EFFECTS OF ANGIOTENSIN-II ON INTRINSIC MYOCARDIAL-CONTRACTILITY IN THE GUINEA-PIG

Analysis of the contraction-relaxation coupling of guinea pig left ventricular papillary muscle was performed with and without angiotensin II (Ang II 10-6 M). The inotropic and lusitropic properties of Ang II were evaluated at 20-degrees-C, 30 beats/mn, CaCl2 6H20 2 mM and pH 7.4, under low load (isotonic conditions) and high load (isometric conditions). The maximum velocity of contraction (maxVc) and relaxation (maxVr) were calculated from isotonic contraction having as its only load that corresponding to the imposed preload at Lmax. The maximum positive (+ dF/dtmax) and negative values (- dF/dtmax) of the derivative of the force were calculated during isometric contraction. The coefficients, R1 = maxVc/maxVr and R2 = (+ dF/dtmax)/ (- dF/dtmax), were calculated. These two coefficients allow the contraction-relaxation coupling to be assessed at low and high loads respectively. In the presence of Ang II, the increase in the isotonic velocity of relaxation (1.93 +/- 0.26 vs 3.15 +/- 0.35 Lmax/sec ; p < 0.001) was greater than that of the isotonic velocity of contraction (0.74 +/- 0.05 vs 1.02 +/- 0.07 Lmax/sec ; p < 0.001). This results in a decrease in the ratio of the velocities of isotonic contraction and relaxation (R1) (0.44 +/- 0.06 vs 0.35 +/- 0.05 ; p < 0.01). Under isometric conditions, Ang II induced a proportional increase in the parameters of contraction and relaxation. Consequently, there was no significant change in the R2 coefficient (1.22 +/- 0.06 vs 1.12 +/- 0.08). Moreover, Ang II did not induce any change in the sensitivity of the relaxation with respect to load. The mechanical effects of Ang II on die contraction-relaxation coupling of guinea pig myocardium are identical to those induced by alpha1-adrenergic stimulation of rat myocardium.