Abstract
The mechanical response of the red and white muscles of the silver carp was investigated using linearly stimulating currents. In the presence of tetrodotoxin, the peak tension increased when the rate of rise was reduced to about 0.3mA/sec in the red muscle, and to about 3mA/sec in the white muscle, provided that the maximum current applied always reached 3mA. Further reduction of the rate of rise decreased the tension, particularly in the white muscle. Low concentrations of caffeine (0.5 mM) potentiated mainly the early phase of contraction. However, the tension produced by a stimulating current with a rate lower than this value was suppressed by caffeine. When the external K ion was cumulatively increased to more than 13.5mM in the red muscle and to more than 10.8mM in the white muscle, the tension elicited by a linearly rising current was reduced. La and Mn ions suppressed mainly the late phase of contraction.
It is postulated that two different mechanisms are involved in increasing the intracellular-free Ca concentration. The early phase of contraction may be a result of Ca release from the surface of terminal cisternae of the sarcoplasmic reticulum triggered by depolarization. The late phase produced by a prolonged depolarization may be a consequence of Ca movement across the plasma membrane or the membrane of sarcoplasmic reticulum. There seems to be an inactivation process for both the early phase and late phase. The difference in mechanical properties between the red and white muscles may be due to the presence of strong inactivation in the excitation-contraction coupling in the white muscle.
