Coordination of cardiac myofibrillar and sarcotubular activities in rats exercised by swimming

We measured the functional activity of both myofibrils and fragmented sarcoplasmic reticulum (FSR) in microsomal fractions and homogenates of hearts of sedentary rats and rats exercised by swimming 75 min twice daily for 8 wk. Ca2+ transport was measured under conditions that restricted uptake of Ca2+ to the SR vesicles in the homogenates or microsomal fractions. There was a significant increase in myosin Ca2+-ATPase activity of myofibrils prepared from hearts of swimmers, indicating that a “training effect” had occurred. The mean rate of Ca2+ transport and mean storage capacity were the same for SR vesicles in the homogenates and microsomal fractions from hearts of controls and swimmers. At the same free Ca2+ concentration, the velocity of Ca2+ transport by FSR in homogenate preparations was inversely related to the myofibrillar ATPase activity in a series of preparations from hearts of swimmers, but there was little correlation between the same activities measured in preparations from a series of sedentary rats. Our results suggest that the increase in the rate of relaxation of hearts from exercised rats is not due to an increase in the rate of Ca2+ transport by the SR but may be due to other factors, which include an increase in the rate of cross-bridge cycling or an alteration in the relation and coordination between cross-bridge cycling and SR Ca2+ transport activity.

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Cross bridge-dependent activation of contraction in cardiac myofibrils at low pH

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1999.276.5.h1460 ◽

1999 ◽

Vol 276 (5) ◽

pp. H1460-H1467 ◽

Cited By ~ 7

Author(s):

D. R. Swartz ◽

D. Zhang ◽

K. W. Yancey

Keyword(s):

Atpase Activity ◽

Calcium Binding ◽

Striated Muscle ◽

Contractile Activity ◽

Low Ph ◽

Myofibrillar Atpase ◽

Peak Activity ◽

Cross Bridge ◽

Myofibrillar Atpase Activity ◽

Cross Bridges

Striated muscle contracts in the absence of calcium at low concentrations of MgATP ([MgATP]), and this has been termed rigor activation because rigor cross bridges attach and activate adjacent actin sites. This process is well characterized in skeletal muscle but not in cardiac muscle. Rigor cross bridges are also thought to increase calcium binding to troponin C and play a synergistic role in activation. We tested the hypothesis that cross bridge-dependent activation results in an increase in contractile activity at normal and low pH values. Myofibrillar ATPase activity was measured as a function of pCa and [MgATP] at pH 7.0, and the data showed that, at pCa values of ≥5.5, there was a biphasic relationship between activity and [MgATP]. Peak activity occurred at 10–50 μM MgATP, and [MgATP] for peak activity was lower with increased pCa. The ATPase activity of rat cardiac myofibrils as a function of [MgATP] at a pCa of 9.0 was measured at several pH levels (pH 5.4–7.0). The ATPase activity as a function of [MgATP] was biphasic with a maximum at 8–10 μM MgATP. Lower pH did not result in a substantial decrease in myofibrillar ATPase activity even at pH 5.4. The extent of shortening, as measured by Z-line spacing, was greatest at 8 μM MgATP and less at both lower and higher [MgATP], and this response was observed at all pH levels. These studies suggest that the peak ATPase activity associated with low [MgATP] was coupled to sarcomere shortening. These results support the hypothesis that cross bridge-dependent activation of contraction may be responsible for contracture in the ischemic heart.

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