Cross-Bridge-Dependent Change in Ca2+ Sensitivity is Involved in the Negative Inotropic Effect of Nifedipine in Aequorin-Injected Ferret Ventricular Muscles

Abstract

ifedipine (Nif), a representative dihydropyridine receptor antagonist, is widely used in research and clinical courses 1-3 because of its well-known effects on the cardiovascular system through L-type Ca 2+ channel inhibition. The intracellular mechanism of the negative inotropic effect of Nif on cardiac muscle contraction is due to a decrease in the magnitude of the Ca 2+ transient (a transient change in intracellular Ca 2+ concentration ([Ca 2+ ]i)) that is induced by a decrease in Ca 2+ influx through L-type Ca 2+ channel, 2,4,5 an essential factor for Ca 2+ release from the sarcoplasmic reticulum (SR), although there is no proof that shows the direct effect of Nif on the Ca 2+ sensitivity of myofilaments. 2,4,5 The regulatory mechanism of cardiac muscle contraction is divided into 3 steps: 6,7 (1) a change in [Ca 2+ ]i (upstream mechanism); (2) Ca 2+ binding to troponin C (TnC) (Ca 2+ sensitivity) (central mechanism); and (3) We postulated that Nif exerts its negative inotropic effect by decreasing the amount of Ca 2+ binding to TnC (central mechanism) through an inhibition of Ca 2+ current (upstream mechanism), and also via a decrease in the Ca 2+ affinity for TnC (central mechanism) due to the feedback mechanism from the cross-bridges to TnC.In order to prove the hypothesis, we evaluated the crossbridge-dependent change in Ca 2+ sensitivity in the presence of Nif in intact ferret papillary muscles injected with aequorin as we previously reported. 9 The preliminary result was already presented at an annual meeting of the Japanese Circulation Society and was reported in abstract form. 10 Methods PreparationAdult male ferrets were anesthetized with pentobarbital sodium (intraperitoneal injection, 100 mg/kg) and the heart was quickly removed. Our experimental procedure was approved by the Guidance for Animal Experiments at The Jikei University School of Medicine. After washing the 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 ResultsWe used aequorin-injected ferret papillary muscles and measured the slope of the extraCa 2+ -tension relation which expresses the change in the Ca 2+ sensitivity through the feedback from the crossbridges. Twitch tension was decreased significantly by 0.5 mol/L Nif accompanying a significant reduction of the Ca 2+ transient peak. When Nif (0.2-0.5 mol/L) was added to the solution with 8 mmol/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 1 mmol/L in the absence of Nif. BAY-K 8644 (0.3 mol/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 (3 mmol/L), an inhibitor of the active cross-bridge...