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BIOL 4510
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BIOL4510 / KINE4510 Excitation-Contraction Coupling The purpose of electrical depolarization (i.e. action potentials) is to initiate contraction. This process is called excitation-contraction coupling (E-C coupling). The2+lectrical changes in the muscle cell (myocyte) induce an increase in intracellular Ca leading to the development of contraction. Muscle relaxation ensues following a decrease in intracellular Ca . There are 2+ significant differences in E-C coupling in skeletal, cardiac and smooth muscle. Top figure showing action potentials (APs) in Cardiac Muscle vs Skeletal Muscle. In sk muscle APs are very short (3-5msecs) and a single AP generates a twitch force lasting 50-100 msecs. In cardiac muscle APs are much longer (~200msecs) and a single AP generates a twitch force lasting ~300 msecs. Bottom figure shows that APs in sk muscle have a rapid recovery time (short refractory time) and therefore sk muscle can be rapidly and repetitively stimulated which causes fusion of twitches and the generation of tetanus. In fact, sk muscle contraction usually involves tetanization of individual motor units. Cardiac muscle has long AP relative to force/pressure generation. Therefore, tetanization is not possible. Cardiac muscle cannot be stimulated faster than the refractory period of the AP. Note: refractory period ~ AP duration since electrical (and thus mechanical stimulation of muscle) can only occur if most Na channels are available to initiate and propagate APs. Therefore refractory period controlled by rate of recovery of Na channels from inactivation (which requires membrane repolarization). Thus, long APs in cardiac muscle prevent tetanization of cardiac muscle. Why is this a critical feature of heart? 1 E-C Coupling in the Heart AP generation occurs in order to induce muscle contraction. The process of membrane depolarization leading to muscle contraction is called excitation-contraction coupling = ECC = the events underlying the mechanical response (muscle contraction) to an electrical 2+ depolarization (action potential). In the heart, ECC is dependent on a rise in Ca which catalyzes the conversion of ATP into force and mechanical work by myosin and the contractile proteins. The Ca 2+originates from both intracellular (sarcoplasmic reticular) and extracellular Ca .+ Figure shows the absence of contraction when extracellular Ca 2+ is removed. Note: Sk,. Muscle will still contract in the absence of extracellular Ca2+ 2+ It has been calculated that there is an increase of 90 120 M total Ca in the cytoplasm with each cardiac contraction. The rise in free [Ca ] which is about 1M. The free Ca 2+rise is the tip-of-the=iceberg. The low free [Ca ] is the result of buffering (100:1) of Ca 2+by intracellular i proteins. The major buffers being troponin-C (~150M) and calmodulin (~20M).as well as NCX, SERCA2a 2 - Ca 2+ transients (i.e. the transient rise in Ca2+associated with ECC) is typically measured 2+ with fluorescent indicators which are molecules that bind Ca and thereby change their flevel of fluorescence. Recent Nobel Prize awarded to Roger Tsien who developed many of these dyes. Photo: U. Montan Osamu Shimomura Martin Chalfie Roger Y. Tsien The Nobel Prize in Chemistry 2008 was awarded jointly to Osamu Shimomura, Martin Chalfie and Roger Y. Tsien "for the discovery and development of the green fluorescent protein, GFP". 2+ - these dyes have a number of carboxylic acid moieties that bind Ca plus a large pi-bond structure that provides a structure for resonating electrons over a relative continuous range of frequencies corresponding to visible light. This allows Ca 2+imagining to be performed. 3 The major car2+ac cellular proteins required 2+r excitation-contraction coupling to occur are: 1) L-type Ca channel (Ca 1V2). allows Ca entry into clefts adjacent to RYR2 channels 2) ryanodine receptor (RyR2) 3) Na /Ca 2+ exchanger (NCX1) 2+ 4) SR Ca ATPase (SERCA2a), phospholamban and sarcolipin 5) contractile protein machinery; myosin, actin, troponins, tropomyosin Excitation-Contraction Coupling (Steps 7-9 take place simultaneously.step 7 is rate-limiting). 4
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