Mg-ATPase and Ca+ activated myosin AtPase activity in ventricular myofibrils from non-failing and diseased human hearts--effects of calcium sensitizing agents MCI-154, DPI 201-106, and caffeine.

We investigated the effects of two purported calcium sensitizing agents, MCI-154 and DPI 201-106, and a known calcium sensitizer caffeine on Mg-ATPase (myofibrillar ATPase) and myosin ATPase activity of left ventricular myofibrils isolated from non-failing, idiopathic (IDCM) and ischemic cardiomyopathic (ISCM) human hearts (i.e. failing hearts). The myofibrillar ATPase activity of non-failing myofibrils was higher than that of diseased myofibrils. MCI-154 increased myofibrillar ATPase Ca2+ sensitivity in myofibrils from non-failing and failing human hearts. Effects of caffeine similarly increased Ca2+ sensitivity. Effects of DPI 201-106 were, however, different. Only at the 10(-6) M concentration was a significant increase in myofibrillar ATPase calcium sensitivity seen in myofibrils from non-failing human hearts. In contrast, in myofibrils from failing hearts, DPI 201-106 caused a concentration-dependent increase in myofibrillar ATPase Ca2+ sensitivity. Myosin ATPase activity in failing myocardium was also decreased. In the presence of MCI-154, myosin ATPase activity increased by 11, 19, and 24% for non-failing, IDCM, and ISCM hearts, respectively. DPI 201-106 caused an increase in the enzymatic activity of less than 5% for all preparations, and caffeine induced an increase of 4, 11, and 10% in non-failing, IDCM and ISCM hearts, respectively. The mechanism of restoring the myofibrillar Ca2+ sensitivity and myosin enzymatic activity in diseased human hearts is most likely due to enhancement of the Ca2+ activation of the contractile apparatus induced by these agents. We propose that myosin light chain-related regulation may play a complementary role to the troponin-related regulation of myocardial contractility.

Defects in calcium control.

METHODS: Multicellular preparations from nonfailing and failing human hearts or animals with cardiac hypertrophy were used to study intracellular calcium mobilization. Left ventricular muscle strips were loaded with the intracellular calcium indicator aequorin. Muscle strips were attached to a force transducer and stretched until there was no further increase in active force and stimulated to contract at varying frequencies. Muscles were placed in an oxygenated bath and studied at 30 degrees C. Pharmacological agents were used to increase intracellular sodium or intracellular calcium directly. Agents with known sites of action were then applied to define the original of resulting changes in the amplitude and shape of the caclium transient. Cellular homogenates were also used to study SR Ca(2+) ATPase activity based in a pyruvate/NADH-coupled reaction. Action potentials were also recorded from isolated muscle strips. Findings from isolated myocytes loaded with an intracellular calcium indicator are also reported. CONCLUSIONS: In failing human cardiomyocytes, decreased SERCA2a activity contributes to abnormal calcium handling, elevated diastolic calcium concentrations, and decreased contractility at higher rates of stimulation. Enhanced sodium calcium exchanger activity when working in the reverse mode (ie, transporting calcium into the cell) can potentially worsen calcium mobilization, induce arrhythmias, and negatively impact muscle contraction. Elevated intracellular sodium concentrations can prolong the action potential duration, as well as the time course of muscle contraction, resulting in increased arrhythmogenesis.