Methods for Measuring Functional Properties of Sarcoplasmic Reticulum and Myofibrils in Small Samples of Myocardium
We describe here methods developed to study properties of sarcoplasmic reticulum (SR) and myofibrils in the same sample of heart muscle. We wanted to do this because regulation of SR and myofibrils appears to be coordinated in both short-term and long-term regulation of the heart. In short-term regulation of beating hearts, both SR and myofibrillar proteins are phosphorylated by activation of cAMP-dependent protein kinases (Solaro et al., 1976, 1980; E. G. Kranias and R. J. Solaro, unpublished observations). These phosphorylations alter cardiac function by increasing the velocity of Ca2+ transport by the SR (Katz, 1979) and by decreasing the Ca2+ sensitivity of myofibrillar activation (Solaro et al., 1976; Holroyde et al., 1979), and these changes are likely to be responsible for the decreased contraction-relaxation cycle time of hearts responding to sympathetic nerve stimulation. During relatively long-term stress, such as exercise (Pagani and Solaro, 1981), pressure overload, or thyrotoxicosis (Alpert et al., 1979), kinetic properties of SR and myofibrils change in a way that suggests that these chronic signals might also induce coordinated changes in the SR and the contractoregulatory protein complex. For example, hearts of hyperthyroid rabbits show increased rates of contraction and relaxation, and this is undoubtedly a manifestation of increased velocity of Ca2+ transport by the SR and the increased rate of myofibrillar cross-bridge cycling. It is also clear that some pharmacological agents, general and local anesthetics, for example, affect both SR and myofibrillar function and thus may affect coordination between these organelles.
KeywordsSarcoplasmic Reticulum Myofibrillar Protein Sarcoplasmic Reticulum Vesicle Myofibrillar ATPase Sympathetic Nerve Stimulation
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