Mechanical restitution and recirculation fraction in cardiac myocytes and left ventricular muscle of adult rats

Features the interaction of functional and metabolic remodeling of myocardium in comorbid course of ischemic heart disease and 2 type diabetes mellitus

Background: Metabolic and structural changes in cardiomyocytes in diabetes mellitus lead to aggravation of contractile myocardial dysfunction in coronary heart disease (CHD). The contractility dysfunction of cardiomyocytes is determined by a change in the levels of sarcoplasmic reticulum (SR) Ca2+-ATPase and energetic supply of the cardiomyocytes. Aims: To study the features of functional remodeling of the heart muscle in coronary heart disease with and without type 2 diabetes mellitus (DM2) depend on the level of Ca2+-ATPase and the activity of enzymes involved in energy metabolism. Materials and methods: The work was performed on the heart biopsy of patients with CHD and patients with CHD combined with DM2. The inotropic reaction of myocardial strips on rest periods was assessed. The expression level of Ca2+-ATPase, the activity of enzymes succinate dehydrogenase (SDH) and lactate dehydrogenase (LDH) and the intensity of oxidative phosphorylation processes were determined. Results: The interval-force relationship in patients with CHD with and without DM2 had both negative and positive dynamics. The positive dynamics corresponds to the "high content" of the Ca2+-ATPase and the negative dynamics corresponds to the "low content" were found. At the combined pathology the positive inotropic dynamics is more pronounced and corresponds to a higher protein level. In the patients myocardium with CHD the activity of SDH and LDH was higher, while the oxygen uptake rate by mitochondria was higher in the myocardium with combined pathology. Conclusions: The potentiation of inotropic response of patient myocardium with CHD with and without DM2 corresponds to the "high level" of Ca2+-ATPase. In the combined pathology the inotropic capabilities of the myocardium are more expressed. In CHD the synthesis of ATP in cardiomyocytes is realized mainly due to glycolytic processes and Krebs cycle. In combined pathology the ATP synthesis is realized to a greater extent due to the oxidative phosphorylation.

Influence of Amiodarone and Dronedarone on the Force-Interval Dependence of Rat Myocardium

The antiarrhythmic effect of amiodarone and its analogue dronedarone is caused by their direct actions on several cardiomyocyte sarcolemmal ion currents. However, whether their effects are related to intracellular calcium levels is not exactly known. Ca²⁺ cycling refers to the release and reuptake of intracellular Ca²⁺, which induces muscle contraction and relaxation and determines the force-interval dependence. This study aimed to evaluate the influence of amiodarone and dronedarone on the force-interval relationship. Materials and Results. The work was performed on the papillary muscles of the left ventricle of male Wistar rats. Muscle perfusion was performed at 36.5°C with oxygenated Krebs-Henseleit solution with baseline stimulation 0.5 Hz. The postrest test (4-60 s) and the extrasystolic exposure (0.2-1.5 s) were evaluated. Inotropic reaction to the test exposure was evaluated before and after muscle perfusion with solution containing amiodarone (10⁻⁶ M) or dronedarone (10⁻⁶ M) during 10 min. Amiodarone or dronedarone led to decrease of the amplitude of extrasystolic contractions of the papillary muscles. The amplitude of postextrasystolic contractions after short extrasystolic intervals on the background of the drugs was increased. Amiodarone and dronedarone led to increase of the amplitude of postrest contractions. Conclusions. Dronedarone reduces the excitability of cardiomyocyte sarcolemma to a greater extent than amiodarone. Amiodarone and dronedarone are able to increase postextrasystolic and postrest potentiation. The effect of amiodarone on postextrasystolic and postrest potentiation is more pronounced in comparison with dronedarone.

Novel means to monitor cardiac performance: the impact of the force-frequency and force-interval relationships on recirculation fraction and potentiation ratio

Insights into intracellular calcium regulation and contractile state can be accomplished by changing pacing rate. Steady-state increases in heart rate (HR) (force-frequency relationship, FFR), and introduction of extrasystoles (ES) (force-interval relationship, FIR) have been used to investigate this relationship. This study focused on the recirculation fraction (RF) and potentiation ratio (PR), obtained from the recovery of the FFR and FIR. These parameters may provide insight on intracellular Ca(2+) regulation. Left ventricular (LV) pressures and HR were assessed in anesthetized canines (n = 7). Intrinsic data were collected prior to and following HR increases to 150, 180, and 200 bpm, as well as following delivery of an ES at 280 ms. The RF was calculated as the slope of dP/dt(max(n + 1)) vs. dP/dt(max(n)), where n = beat number. The PR was calculated by normalizing dP/dt(max) from the first beat following the ES (or the last paced beat) to the steady-state dP/dt(max). The RF due to an ES was not significantly different than that from a HR of 200 bpm. The PR from an ES was not significantly different than from a HR of 150 bpm. The impact of an ES delivered at an interval of 280 ms produces a PR similar to that from a HR of 150 bpm; yet, it recovers similarly to the termination of pacing at 200 bpm, eliciting a similar RF value. The method of measuring RF by an ES versus an increased HR may provide a safer and more feasible approach to collecting diagnostic information.

Effects of interleukin-2 on the mechanical restitution and post-rest contraction in rat ventricular papillary muscle

To determine whether application of interleukin-2 (IL-2) alters function of sarcoplasmic reticulum (SR), we measured mechanical restitution and post-rest potentiation (PRP) in isolated rat papillary muscles. Mechanical restitution curves were constructed by interpolating extrasystoles at different test intervals following a train of steady state beats. In control group, the maximal PRP was reached after 60-120s of rest and the maximal potentiation factor was 2.36 +/- 0.23. IL-2 at 200 U/ml decreased the steady-state force of contraction to 56.4 +/- 7.2% of pre-drug control. But the time constant of recovery of steady-state force was not altered after IL-2. IL-2 decreased PRP at all intervals, shifted the potentiation curve parallel to lower values. But the potentiation was enhanced when compared with pre-rest control value in the presence of IL-2. In papillary muscle treated with IL-2, the onset of maximal PRP was delayed and the potentiation factor after 300s was 4.72 +/- 0.58 times that at the steady-state. Recirculation fraction of calcium calculated from the decay of PRP was 0.78 +/- 0.09 in control and 0.59 +/- 0.08 after IL-2 treatment. We conclude that IL-2 decreases the function of SR, which suggests that an impaired function of SR may contribute to the negative inotropic effect of IL-2.

Exponential fitting of postextrasystolic potentiation may underestimate the cardiac Ca2+ recirculation fraction: a theoretical analysis

The recirculation fraction of intramyocardial Ca(2+) (RF) has conventionally been obtained from the monotonic decay of postextrasystolic potentiation (PESP). The used assumption is that the decay is exponential. However, we have found that PESP usually decays in alternans even at spontaneous heart rates (>100 beats/min) in excised, cross-circulated canine heart preparations under normal coronary perfusion and normothermia. We have already devised a means of extracting the exponential decay component for RF calculation by subtracting the oscillatory component from the alternans PESP decay by a curve-fitting method. Using mathematics, we assessed the possible error in estimated RF when an exponential curve was naively fit to the alternans PESP decay. We obtained results showing that the exponential assumption may considerably underestimate RF even when the alternans is trivial with the oscillatory component of only 10% of the exponential component.

Mechanical restitution in atrial muscle from human and rat hearts: effects of agents that modify sarcoplasmic reticulum function

Force of contraction (Fc) of isolated human and rat atrial myocardium shows characteristic patterns of mechanical restitution when single test intervals are interposed in regular stimulation. With several pharmacological agents that modify the function of the sarcoplasmic reticulum we have investigated the role of the sarcoplasmic reticulum in mechanical restitution in these two species. Caffeine, thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (BHQ) were used to reduce Ca2+ uptake, ryanodine to open Ca2+ release channels, and forskolin to stimulate Ca2+ uptake. Under control conditions, Fc recovered rapidly with test intervals shorter than steady-state, and was potentiated with longer than steady-state intervals. In human atrial tissue the maximum potentiation factor was 1.26 +/- 0.03 after a mean test interval of 9.70 +/- 1.55 s (n = 43) as compared to 3.07 +/- 0.45 after 30 sec. in rat atria (n = 48). Caffeine (3 mM) did not significantly affect steady-state Fc but abolished post-rest potentiation in human and rat preparations. Forskolin (1 microM) enhanced and accentuated the mechanical restitution curve in particular for short test intervals. In the presence of thapsigargin (10 microM), steady-state Fc and mechanical restitution could not be distinguished from time-matched controls exposed to solvent only, indicating that this agent is ineffective in human and rat atrial tissue. In contrast, the putative Ca2+ uptake inhibitor BHQ (100 microM) strongly reduced steady-state Fc and decreased potentiation at all intervals in human muscle, but shifted the mechanical restitution curve in parallel to lower values in rat atria. Ryanodine (10 nM) induced post-rest decay in human and depressed both steady-state Fc and post-rest potentiation in rat atrial muscle. From these results it is concluded that human and rat atrial muscle differ in the Ca2+ handling by the sarcoplasmic reticulum during mechanical restitution.