The isolated working guinea pig heart: A functional and electrophysiological characterisation

Figures

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  FIGURE 1. Experimental set‐up and perfusion circuit used for working guinea pig hearts. This recirculating system allows for hearts to be initially perfused in Langendorff mode while a cannula is secured in the left atrium via the opening for the pulmonary veins. Hearts are then switched to working mode whereby the left side of the heart is filled via the atrial cannula and the Langendorff reservoir is closed off. Buffer is ejected via the aorta into a Windkessel compliance chamber containing a fixed volume of air (10 mL) and from there against a fixed hydrostatic column. Aortic flow is measured by an appropriate rotameter (12–150 mL min−1) and afterload set by adjusting the height of the out‐flow column. Buffer is then returned to a perfusate reservoir where it is gassed and pumped via a cellulose acetate in‐line filter (porosity 5 µm) to the left atrial venous reservoir. Preload is set by adjusting the height of a sipper in this reservoir connected to a peristaltic pump set at a flow rate exceeding that of the expected cardiac output and the venous return pump. Coronary flow drips from the heart into a heart chamber and coronary flow rate can be measured with a measuring cylinder or…

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  FIGURE 2. Working heart instrumentation. (1) Pressure–volume catheter within the aortic cannula. (2) Pulmonary vein cannula. (3) Bipolar pacing electrodes positioned on the right atrium. (4) ECG wires positioned on the ventricular apex (positive), right atrium (negative) and connective tissue surrounding aorta (earth).

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  FIGURE 3. Example traces showing left ventricular pressure (LVP) measured using a pressure–volume tip‐transducer (a) and an intraventricular balloon (b) and the variables measured (c). In (a), pressure was measured using a tip‐transducer as described in an ejecting working heart and in (b), in an isovolumically contracting Langendorff preparation. Note: the symmetrical appearance and lack of true diastolic pause in the Langendorff pressure trace (b), whereas the working heart trace (a) shows a true EDP morphology and atrial ‘kick’, different rates of contraction and relaxation and an identifiable diastolic interval. Panel (c) shows example LVP, dP/dt and ECG traces and the definition of the intervals and variables measured. Cyclic variables were defined automatically in real‐time by the LabChart software, where systolic duration is defined as left ventricular end‐diastolic pressure (LVEDP) to dP/dt min and diastolic duration as dP/dt min to end of cycle. LVDP, left ventricular developed pressure; Tp–Te, T‐peak to T‐end interval.

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  FIGURE 4. Example PV loop and ‘IVC’ occlusion from working heart set‐up. Representative examples of (a) pressure–volume (PV) loops obtained using a PV catheter within the left ventricle of an isolated working guinea pig heart and (b) modified ‘IVC’ occlusions performed by abruptly partially occluding inflow to the left side of the working heart to enable end diastolic‐ and end systolic PV relationships to be determined – referred to as preload occlusions (blue and red lines, respectively). Note: raw LV conductance data are displayed on the x‐axis owing to the inability to correctly calibrate volume calculation on the ADV500 PV system used. EDPVR, end‐diastolic pressure–volume relationship; ESPVR, end‐systolic pressure–volume relationship; IVC, inferior vena cava; LVP, left ventricular pressure.

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  FIGURE 5. Relationships between RR interval, heart rate, QT and QTc intervals in working guinea pig hearts paced across a range of heart rates. (a) Example data showing the non‐linear relationship between heart rate and RR interval. (b) Relationship between heart rate and QT interval recorded at different pacing rates and in the presence of varying concentrations of ivabradine (0–0.6 µmol.L−1). (c) Data as in (b) plotted as a function of RR interval. The linear regression fit is shown in black while the red dashed line indicates the line describing a single exponential plateaux function. Note: the higher correlation coefficient (r 2) of the linear regression in (b) compared to (c) (black line) indicates QT interval data are best described as a linear function of heart rate rather than RR interval. (d) QT interval corrected for changes in heart rate (QTc) using the data in (b) and Equation 1 (see text).

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  FIGURE 6. Relationship between heart rate and other ECG variables. ECG variables were measured in working guinea pig hearts paced across a range of heart rates in the presence of varying concentrations of ivabradine (0.6 µmol L−1). Relationships between heart rate and Tp–Te interval (a), PR interval (b), and QRS duration (c) are shown. The slope of the linear regression analysis and correlation coefficient (r 2) for each data set are shown for grouped data from all 8 hearts.

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  FIGURE 7. Relationship between RR interval and indices of systolic function. Systolic functional variables were measured in working guinea pig hearts paced across a range of heart rates in the presence of ivabradine (0.6 µmol L−1). Relationships between the RR interval and LVDP (a), dP/dt max (b), and systolic duration (c) are shown. The slope of the linear regression analysis and correlation coefficient (r 2) for each data set are shown for grouped data from all 8 hearts.

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  FIGURE 8. Relationship between RR interval and indices of diastolic function. Diastolic functional variables were measured in working guinea pig hearts paced across a range of heart rates in the presence of ivabradine (max. 0.6 µmol L−1). Relationships between RR interval and LVEDP (a), dP/dt min (b), tau (c), and diastolic duration (d) are shown. The slope of the linear regression analysis and correlation coefficient (r 2) for each data set are shown for grouped data from all 8 hearts.

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  FIGURE 9. Effect of CPA on function in the working heart. Effect of incremental doses (max. 10 µmol L−1) of cyclopiazonic acid (CPA) on function in the isolated working guinea pig heart. The effects of each dose of CPA on heart rate (a), cardiac output (b), and stroke volume (c) are shown. Data presented as means ± SD, n = 5 hearts. Statistical comparisons and P‐values are as indicated with respect to baseline – non‐significant comparisons are not shown.

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  FIGURE 10. Effect of CPA on function in the working heart. Effect of incremental doses (max. 10 µmol L−1) of cyclopiazonic acid (CPA) on function in the isolated working guinea pig heart, as measured by pressure–volume loop parameters. The effects of each dose of CPA on stroke work (SW) (a), end systolic pressure–volume relationship (ESPVR) (b), and end diastolic pressure–volume relationship (EDPVR) (c) are shown. Data presented as means ± SD, n = 4–5 hearts (parameters only recorded if high‐quality pressure–volume loops were obtained). Statistical comparisons and P‐values are as indicated with respect to baseline – non‐significant comparisons are not shown.

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  FIGURE 11. Effect of CPA on systolic function in the working heart. Effect of incremental doses (max. 10 µmol L−1) of cyclopiazonic acid (CPA) on systolic function in the isolated working guinea pig heart. The effects of each dose of CPA on rate‐corrected LVDP (a), dP/dt max (b), and systolic duration (SD) (c). Data presented as means ± SD, n = 4–5 hearts. Statistical comparisons and P‐values are as indicated with respect to baseline – non‐significant comparisons are not shown.

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  FIGURE 12. Effect of CPA on diastolic function in the working heart. Effect of incremental doses (max. 10 µmol L−1) of cyclopiazonic acid (CPA) on diastolic function in the isolated working guinea pig heart. The effects of each dose of CPA on rate‐corrected LVEDP (a), dP/dt min (b), diastolic duration (DD) (c), and tau (d). Data presented as means ± SD, n = 4–5 hearts. Statistical comparisons and P‐values are as indicated with respect to baseline – non‐significant comparisons are not shown.