Effects of various calmodulin antagonists on Na/Ca exchange current of single ventricular cells of guinea-pig.

The effects of various calmodulin inhibitors were examined on the Na/Ca exchange current in single cardiac ventricular cells of the guinea-pig using the whole-cell patch-clamp technique. External application of W-7 and trifluoperazine inhibited Na/Ca exchange current in a dose-dependent manner with IC50 values of 13 and 7 microM, respectively. W-5 inhibited the exchange current but less potently than W-7. More specific calmodulin inhibitors such as CGS 9343B and calmidazolium did not, however, decrease the current as significantly as expected. All these drugs inhibited the Na current more strongly than the Na/Ca exchange current. Ruthenium red (RR), another type of calmodulin inhibitor, did not decrease the exchange current by internal application. Neither mastoparan or melittin (calmodulin-binding peptides) inhibited the exchange current appreciably. RR and the peptides did not affect the Na current either. These results indicate that calmodulin may not involved in the activation of cardiac Na/Ca exchange or the Na current. Internal application of chymotrypsin inhibited the blocking effect of W-7 on the Na/Ca exchange current but not that on the Na current. These results indicate that W-7 blocks the Na/Ca exchange current not by binding to calmodulin but possibly by directly affecting an internal site of the exchanger itself and that the inhibitory action of W-7 is different on the Na/Ca exchange current and the Na current.