Transport Troponin

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

Eosin emission characteristics depend strongly on the solvent. Specifically transfer from aqueous solution to a nonaqueous solvent shifts the emission of Eosin toward longer wavelengths and increases the emission intensity. Wang and Cheung [142] have used the fluorescence enhancement of the Eosin label to study the association of troponin I with troponin C. Similarly Skou and Esmann [143] and Helmich de Jong et al. [144] have used Eosin itself as a fluorescent probe to study the conformational changes of enzymes involved in ionic transport. 

Schematic diagram of a cardiac muscle sarcomere, with sites of action of several drugs that alter contractility (numbered structures). Site 1 is Na+/K+ ATPase, the sodium pump. Site 2 is the sodium/calcium exchanger. Site 3 is the voltage-gated calcium channel. Site 4 is a calcium transporter that pumps calcium into the sarcoplasmic reticulum (SR). Site 5 is a calcium channel in the membrane of the SR that is triggered to release stored calcium by activator calcium. Site 6 is the actin-troponin-tropomyosin complex at which activator calcium brings about the contractile interaction of actin and myosin.

Actin and tubulin are two important cellular components that are involved in cell shape and movement. Actin is present in all mammalian cells and is involved in cellular transport and phagocytosis (eating of extracellular materials), provides rigidity to cell membranes, and when bonded to tropomyosin and troponin, forms the thin filaments of muscle. Thbulin is the subunit from which microtubules are self-assembled. Microtubules are most commonly known for their role in cell division. The mechanisms of self-assembly of these macromolecules have been well studied and are important models of biological assembly processes. Below we examine each of these processes. 

Relaxation of the muscle is brought about by removal of the ionic calcium from the sarcoplasm. This calcium is transported across the membrane of the sarcoplasmic reticulum, in an energy requiring process. In addition to the calcium pumping ATPase, the sarcoplasmic reticulum also contains a calcium binding protein called calsequestrin (Section 4.3.3). Some of the calcium segregated by the sarcoplasmic reticulum is apparently bound to this protein within the lumen of the sarcoplasmic reticulum. As sequestration of calcium ions into sarcoplasmic reticulum proceeds, more calcium ions dissociate from their binding sites on troponin C, re-... 

By stimulating p-receptors, and hence cAMP production, catecholamines augment all heart functions including systolic force, velocity of myocyte shortening, sinoatrial rate, conduction velocity, and excitability. In pacemaker fibers, cAMP-gated channels ( pacemaker channels ) are activated, whereby diastolic depolarization is hastened and the firing threshold for the action potential is reached sooner (B). cAMP activates protein kinase A, which phosphorylates different Ca2+ transport proteins. In this way, contraction of heart muscle cells is accelerated, as more Ca2 enters the cell from the extracellular space via L-type Ca2 channels and release of Ca2 from the sarcoplasmic reticulum (via ryanodine receptors, RyR) is augmented. Faster relaxation of heart muscle cells is effected by phosphorylation of troponin and phospholamban. 

Ferm VH, Layton WM Jr (1979) Reduction in cadmium teratogenesis by prior cadmium exposure. Environ Res 18 347-350 Forsen S, Thulin E, Lilja H (1979) Cd NMR in the study of calcium binding proteins troponin C. FEBS Lett 104 123-126 Foulkes EC (1978) Renal tubular transport of cadmium-metallothionein. Toxicol Appl Pharmacol 45 505-512... 

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