title: Electric currents applied during refractory period enhance contractility and systolic calcium in the ferret heart.
abstract: We investigated the mechanism of positive inotropism of electric currents applied during the absolute refractory period. Ten Langendorff-perfused ferret hearts were instrumented to measure isovolumic left ventricular pressure (LVP) and the aequorin luminescence. Biphasic square-wave electric currents (+/-20 mA, total duration 30 ms) were delivered between pairs of electrodes. Six hearts were perfused at different extracellular Ca(2+) concentrations ([Ca(2+)](o); 1, 2, 4, and 8 mM). These signals increased LVP from 50.0 +/- 9.4 to 70.1 +/- 14.7, from 67.5 +/- 11.0 to 79.0 +/- 15.6, from 79.3 +/- 21.0 to 87.1 +/- 22.8, and from 84.6 +/- 24.0 to 91.8 +/- 28.5 mmHg at the respective [Ca(2+)](o) (P < 0.05). Peak free intracellular [Ca(2+)] ([Ca(2+)](i)) increased from 0.52 +/- 0.13 to 1.37 +/- 0.23, from 0.76 +/- 0.23 to 1.73 +/- 0.14, from 1.10 +/- 0.24 to 2.05 +/- 0.33, and from 1.41 +/- 0.36 to 2.24 +/- 0.36 microM/ml, respectively (P < 0.001). With the use of 1 mg/l propranolol with 1 mM [Ca(2+)](o), LVP and [Ca(2+)](i) were increased significantly from 48.7 +/- 8.18 to 56.3 +/- 6.11 mmHg and from 0.61 +/- 0.11 to 1.17 +/- 0.20 microM, respectively (P < 0.05). In conclusion, positive inotropism of such electrical currents was due to increased peak [Ca(2+)](i) and Ca(2+) responsiveness of the myofilaments did not change significantly.
authors.affiliation: Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, New York 10032, USA.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Mohri
authors.forename: Juichiro
authors.initials: J
authors.lastname: Shimizu
authors.forename: Yuval
authors.initials: Y
authors.lastname: Mika
authors.forename: Itzhak
authors.initials: I
authors.lastname: Shemer
authors.forename: Jie
authors.initials: J
authors.lastname: Wang
authors.forename: Shlomo
authors.initials: S
authors.lastname: Ben-Haim
authors.forename: Daniel
authors.initials: D
authors.lastname: Burkhoff
mesh_terms: D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000331:Aequorin; D000818:Animals; D001794:Blood Pressure; D002118:Calcium / Q000032:analysis* / Q000378:metabolism; D004553:Electric Conductivity; D005289:Ferrets / Q000502:physiology*; D005456:Fluorescent Dyes; D008163:Luminescent Measurements; D009200:Myocardial Contraction; D009638:Norepinephrine / Q000378:metabolism; D011433:Propranolol / Q000494:pharmacology; D013268:Stimulation, Chemical; D013599:Systole / Q000502:physiology*; D016277:Ventricular Function, Left
chemical_list: D000319:Adrenergic beta-Antagonists; D005456:Fluorescent Dyes; D000331:Aequorin; D011433:Propranolol; D002118:Calcium; D009638:Norepinephrine