Background: We have previously shown that chronic elevation of plasma norepinephrine leads to a functional independent increase in left ventricular weight. The goals of the present study were to determine quantitatively the component of the myocardium that accounted for the observed structural changes and to determine the function of the hypertrophied myocardium.
Methods and results: Mongrel dogs were chronically instrumented for measurement of arterial and left ventricular pressures, left ventricular internal diameter, and left ventricular wall thickness. Subcutaneous osmotic pumps were implanted to release norepinephrine continuously for 28 days. Hemodynamics were measured with dogs in the quietly resting state and during infusions of isoproterenol at 0.1 and 0.5 micrograms/kg/min before and on days 14 and 28 during the infusion of norepinephrine. The hemodynamic response to 10 micrograms/kg phenylephrine, given as a bolus, was also assessed before norepinephrine and 28 days during the infusion of norepinephrine, and the end-systolic pressure-diameter or wall-stress-diameter relations were calculated. On day 28, hearts were arrested in diastole and perfusion fixed in situ. Tissue samples were prepared for electron microscopy and morphometry. Hemodynamic studies showed that isoproterenol (0.5 micrograms/kg/min) reduced mean arterial pressure (MAP) to the same point on each experimental day, and the increases in indexes of contractility were reduced during norepinephrine infusion. Left ventricular dP/dtmax increased 131 +/- 24% on control day, only 67 +/- 20% on day 14, and 55 +/- 18% on day 28. Similar changes were observed in dP/dt/DP40 and dP/dt/end-diastolic circumference. However, Emax, the slope of the end-systolic pressure-diameter or wall stress diameter relations, was unchanged, suggesting that inotropic state was not altered. Morphometric studies showed that the cross-sectional area of myocytes increased by 55%, but myocyte and capillary densities decreased by 34% and 29%, respectively (p less than 0.05) in dogs with high norepinephrine levels. There were no differences in volume fractions of myocytes, capillary lumen, or interstitium or capillary-to-myocyte ratio.
Conclusions: The myocardium of dogs with high norepinephrine levels shows reduced inotropic response to beta-adrenergic stimulation despite the increases in left ventricular mass and left ventricular wall thickness, which are a result of growth of the cardiac myocytes and characteristic of concentric hypertrophy. These data suggest that chronic adrenergic stimulation of the heart reduces the beta-receptor coupling to the contractile response without importantly compromising left ventricular function.