Correlation between total catecholamine content and redistribution of myosin isoenzymes in pressure loaded ventricular myocardium of the spontaneously hypertensive rat.

In the spontaneously hypertensive rat (SHR) with established hypertension left ventricular mass closely correlated with the proportion of ventricular myosin (VM)-3. A 60% VM-3 content was reached corresponding, under the present housing conditions, to an average age of 40 weeks without any significant change in total noradrenaline content. However, all 72-week-old SHR showed marked reduction in total noradrenaline content which corresponded to noradrenaline depletion. At this stage, the proportion of VM-3 varied greatly; some of the SHR exhibited a higher proportion of VM-3 than would be expected from their ventricular weight. The excessive redistribution in the direction of VM-3 might, therefore, be considered as a reaction secondary to noradrenaline depletion. Total dopamine content was also reduced in 72-week-old SHR. In younger SHR, a positive correlation was found for dopamine content and left ventricular weight. It is concluded that a redistribution of the myosin isoenzymes up to 60% VM-3 in SHR is not associated with deterioration of ventricular performance to the extent that an excessive neurohumoral drive ensues. However, in the late stage of haemodynamic overload, a functional state is reached which is prone to noradrenaline depletion. A causative factor involved in this process could be the extreme redistribution of the myosin isoenzyme population, besides other factors such as fibrosis and dilatation. Which functional determinants or structural elements are finally decisive for the transition into a myocardium with depleted noradrenaline stores requires further investigation.