[Ionic mechanism of the Woodwors staircase]

Analysis of single-chamber model of electromechanical coupling in the myocardial cell has shown that Woodwors staircase can be imitated in two cases: 1) stationary input current Ca2+ strongly exceeds the potential-dependent uptake of Ca2+ into the cell through the sarcolemma; 2) the action potential (AP) is shortened abruptly with an increase of the myocardium stimulation frequency. The experiments performed on a fragment of the frog heart ventricle supported the conclusions of the model. Blocking of Ca-channels with nifedipine (10(-6) g/mol) at the background of isotonic substitution of 70% of NaCl resulted in the development of "negative staircase" with an increase of stimulation rhythm. An abrupt shortening of AP after rest at joint action of adrenaline (10(-6) g/ml) and blocker of Ca-channels D-600 (10(-6) g/ml) was accompanied by Woodwors staircase.