Effect of ischemia on calcium-dependent fluorescence transients in rabbit hearts containing indo 1. Correlation with monophasic action potentials and contraction
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1Division of Cardiology, Stanford University School of Medicine, California 94305.
Insights
Acute global ischemia in rabbit hearts disrupts cytosolic calcium levels, increasing both systolic and diastolic calcium. This calcium imbalance contributes to the loss of electrical and mechanical synchrony during ischemia.
Area of Science:
- Cardiology
- Physiology
- Biochemistry
Background:
- Cytosolic calcium transients regulate cardiac excitation-contraction coupling.
- Acute global ischemia significantly impacts myocardial function and cellular homeostasis.
Purpose of the Study:
- To investigate the effects of acute global ischemia on cytosolic calcium transients in rabbit hearts.
- To explore the relationship between altered calcium dynamics and mechanical/electrical dysfunction during ischemia.
Main Methods:
- Utilized perfused rabbit hearts loaded with the fluorescent calcium indicator indo 1.
- Recorded fluorescence ratios (F400/F550) from the left ventricle epicardium to measure calcium transients.
- Employed an analog circuit to cancel optical motion artifacts and analyzed intracellular calcium alternans.
Main Results:
- Global ischemia suppressed contraction while increasing systolic and diastolic cytosolic calcium levels.
- Ischemia induced intracellular calcium alternans and led to nonuniformities in contraction and action potentials.
- Ischemic effects were reversible upon reperfusion, inhibited by verapamil, and mimicked by acidified perfusion.
Conclusions:
- Elevated cytosolic calcium and calcium alternans are key consequences of acute global ischemia.
- Intracellular calcium abnormalities are implicated as a causal factor in the loss of electrical and mechanical synchrony in ischemic hearts.
Abstract:
The effects of acute global ischemia on cytosolic calcium transients were studied in perfused rabbit hearts loaded with the fluorescent calcium indicator indo 1. Indo 1-loaded hearts were illuminated at 360 nm, and fluorescence was recorded simultaneously at 400 and 550 nm from the epicardial surface of the left ventricle. The F400/F550 ratio was calculated by an analog circuit, which allowed cancellation of optical motion artifact. Resulting calcium transients demonstrated a rapid upstroke and slow decay similar to those recorded in isolated ventricular myocytes. Global ischemia rapidly suppressed contraction, but it produced a concurrent increase in the systolic and diastolic levels of the calcium transients, together with an increase in the duration of the peak. The effects of ischemia were reversed by reperfusion, inhibited by verapamil, and mimicked by perfusion of nonischemic hearts with acidified (CO2-rich) solution. In addition to elevation of the calcium transients, ischemia caused a pattern of intracellular calcium alternans that was discernible after 2-3 minutes. The pattern of alternans was stable at a given epicardial site, but it could be out of phase at different sites. Similar nonuniformities were observed in contraction strength and in the duration of monophasic action potentials recorded immediately adjacent to the fiber-optic probe. Abnormalities in intracellular calcium may be a causal factor in the loss of electrical and mechanical synchrony in the acutely ischemic heart.