Cross-Bridge-Dependent Change in Ca^<2+> Sensitivity is Involved in the Negative Inotropic Effect of Nifedipine in Aequorin-Injected Ferret Ventricular Muscles

Background We hypothesized that the negative inotropic effect of nifedipine (Nif) on cardiac ventricular muscle is partly due to the cross-bridge-dependent decrease of Ca^<2+> sensitivity of the myofilaments as well as the decrease in Ca^<2+> influx. Method and Results We used aequorin-injected ferret papillary muscles and measured the slope of the extra-Ca^<2+>-tension relation which expresses the change in the Ca^<2+> sensitivity through the feedback from the cross-bridges. Twitch tension was decreased significantly by 0.5μmol/L Nif accompanying a significant reduction of the Ca^<2+> transient peak. When Nif (0.2-0.5μmol/L) was added to the solution with 8mmol/L Ca^<2+>, the slope of the extra-Ca^<2+>-tension relation became steeper in a concentration-dependent manner, which was similar to the change in the slope when the concentration of Ca^<2+> was decreased from 8 to 1mmol/L in the absence of Nif. BAY-K 8644 (0.3μmol/L), a dihydropyridine receptor agonist, showed the opposite effect on the slope of the extra-Ca^<2+>-tension relation to that observed in Nif. However, 2, 3-butanedione monoxime (3mmol/L), an inhibitor of the active cross-bridges, antagonized the effect of BAY-K 8644. Conclusion Nif exerts its negative inotropic effect on cardiac muscle by suppressing Ca^<2+> binding to troponin C via the inhibition of the L-type Ca^<2+> channel, and by the cross-bridge-dependent decrease in the Ca^<2+> sensitivity, as in low extracellular Ca^<2+> concentration.