Ventricular myosin pattern of spontaneously hypertensive turkeys is unaffected by labetalol treatment

In most animal species, left ventricular hypertrophy due to pressure overload is associated with an advantageous increase of the "slow" V₃ isomyosin. In contrast, in spontaneously hypertensive turkeys, the development of left ventricular hypertrophy is associated with the synthesis of a "fast" V₁-like isomyosin, with high incidence of cardiac failure. This could be related to the high catecholamine levels found in these animals. This is why we studied the ventricular myosin pattern after lowering of blood pressure and regression of cardiac hypertrophy obtained by means of labetalol, an α- and β-blocking drug which inhibits the effects of catecholamines. From the 2nd to the 32nd week of age, 22 turkeys were treated with increasing doses of p.o. labetalol (from 20 to 35 mg/kg body weight daily) and 16 other turkeys were given daily p.o. placebo. Blood pressure and heart rate were periodically measured by an indirect method. After sacrifice, the degree of cardiac hypertrophy was evaluated by the biventricular weight to body weight ratio, ventricular myosin was purified, Ca⁺⁺-activated ATPase activity assessed, and ventricular myosin pattern was determined by two-dimensional gel electrophoresis of myosin heavy chains. Plasma and cardiac catecholamines were measured by high performance liquid chromatography. Throughout the study period, blood pressure and heart rate were significantly reduced in the labetalol-treated animals as compared to the untreated ones. At the end of the study period, the ventricular mass was significantly lower in the labetalol group. Nevertheless, no differences were obscrved in ventricular myosin pattern and Ca⁺⁺-activated ATPase activity levels between the two groups. In the labetalol group, an increase in plasma catecholamines and only a slight, but not significant, increase in cardiac catecholamines was found. These data indicate that in spontancously hypertensive turkeys, the synthesis of the "fast" V₁-like isomyosin is not influenced by known pathophysiological stimuli like blood pressure, cardiac hypertrophy and catecholamines.