Intracellular calcium handling heterogeneities in intact guinea pig hearts.

Regional heterogeneities of ventricular repolarizing currents and their role in arrhythmogenesis have received much attention; however, relatively little is known regarding heterogeneities of intracellular calcium handling. Because repolarization properties and contractile function are heterogeneous from base to apex of the intact heart, we hypothesize that calcium handling is also heterogeneous from base to apex. To test this hypothesis, we developed a novel ratiometric optical mapping system capable of measuring calcium fluorescence of indo-1 at two separate wavelengths from 256 sites simultaneously. With the use of intact Langendorff-perfused guinea pig hearts, ratiometric calcium transients were recorded under normal conditions and during administration of known inotropic agents. Ratiometric calcium transients were insensitive to changes in excitation light intensity and fluorescence over time. Under control conditions, calcium transient amplitude near the apex was significantly larger (60%, P < 0.01) compared with the base. In contrast, calcium transient duration was significantly longer (7.5%, P < 0.03) near the base compared with the apex. During isoproterenol (0.05 microM) and verapamil (2.5 microM) administration, ratiometric calcium transients accurately reflected changes in contractile function, and, the direction of base-to-apex heterogeneities remained unchanged compared with control. Ratiometric optical mapping techniques can be used to accurately quantify heterogeneities of calcium handling in the intact heart. Significant heterogeneities of calcium release and sequestration exist from base to apex of the intact heart. These heterogeneities are consistent with base-to-apex heterogeneities of contraction observed in the intact heart and may play a role in arrhythmogenesis under abnormal conditions

Molecular correlates of repolarization alternans in cardiac myocytes

Arrhythmogenic action potential alternans (APD-ALT) is thought to arise from beat to beat alteration in cellular Ca(2+) cycling. Previously, we found that spatial heterogeneity in APD-ALT between ventricular myocytes is key to the mechanism linking APD-ALT to cardiac arrhythmogenesis. However, the cellular and molecular basis for APD-ALT is poorly understood. To test the hypothesis that spatial heterogeneities in expression and function of calcium cycling proteins underlies heterogeneities in APD-ALT, endocardial and epicardial myocytes were isolated from left ventricular free wall of 20 guinea pig hearts. APD-ALT and Ca(2+) transient alternans (Ca-ALT) were measured simultaneously as stimulus rate was increased progressively. Endocardial myocytes exhibited greater susceptibility to cellular alternans than epicardial myocytes as evidenced by a significantly lower pacing rate threshold for APD-ALT (113 +/ -9 bpm vs. 151 +/- 8 bpm, respectively, P < 0.05) and for Ca-ALT (110 +/- 8 bpm vs. 149 +/- 8 bpm, respectively, P < 0.05). APD-ALT never occurred without Ca-ALT, whereas Ca-ALT was readily induced in the absence of APD-ALT by repetitive constant action potential waveform, suggesting that Ca-ALT was not secondary to APD-ALT. Importantly, there were significant voltage-independent differences in Ca(2+) cycling between endocardial and epicardial myocytes as evidenced by weaker Ca(2+) release (32% lower Ca(2+) amplitude, and 16% longer rise time), and slower Ca(2+) reuptake (24% larger Ca(2+) decay time constant, and 9% longer Ca(2+) transient duration) in endocardial compared to epicardial myocytes. Taken together these data indicate that myocytes that are most susceptible to APD-ALT exhibit impaired Ca(2+) release and reuptake. Moreover, transmural differences in Ca(2+) cycling function was associated with significantly reduced endocardial expression of ryanodine release channel (by 22%) and SERCA2 (by 40%), suggesting a potential molecular basis for spatially heterogeneous APD-ALT. Moreover, transmural differences in expression and function of key SR Ca(2+) cycling proteins may underlie spatial heterogeneity of APD-ALT that has been closely linked to cardiac arrhythmogenesis

Mechanisms Underlying Electro-Mechanical Cardiac Alternans

Electro-mechanical cardiac alternans consists in beat-to-beat changes in the strength of cardiac contraction. Despite its important role in cardiac arrhythmogenesis, its molecular origin is not well understood. The appearance of calcium alternans has often been associated to fluctuations in the sarcoplasmic reticulum calcium level (SR Ca load). However, cytosolic calcium alternans observed without concurrent oscillations in the SR Ca content suggests an alternative mechanism related to a dysfunction in the dynamics of the ryanodine receptor (RyR2). In this chapter we review recent results regarding the relative role of SR Ca content fluctuations and SR refractoriness for the appearance of alternans in both ventricular and atrial cells