INHIBITION BY EXTERNAL CD2+ OF NA CA EXCHANGE AND L-TYPE CA CHANNEL IN RABBIT VENTRICULAR MYOCYTES/

We investigated the effect of external Cd2+ on the Na/Ca exchange and the L-type Ca channel current (I-Ca,I-L) in whole cell patch-clamped r abbit ventricular myocytes at 36 degrees C. After the interfering ion channels and the Na/K pump were blocked, the exchange current was meas ured as the membrane current that was inhibited by 5 mM nickel. Extern al Cd2+ inhibited Na/Ca exchange with a dissociation constant (K-D) of 320.6 +/- 12.4 mu M and a Hill coefficient of 1.5 +/- 0.09 (n = 13 ce lls) and I-Ca,I-L with a K-D of 2.14 +/- 0.15 mu M and a Hill coeffici ent of 0.74 +/- 0.03 (n = 11 cells). We observed some overlap in the C d2+ concentration that blocked each mechanism. Cd2+ (100-500 mu M) is used commonly to block I-Ca,I-L completely. However, 100 mu M Cd2+ als o inhibits 20% of the Na/Ca exchange activity, whereas 500 mu M Cd2+ i nhibits 60%.