[Cationic regulation of myocardial contraction and relaxation when contraction frequency is increased]

During augmentation of frequency of contractions or Ca2+ perfusate concentration the velocity of isotonic relaxation increased much more than the velocity or amplitude of contraction. The phenomenon of predominant activation of relaxation ( PPAR ) was less prominent at decreased Ca2+ concentration acidosis (pH 6,85), under action of calmodulin inhibitor, trifluoperazine, or replacement of Ca2+ ions by Sr2+. Diminution of Na2+ concentration to 25 mM was followed by apparent signs of cellular Ca2+--overload and elimination of PPAR . The results coincide with the suggestion that PPAR is caused by activation of calmodulin-dependent Ca2+ removal from the myoplasma and myofibrils.