흰쥐 심근의 역 사다리 효과에 있어서 생후 연령에 따른 Na+-Ca2+ 교환의 역할에 관한 연구

Studies on the Roles of Na+-Ca2+ Exchange according to Postnatal Age in the Negative Staircase Effect of the Rat Heart

<P> Recent reports revealed that the Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> 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<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> exchanger may decrease the tension development by transporting the Ca<SUP>2&#8290;</SUP> out of the cell right after it released from SR, on the basis of this anatomical proximity. We exammined 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(3x10<SUP>-5</SUP> M) treatment decreased the developed tension further according to postnatal age. Monensin(3x10<SUP>-6</SUP> 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. Ca<SUP>2&#8290;</SUP> 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<SUP>&#8290;</SUP> and Ca<SUP>2&#8290;</SUP> depletion was only a half of those of the yonger groups. Taken together, it is concluded that the Na<SUP>&#8290;</SUP>-Ca<SUP>2&#8290;</SUP> exchange transports more Ca<SUP>2&#8290;</SUP> 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.