ABSTRACT
Recent reports revealed that the Na+/-Ca2+ exchangers and feet structures of sarcoplasmic reticulum (SR) are located in close vicinity in the specific compartment. Therefore, we investigated the possibility that the Na+/-Ca2+ exchanger may decrease the tension development by transporting the Ca2+ out of the cell right after it released from SR, on the basis of this anatomical proximity. We examined the negative force-frequency relationship of the developed tension in the electrically field stimulated left atria of postnatal developing rat (1, 3 day, 1 week and 4 week old after birth). Cyclopiazonic acid (3 X 10(-5) M) treatment decreased the developed tension further according to postnatal age. Monensin (3 X 10(-6) M) treatment did not increase the maximal tension in 4 week-old rat, preserving negative staircase, while the negative staircase in the younger rat were flattened. Ca2+ depletion in the buffer elicited more suppression of the maximal tension according to the frequency in all groups except the 4 week-old group. The % decrease of the maximal developed tension of 4 week-old group at 1 Hz to that of 0.1 Hz after Na+ and Ca2+ depletion was only a half of those of the younger groups. Taken together, it is concluded that the Na+/-Ca2+ exchange transports more Ca2+ released from SR out of the cell in proportion to the frequency, and this is responsible for the negative staircase effect of the rat heart.