Are there stress resistant and susceptible myocardia?

The satisfactory analysis of the Na/K ATPase, its pumping component and the mechanism of action of the inhibitor digitalis remains elusive; yet the controversial inotropic effect of digitalis in the clinical setting has been known for over a century. There are also conflicting reports of the effect of urea and uremia on the cardiovascular system, and the evidence as it exists, suggests that urea may have two effects on the intact heart, by virtue of its extent of action on hydrogen bonding of water molecules, determined by which type of muscle constitutes the myocardium. If different types of myocardium do exist, they could well respond differently to inotropic agents. Evidence suggests that two types of myocardia, relatively stress resistant or susceptible may exist, analagous to known skeletal muscle differences.