Comments on "Quantal mechanisms in cardiac contraction".

T HE phenomenon to which Pollack gives the name "stepwise shortening" (though he claims that similar steps occur also during passive lengthening) is clearly of great interest if it is genuine; indeed, Pollack claims that it requires fundamental rethinking of current ideas about the mechanism of contraction. Before any such conclusion can be reached, however, three questions must be answered: First, is the phenomenon genuine? Two of Pollack's methods have been severely criticised, and in my view he has not answered those criticisms. Second, if the phenomenon is genuine, is it related to events in the contractile mechanism itself? The fact that it is claimed to occur during stretch and release of unstimulated fibers, and even in fibers so far extended that there is no overlap of thick and thin filaments, suggests that passive elements are responsible. Third, if in some cases the steps do arise in the contraction process, does this imply that current theories are wrong? They could be manifestations of the phenomenon well known under the name "isotohic transients". I will address myself to these three questions in that order. I may as well say at the outset that I am not convinced that the phenomenon is real. The deviations from smooth shortening are small (order of ± 0.1% of sarcomere length) and none of the methods used by Pollack and his collaborators has been shown to be free from errors sufficient to give the appearance of "steps" of this magnitude.