nigromaculata, were used in this experiment. Isometric tension was recorded by force transducer, and membrane potentials of ventricular muscles were measured through the intracellular glass microelectrodes, which were filled with 3M KCI and had resistance of 30±50MΩ. All experiments were performed at room temperature in a tris¡¤buffered Ringer solution which was aerated with 100% O2. Isotonic high K, low Na solution was used to induce K-contracture, K-contracture appeared at the concentration of 20 to 30mM-KCI and was potentiated in parallel with the increase in KCI concentration. The contracture had two components: an initial rapid phasic and a subsequent slow tonic contractile responses. Membrane Potentials measured at normal Ringer solution(2.5mM KCI) was -90 to -100 mV, and decreased linearly as the KCI concentration increased; -55mV at 20mM.KCI, -45mV at 30 mM.KCI, -30 mY at 50 mM.KCI, and -12 mV at 100 mM.KCI. K-contracture was evoked firstly at the membrane potential of -45 mV. The contracture was potentiated by the increase of bathing extracellular Ca++ concentration. However, in the absence of Ca++ the contracture was almost not induced by 50 mM.KCI solution. Caffeine(20mM) in normal Ringer solution, which is known to release Ca++ from SR without substantial effects on the Ca++ fluxes across the surface membrane, did not affect membrane potential and also not initiate contracture, but the caffeine in 20 mM-KCI Ringer solution produced a contracture. Above results suggest that the main Ca++ source for the K¡¤contracture of frog ventricle is Ca++ influx through the voltage-dependent Ca-channel, and that in the K-contracture at the concentration of 100 mM-KCI, the mechanism of Na/ca exchange also partly contributs, in addition to the Ca++ influx.
