The Mechanosensory Heart

The cardiac muscle has an intrinsic ability to sense its filling state and react to its changes, independently of cardiac innervation that may partially serve the same functions. This ability, interesting by itself, has also a medical significance because it is associated with disturbances that may develop if a sustained loading of the myocytes will change their function. This may lead to adaptational growth of the cardiac muscle, but also to serious diseases like myocardial left ventricular hypertrophy and heart failure. In this book, and in this introductory chapter, we will focus on the nature of the sensory mechanisms of the cardiac myocytes, based on the mechanism that can be called mechanotransduction. We will look at the ability of cardiac cells to sense the filling state of the heart as a process where a mechanical stimulus is transformed into a change in the cells functions, be it in membrane voltage, contraction force, ion balance, exocytosis or in gene expression. One possibility to do this is to divide the sensation process into limited series of more or less accurately timed events, from coding of the mechanical stimuli to both signalling via second messengers and to decoding of the information into changes in heart function, as proposed earlier. As in other physiological functions, also mechanotransduction is controlled by feedback. In the heart it consists of exocytosis of vasoactive peptides and growth of the heart muscle, both tending to decrease the initial (volume) load, and of the coactivation of regulatory pathways in the nervous system. Under some circumstances the physiological regulatory loops may become maladaptive, leading to development of pathological hypertrophy and heart failure.