Causes of Abnormal Ca2+ Transients in Guinea Pig Pathophysiological Ventricular Muscle Revealed by Ca2+ and Action Potential Imaging at Cellular Level

Background: Abnormal Ca2+ transients are often observed in heart muscles under a variety of pathophysiological conditions including ventricular tachycardia. To clarify whether these abnormal Ca2+ transients can be attributed to abnormal action potential generation or abnormal Ca2+ handling/excitation-contraction (EC) coupling, we developed a procedure to determine Ca2+ and action potential signals at the cellular level in isolated heart tissues. Methodology/Principal Findings: After loading ventricular papillary muscle with rhod-2 and di-4-ANEPPS, mono-wavelength fluorescence images from rhod-2 and ratiometric images of two wavelengths of emission from di-4-ANEPPS were sequentially obtained. To mimic the ventricular tachycardia, the ventricular muscles were field-stimulated in nonflowing Krebs solution which elicited abnormal Ca2+ transients. For the failed and alternating Ca2+ transient generation, there were two types of causes, i.e., failed or abnormal action potential generation and abnormal EC coupling. In cells showing delayed initiation of Ca2+ transients with field stimulation, action potential onset was delayed and the rate of rise was slower than in healthy cells. Similar delayed onset was also observed in the presence of heptanol, an inhibitor of gap junction channels but having a non-specific channel blocking effect. A Na+ channel blocker, on the other hand, reduced the rate of rise of the action potentials but did not result in desynchronization of the action potentials. The delayed onset of action potentials can be explained primarily by impaired gap junctions and partly by Na+ channel inactivation. Conclusions/Significance: Our results indicate that there are multiple patterns for the causes of abnormal Ca2+ signals and that our methods are useful for investigating the physiology and pathophysiology of heart muscle.