Sensitivity Of Contractile Machinery Research Articles

Signal transduction and protein phosphorylation in smooth muscle contraction.

Smooth muscles are important constituents of vertebrate organisms that provide for contractile activity of internal organs and blood vessels. Basic molecular mechanism of both smooth and striated muscle contractility is the force-producing ATP-dependent interaction of the major contractile proteins, actin and myosin II molecular motor, activated upon elevation of the free intracellular Ca2+ concentration ([Ca2+](i)). However, whereas striated muscles display a proportionality of generated force to the [Ca2+](i) level, smooth muscles feature molecular mechanisms that modulate sensitivity of contractile machinery to [Ca2+](i). Phosphorylation of proteins that regulate functional activity of actomyosin plays an essential role in these modulatory mechanisms. This provides an ability for smooth muscle to contract and maintain tension within a broad range of [Ca2+](i) and with a low energy cost, unavailable to a striated muscle. Detailed exploration of these mechanisms is required to understand the molecular organization and functioning of vertebrate contractile systems and for development of novel advances for treating cardiovascular and many other disorders. This review summarizes the currently known and hypothetical mechanisms involved in regulation of smooth muscle Ca2+-sensitivity with a special reference to phosphorylation of regulatory proteins of the contractile machinery as a means to modulate their activity.

Length Modulation of Active Force in Rat Cardiac Myocytes: is Titin the Sensor?

The intrinsic cellular mechanisms by which length regulates myocardial contraction, the basis of the Frank–Starling relation, are uncertain. The aim of this work was to test the hypothesis that passive force, possibly via titin, participates in the modulation of Ca2+sensitivity of cardiac contractile proteins induced by stretch. Titin degradation by a mild trypsin digestion modulated passive force induced by increasing from 1.9 to 2.3 μ m sarcomere length in skinned rat cardiac cells. Force–pCa curves were established at these two sarcomere lengths after various durations of trypsin application that induced different passive force levels. They allowed us to evaluate myofilament Ca2+sensitivity by the pCa of half-maximal activation (pCa50). In control conditions, stretching cells from 1.9 to 2.3 μ m induced a leftward shift of pCa50(ΔpCa50) of 0.39±0.03 pCa units (mean±SEM, n=8 cells), reflecting an increase in Ca2+sensitivity of the contractile machinery. Passive force measured every 2 min decreased exponentially after the beginning of the trypsin application (t1/2@12 min). The first 30% decrease of passive force did not affect the stretch-induced variation in Ca2+sensitivity. Then, with further decrease in passive force, ΔpCa50decreased. At the lowest passive force investigated 20% of initial passive force, ΔpCa50decreased by approximately 55%. These effects were not accompanied by a significant modification of either maximal activated force at pCa 4.5 solution or pCa50at 1.9 μ m sarcomere length. This indicates that there was no major functional alteration of the contractile machinery during the protocol as also suggested by contractile and regulatory protein electrophoresis on 2.5–12% gradient and 15% SDS–PAGE gels. Thus, besides modulation induced by the reduced lattice spacing during enhanced heart refilling, Ca2+sensitivity of the cardiac contractile machinery may be controlled at least partially by internal passive load, which is known to be largely attributable to titin.

Ca2+ sensitivity of contractile machinery and Ca2+ handling energy. Simulation.

Myocardial Ca2+ handling during excitation-contraction coupling has been modelled mathematically to gain a better insight into the expectation that Ca2+ sensitization of contractile machinery may save myocardial energy utilization for Ca2+ handling. The basic model of myocardial Ca2+ kinetics and mechanoenergetics involved the sarcoplasmic reticulum (SR), sarcoplasm, troponin C (Tn) and crossbridges (CB). The relations among the released Ca2+ ions from the SR, peak concentrations of sarcoplasmic free Ca2+ ([Ca2+]i) and Ca(2+)-bound troponin ([TnCa]) and peak contractile force were computed, based upon the assumptions that the released Ca2+ ions diffuse as free Ca2+ in sarcoplasm, bind kinetically with Tn with an association rate constant of k1, dissociate from TnCa with a dissociation rate constant of k2, and are sequestered into the SR with consumption of ATP. TnCa was associated with CB cycling to develop force with a set of given on and off rate constants. The association constant Ka (= k1/k2) of TnCa as an index of Ca2+ sensitivity of Tn was varied 32-fold from 0.25 to 8/microM. Results showed that Ca2+ sensitization from a lower Ka level could most sharply decrease the total Ca2+ release required to develop the same contractile force. Thus, it would reduce the total Ca2+ handling energy that the SR uses to maintain the same contractility.