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Frequency dependence of Ca2+ release from the sarcoplasmic reticulum in human ventricular myocytes from end-stage
K R Sipido1, T Stankovicova, W Flameng
1Laboratory of Experimental Cardiology, University of Leuven, Belgium. karin.sipido@med.kuleuven.ac.be
Insights
Failing human hearts show reduced calcium release from the sarcoplasmic reticulum (SR) at higher heart rates, leading to impaired contractility. This calcium dysregulation is more severe in dilated cardiomyopathy than ischemic cardiomyopathy.
Area of Science:
- Cardiology
- Cellular Physiology
- Biochemistry
Background:
- Human cardiac muscle dysfunction in heart failure is characterized by decreased active tension and increased diastolic tension at higher stimulation frequencies.
- This phenomenon is linked to underlying systolic and diastolic dysfunction, necessitating an investigation into cellular calcium ion ([Ca2+]i) regulation.
Purpose of the Study:
- To investigate the underlying changes in cellular calcium ion ([Ca2+]i) regulation in failing human cardiac muscle.
- To elucidate the mechanisms contributing to the negative force-frequency relationship in cardiomyopathies.
Main Methods:
- Single ventricular myocytes were isolated from explanted human hearts of patients with ischemic or dilated cardiomyopathy.
- Cellular calcium dynamics were studied using whole-cell patch clamp electrophysiology with fluo-3 and fura-red indicators.
Main Results:
- Calcium release from the sarcoplasmic reticulum (SR) decreased at stimulation frequencies above 0.5 Hz, with a more pronounced effect in dilated cardiomyopathy.
- Diastolic intracellular calcium ([Ca2+]i) increased, and action potential duration (APD90) decreased with increasing stimulation frequency.
- L-type calcium current (ICaL) showed reversible decreases at higher frequencies, with slow recovery from inactivation linked to the decay of the calcium transient.
Conclusions:
- The negative force-frequency relation in failing human hearts is primarily caused by reduced myocyte calcium release at frequencies ≥ 0.5 Hz, particularly in dilated cardiomyopathy.
- Inhibition of L-type calcium current (ICaL) at higher frequencies, exacerbated by elevated diastolic [Ca2+]i, contributes to this phenomenon by reducing calcium release triggers and SR calcium loading.
Objectives:
Human cardiac muscle from failing heart shows a decrease in active tension development and a rise in diastolic tension at stimulation frequencies above 50-60 beats/min due to both systolic and diastolic dysfunction. We have investigated underlying changes in cellular [Ca2+]i regulation.
Methods:
Single ventricular myocytes were isolated enzymatically from the explanted hearts of transplant recipients with ischemic cardiomyopathy (nhearts = 5 ncells = 15) or dilated cardiomyopathy (nhearts = 6, ncells = 19). Cells were studied during whole-cell patch clamp with fluo-3 and fura-red as [Ca2+]i indicators (36 +/- 1 degrees C).
Results:
In current clamp mode (action potential recording), the amplitude of Ca2+ release from the sarcoplasmic reticulum (SR) decreased at stimulation frequencies above 0.5 Hz; this decrease was more pronounced for cells from dilated cardiomyopathy. Diastolic [Ca2+]i increased at 1 and 2 Hz for both groups. Action potential duration (APD90) decreased with frequency in all cells; in addition there was a drop in plateau potential of 10 +/- 1 mV for cells from ischemic cardiomyopathy and of 13 +/- 2 mV for cells from dilated cardiomyopathy. In voltage clamp mode the L-type Ca2+ current showed reversible decrease during stimulation at 1 and 2 Hz. Recovery from inactivation during a double pulse protocol was slow (75 +/- 3% at 500 ms, 89 +/- 3% at 1000 ms) and followed the decay of the [Ca2+]i transient.
Conclusions:
The negative force-frequency relation of the failing human heart is due to a decrease in Ca2+ release of the cardiac myocytes at frequencies > or = 0.5 Hz, more pronounced in dilated than in ischemic cardiomyopathy. Inhibition of ICaL at higher frequencies, at least partially related to an increase in diastolic [Ca2+]i, will contribute to this negative staircase because of a decrease in the trigger for Ca2+ release, and of decreased loading of the SR.