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Updated: Aug 25, 2026
Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling
Published on: August 1, 2016
Diminished inotropic response to amrinone in ventricular myocytes from myopathic hamsters is linked to depression of
Wei Xiong1, Gregory R Ferrier, Susan E Howlett
1Department of Pharmacology, Sir Charles Tupper Medical Building, Dalhousie University, Halifax, Nova Scotia Canada B3H 4H7.
Insights
Amrinone, a phosphodiesterase-III inhibitor, enhances contractions in normal heart cells by boosting calcium release but fails to improve cardiomyopathic cells. Its positive inotropic effect relies on sarcoplasmic reticulum calcium stores, which are not restored in disease.
Area of Science:
- Cardiovascular Physiology
- Pharmacology
- Cellular Biology
Background:
- Heart failure involves depressed contractility in ventricular myocytes.
- Phosphodiesterase-III inhibitors like amrinone are explored for cardiac dysfunction.
- Calcium handling is critical for myocyte contraction and is often impaired in heart disease.
Purpose of the Study:
- To investigate amrinone's effect on contraction and calcium handling in prefailure cardiomyopathic (CM) hamster ventricular myocytes.
- To determine the mechanisms underlying amrinone's inotropic effects, specifically its interaction with Ca(2+)-induced Ca(2+) release (CICR) and L-type Ca(2+) current (I(Ca-L)).
Main Methods:
- Ventricular myocytes from normal and cardiomyopathic hamsters were studied.
- Cell shortening and ion currents (I(Ca-L)) were measured using voltage-clamp techniques.
- Amrinone's effects on high-gain and low-gain CICR and sarcoplasmic reticulum (SR) Ca(2+) stores (assessed by caffeine) were evaluated.
Main Results:
- Amrinone selectively enhanced high-gain CICR and contractions in normal myocytes, but not low-gain CICR.
- Amrinone inhibited I(Ca-L) in normal myocytes and had a lesser inhibitory effect in CM myocytes.
- Amrinone increased SR Ca(2+) stores in normal myocytes but not in CM myocytes, where high-gain CICR was depressed.
Conclusions:
- Amrinone's positive inotropic effect in normal myocytes is mediated by enhanced high-gain CICR and increased SR Ca(2+) stores, not I(Ca-L) stimulation.
- Amrinone does not restore depressed high-gain CICR or improve contractility in prefailure cardiomyopathic myocytes.
- Restoration of amrinone's inotropic effect in CM cells was observed with forskolin, suggesting a role for adenylyl cyclase modulation.
Abstract:
This study investigates whether amrinone (100-1000 microM), a phosphodiesterase-III inhibitor, can alleviate depression of contractions in ventricular myocytes from prefailure cardiomyopathic (CM) hamsters (80-100 days). Cell shortening and ion currents were measured in voltage-clamped cells at 37 degrees C. Normal myocytes exhibited low-gain Ca(2+)-induced Ca(2+) release (CICR) initiated by test steps from -40 mV and high-gain CICR initiated from more negative potentials. In normal myocytes, amrinone selectively increased contractions initiated by high-gain CICR (fractional shortening increased from 3.6 +/- 0.5% to 5.3 +/- 0.6%, 300 microM amrinone) but had no effect on low-gain CICR. Amrinone decreased L-type Ca(2+) current (I(Ca-L); -5.5 +/- 0.8 to -3.7 +/- 0.5 picoAmp/picoFarad, 300 microM amrinone). In contrast, in CM myocytes, high-gain CICR was virtually absent, and amrinone had no inotropic effect. Amrinone inhibited I(Ca-L) less in CM than normal myocytes. Sarcoplasmic reticulum (SR) Ca(2+) stores, assessed by caffeine, were significantly increased by amrinone in normal but not CM myocytes. Thus, the positive inotropic effect of amrinone in normal hamster myocytes was mediated by selective enhancement of high-gain CICR. This effect was not mediated by stimulation of I(Ca-L) because I(Ca-L) is inhibited by this drug in hamster. High-gain CICR, which is depressed in CM myocytes, cannot be restored by amrinone. However, minimal stimulation of adenylyl cyclase with forskolin restored the positive inotropic effect of amrinone in CM cells. This positive inotropic effect of amrinone may reflect increased SR Ca(2+) stores because increased stores accompanied the positive inotropic effect in normal myocytes but were absent in CM myocytes.
