The effects of ouabain on the contractile and electrical activities were investigated in the isolated preparations of guinea-pig taenia coli, and compared with those of vanadate. Spontaneous contractions were recorded with force transducer, and electrical activites were measured by use of suction electrode, or single sucrose-gap technique. The contractions were induced by the electrical stimulation for 5 seconds every 1 minute with alternating current (60 Hz, 3.0 V/cm) through the platinum electrodes located in parallel with the long axis of the preparation. All experiments were performed in tris-buffered Tyrode solution which was aerated with 100% O<sub>2</sub> and kept at 35℃. The results obtained were as follows: 1) Responses of spontaneous contractions to ouabain were concentration-dependent; 10<sup>-7</sup>M ouabain caused a rise of basal tone. Above the concentration of 10<sup>-6</sup>M ouabain, an initial increase followed by a decrease in tension was observed. 2) A continuous spike discharge was induced by the administration of 10<sup>-7</sup>M ouabain. Above 10<sup>-6</sup>M ouabain, a transient initial increase followed by a decrease in spike frequency and amplitude was produced, and finally membrane potential was sustained at a certain level without a spike discharge. 3) The characteristic response to 10<sup>-7</sup>M ouabain was not blocked by the pretreatment with 10<sup>-7</sup>M atropine. 4) The electrically induced contractions were completely suppressed at the concentration of 2 X 10<sup>-7</sup>M ouabain. These contractions were blocked more rapidly in paralled with the increase in ouabain concentration. 5) Effects of vanadate on the spontaneous activities were quite different from those of ouabain; 10<sup>-6</sup>M vanadate increased the amplitude of contractions and 10<sup>-5</sup>M vanadate increased slightly both amplitude and frequency of spontaneous contractions. 10<sup>-4</sup>M vanadate showed irregular phasic contractions superimposed on the increased basal tone. 6) 10<sup>-5</sup>M vanadate depolarized the membrane potential and shortened the interval between the bursts of spike discharge, whereas 10<sup>-4</sup>M vanadate induced continuous spike discharge with membrane depolarization. 7) Vanadate caused a characteristic inhibitory response to the contractions induced by electrical stimulation; An initial rapid inhibition of tension development and then gradual recovery to a certain level. From the above results, the following conclusions could be made: 1) The rise of basal tone at 10<sup>-7</sup>M ouabain is due to continuous spike discharge without a silent period. The continuous spike discharge is likely to be associated with a slight membrane depolarization caused by the blockage of Na pump. 2) The biphasic response induced by above 10<sup>-6</sup>M> ouabain seems to occur by the different mechanisms. The initial increase in tension is associated with depolarization along with an increase in spike frquency, whereas the subsequent relaxation occurs through a non-electrical mechanism. 3) The characteristic response to 10<sup>-7</sup>M ouabain is resulted not from the action on intrinsic nerve terminal, but from its direct action on the membrane of smooth muscle cells. 4) The phasic contractions superimposed on the increased basal tone at the concentration of 10<sup>-4</sup>M vanadate is resulted from the continuous spike discharge with membrane depolarization, of which mechanism remains unknown. 5) The inhibitory action of ouabain on the electrically induced contractions suggests that the increasein intracellular Na in some way inhibits the electrically induced Ca<sup>2+</sup> influx. The mechanism of vanadate action on the induced contractions remains unknown.
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