Effects of Myosin Isozyme Shift on Curvilinearity of the Left Ventricular End-Systolic Pressure-Volume Relation of In Situ Rat Hearts.

Abstract

Recently we have shown that the left ventricular end-systolic pressure-volume relation (ESPVR) of in situ rat hearts is an upward convex curve in contrast to the linear left ventricular ESPVR in dog and human hearts. Within the smaller left ventricular volume range, the left ventricular end-systolic pressure rose steeply with increases in left ventricular volume, but it gradually reached a plateau at the larger left ventricular volumes. In adult rat hearts, the myosin isozyme is V1, unlike V3 in dog and human hearts. To investigate whether myosin isozyme affects the curvilinearity of the left ventricular ESPVR, we evaluated the left ventricular ESPVR in hypothyroid rats in which the left ventricular myosin isozyme had been shifted to V3. In the hypothyroid rats, the left ventricular contractility was depressed and the ESPVR became closer to linear. However, after dobutamine administration the ESPVR returned to curvilinear. In nor-mal rats the curvilinearity of the left ventricular ESPVR was decreased by negative inotropic agents such as adrenergic blockers. These results indicate that the depressed left ventricular contractility in the hypothyroidism make ESPVR linear and that the enhanced left ventricular contractility from dobutamine make it curvilinear. We concluded that the curvilinearity of the rat left ventricular ESPVR is not determined by myosin isozyme per se, but by the left ventricular contractility.

New Index for Oxygen Cost of Contractility from Curved End-Systolic Pressure-Volume Relations in Cross-Circulated Rat Hearts.

“…We determined PVA by the method described below because the ESPVR of the rat LV was an upward convex curve [1][2][3][4][5][6]. We fitted the end-systolic and end-diastolic P-V points by the following equations:…”

“…Equation 1 is the same as used in our previous studies [1,2,4,5,7]. Equation 2 is substantially the same as used in our previous study [7].…”

“…The values of mLVV and V 0 are normalized by LV mass to 1 g. Our new index for quantifying LV contractility is based on PVA mLVV (a total mechanical energy per beat at a mLVV) that has been proposed to be a good index for mechanoenergetic evaluation of rat LV contractility [1,2,[4][5][6]. Therefore, the value of (eESP/ ESV) mLVV is mechanoenergetically reasonable.…”

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Recently, we have consistently observed curved endsystolic pressure-volume relations (ESPVRs) of the left ventricle (LV) in rat blood-perfused [1][2][3] and crystalloid-perfused whole heart preparations [4] and in situ ejecting rat hearts [5,6], like those of the puppy LV [7] and adult canine LV in supernormal contractility [8,9]. These studies suggest more generality of the curvilinear than linear ESPVR in different animal species.

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“…Therefore, we were not able to use E max in canine hearts for assessing LV contractility. Instead, we proposed ESP at a mLVV for assessing LV contractility in rat hearts [4][5][6]. However, this index was insufficient to compare LV contractility among left ventricles with different volumes.…”