Understanding the effects of thin and thick filament proteins on the kinetics of Ca(2+) exchange with cardiac troponin C is essential to elucidating the Ca(2+)-dependent mechanisms controlling cardiac muscle contraction and relaxation. Unlike labeling of the endogenous Cys-84, labeling of cardiac troponin C at a novel engineered Cys-53 with 2-(4'-iodoacetamidoanilo)napthalene-6-sulfonic acid allowed us to accurately measure the rate of calcium dissociation from the regulatory domain of troponin C upon incorporation into the troponin complex. Neither tropomyosin nor actin alone affected the Ca(2+) binding properties of the troponin complex. However, addition of actin-tropomyosin to the troponin complex decreased the Ca(2+) sensitivity ( approximately 7.4-fold) and accelerated the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 2.5-fold). Subsequent addition of myosin S1 to the reconstituted thin filaments (actin-tropomyosin-troponin) increased the Ca(2+) sensitivity ( approximately 6.2-fold) and decreased the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 8.1-fold), which was completely reversed by ATP. Consistent with physiological data, replacement of cardiac troponin I with slow skeletal troponin I led to higher Ca(2+) sensitivities and slower Ca(2+) dissociation rates from troponin C in all the systems studied. Thus, both thin and thick filament proteins influence the ability of cardiac troponin C to sense and respond to Ca(2+). These results imply that both cross-bridge kinetics and Ca(2+) dissociation from troponin C work together to modulate the rate of cardiac muscle relaxation.

No abstract

Rho-associated serine/threonine kinase (Rho-kinase) is a downstream effector of small GTPase RhoA that has recently been shown to play an important role in regulating smooth muscle contraction. The present study investigated the role of Rho/ Rho-kinase in hypoxia-induced pulmonary vasoconstriction (HPV). Small pulmonary resistance vessels and cultured pulmonary arterial smooth muscle cells (PASMCs) from the rat were used. PASMCs exposed to hypoxia (PO(2) = 26 +/- 2 mm Hg) showed a significant increase in Rho-kinase activity. Exposure to hypoxia for 20, 40, 60, 90, and 120 min also resulted in a significant increase in myosin light chain (MLC) phosphorylation at all time points in PASMCs. Hypoxia-induced MLC phosphorylation was inhibited by Y-27632 (a Rho-kinase inhibitor), exoenzyme C3 (a specific Rho inhibitor), or toxin B (an inhibitor for Rho proteins). In addition, hypoxia-induced Rho-kinase activation was blocked by C3 and toxin B. Small rat intrapulmonary arterial rings, which were made hypoxic (PO(2) = 30 +/- 3 mm Hg), showed a slow sustained contraction, and Y-27632 caused a significant relaxation during the sustained phase of HPV in a concentration-dependent manner. In summary, the data show that Rho-kinase is activated by hypoxia in PASMCs, and Rho/Rho-kinase is functionally linked to hypoxia-induced MLC phosphorylation and plays a role in the sustained phase of HPV.

The calcium dependent interactions between troponin C (TnC) and other thin and thick filament proteins play a key role in regulation of cardiac muscle contraction. Five hydrophobic residues Phe 20 , Val 44 , Met 45 , Leu 48 and Met 81 in the regulatory domain of TnC were individually substituted with polar Gln, to examine the effect of these mutations that sensitized isolated TnC to calcium on: 1) calcium binding and exchange with TnC in increasingly complex biochemical systems and 2) calcium sensitivity of actomyosin ATPase. The hydrophobic residue mutations drastically affected calcium binding and exchange with TnC in increasingly complex biochemical systems, indicating that side chain intra-and inter-molecular interactions of these residues play a crucial role in determining how TnC responds to calcium. However, the mutations that sensitized isolated TnC to calcium did not necessarily increase the calcium sensitivity of the troponin (Tn) complex or reconstituted thin filaments with or without myosin S1. Furthermore, the calcium sensitivity of reconstituted thin filaments (in the absence of myosin S1) was a better predictor of the calcium dependence of actomyosin ATPase activity than that of TnC or the Tn complex. Thus, both the intrinsic properties of TnC and its interactions with the other contractile proteins play a crucial role in modulating calcium binding to TnC in increasingly complex biochemical systemsThe processes of cardiac muscle contraction and relaxation can be regulated by multiple physiological and patho-physiological stimuli. It is clear that protein alterations associated with heart disease, isoform switching and post translational modifications can affect both the Ca 2+ sensitivity of muscle force generation and relaxation kinetics (for review, see (1-4)). Since cardiac troponin C (TnC) 1 is the Ca 2+ sensor responsible for initiating the contraction / relaxation cycle (for review, see (5,6)), a potentially important mechanism to alter cardiac † This research was funded by NIH grants 5R00HL087462 (to S.B.T) and 5R01HL073828 (to D.R.S.); by Predoctoral Fellowship Award from the American Heart Association (to B. L.) and by National Scientist Development Award from the American Heart Association (to J.P.D).*Address correspondence to: Jonathan P. Davis, Department of Physiology and Cell Biology, The Ohio State University, 304 Hamilton Hall, 1645 Neil Avenue, Columbus, OH 43210, Tel. 614-247-2559; Fax. 614-292-4888; davis.812@osu.edu. ‡ These two authors contributed equally to the manuscript NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2011 March 9. Published in final edited form as:Biochemistry. 2010 March 9; 49(9): 1975-1984. doi:10.1021/bi901867s. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript muscle performance is through directly modifying the properties of TnC. As it has been difficult to find specific pharmacological modulators of TnC, we have taken a genetic approach to modify Ca 2+ binding and exchange with TnC.I...