Ruthenium red is a well known inhibitor of Ca2؉ uptake into mitochondria in vitro. However, its utility as an inhibitor of Ca 2؉ uptake into mitochondria in vivo or in situ in intact cells is limited because of its inhibitory effects on sarcoplasmic reticulum Ca 2؉ release channel and other cellular processes. We have synthesized a ruthenium derivative and found it to be an oxygen-bridged dinuclear ruthenium amine complex. It has the same chemical structure as Ru360 reported previously (Emerson, J., Clarke, M. J., Ying, W-L., and Sanadi, D. R. (1993) J. Am. Chem. Soc. 115, 11799 -11805). Ru360 has been shown to be a potent inhibitor of Ca 2؉ -stimulated respiration of liver mitochondria in vitro. However, the specificity of Ru360 on Ca 2؉ uptake into mitochondria in vitro or in intact cells has not been determined. The present study reports in detail the potency, the effectiveness, and the mechanism of inhibition of mitochondrial Ca 2؉ uptake by Ru360 and its specificity in vitro in isolated mitochondria and in situ in isolated cardiac myocytes. Ru360 was more potent (IC 50 ‫؍‬ 0.184 nM) than ruthenium red (IC 50

We compared the dynamics of the contraction and relaxation of single myocytes isolated from nontransgenic (NTG) mouse hearts and from transgenic (TG-beta-Tm) mouse hearts that overexpress the skeletal isoform of tropomyosin (Tm). Compared with NTG controls, TG-beta-Tm myocytes showed significantly reduced maximal rates of contraction and relaxation with no change in the extent of shortening. This result indicated that the depression in contraction dynamics determined in TG-beta-Tm isolated hearts is intrinsic to the cells. To further investigate the effect of Tm isoform switching on myofilament activity and regulation, we measured myofilament force and ATPase rate as functions of pCa (-log of [Ca2+]). Compared with controls, force generated by myofilaments from TG-beta-Tm hearts and myofibrillar ATPase activity were both more sensitive to Ca2+. However, the shift in pCa50 (half-maximally activating pCa) caused by changing sarcomere length from 1.8 to 2.4 microm was not significantly different between NTG and TG-beta-Tm fiber preparations. To test directly whether isoform switching affected the economy of contraction, force versus ATPase rate relationships were measured in detergent-extracted fiber bundles. In both NTG and TG-beta-Tm preparations, force and ATPase rate were linear and identically correlated, which indicated that crossbridge turnover was unaffected by Tm isoform switching. However, detergent extracted fibers from TG-beta-Tm demonstrated significantly less maximum tension and ATPase activity than NTG controls. Our results provide the first evidence that the Tm isoform population modulates the dynamics of contraction and relaxation of single myocytes by a mechanism that does not alter the rate-limiting step of crossbridge detachment. Our results also indicate that differences in sarcomere-length dependence of activation between cardiac and skeletal muscle are not likely due to differences in the isoform population of Tm.

Acidosis in cardiac muscle is associated with a decrease in developed force. We hypothesized that slow skeletal troponin I (ssTnI), which is expressed in neonatal hearts, is responsible for the observed decreased response to acidic conditions. To test this hypothesis directly, we used adult transgenic (TG) mice that express ssTnI in the heart. Cardiac TnI (cTnI) was completely replaced by ssTnI either with a FLAG epitope introduced into the N‐terminus (TG‐ssTnI*) or without the epitope (TG‐ssTnI) in these mice. TG mice that express cTnI were also generated as a control TG line (TG‐cTnI). Non‐transgenic (NTG) littermates were used as controls. We measured the force‐calcium relationship in all four groups at pH 7.0 and pH 6.5 in detergent‐extracted fibre bundles prepared from left ventricular papillary muscles. The force‐calcium relationship was identical in fibre bundles from NTG and TG‐cTnI mouse hearts, therefore NTG mice served as controls for TG‐ssTnI* and TG‐ssTnI mice. Compared to NTG controls, the force generated by fibre bundles from TG mice expressing ssTnI was more sensitive to Ca2+. The shift in EC50 (the concentration of Ca2+ at which half‐maximal force is generated) caused by acidic pH was significantly smaller in fibre bundles isolated from TG hearts compared to those from NTG hearts. However, there was no difference in the force‐calcium relationship between hearts from the TG‐ssTnI* and TG‐ssTnI groups. We also isolated papillary muscles from the right ventricle of NTG and TG mouse hearts expressing ssTnI and measured isometric force at extracellular pH 7.33 and pH 6.75. At acidic pH, after an initial decline, twitch force recovered to 60 ± 3 % (n= 7) in NTG papillary muscles, 98 ± 2 % (n= 5) in muscles from TG‐ssTnI* and 96 ± 3 % (n= 7) in muscles from TG‐ssTnI hearts. Our results indicate that TnI isoform composition plays a crucial role in the determination of myocardial force sensitivity to acidosis.

Abstract-␤-Adrenergic stimulation of the heart results in an enhanced relaxation rate in association with phosphorylation of both cardiac troponin I (cTnI) and phospholamban (PLB). We studied new lines of mice generated by crossbreeding mice that express slow skeletal troponin I (ssTnI) with PLB knockout (PLBKO) mice. This crossbreeding resulted in the generation of PLB/cTnI, PLB/ssTnI, PLBKO/cTnI, and PLBKO/ssTnI mice. Perfusion with isoproterenol (ISO) significantly increased the peak amplitude of fura-2 ratio in PLB/cTnI, PLBKO/cTnI, and PLBKO/ssTnI groups of mice. However, in the presence of ISO, there were no differences in the peak amplitude of fura-2 ratio among cells isolated from hearts of PLB/cTnI, PLBKO/cTnI, and PLBKO/ssTnI mice. In cells from PLB/cTnI mice, the extent of shortening was increased and the time of relaxation was significantly decreased during ␤-adrenergic stimulation. In PLBKO/cTnI cells, stimulation with ISO resulted in an increased extent of shortening and no change in time of relaxation. However, stimulation with ISO in cells isolated from PLBKO/ssTnI mice not only significantly increased the extent of cell shortening but also increased the time of relaxation. We also determined the kinetics of relaxation of papillary muscles isolated from all four groups of animals in the presence and absence of ISO. Perfusion with ISO increased the rate of relaxation only in PLB/cTnI, PLB/ssTnI, and PLBKO/cTnI muscles. During ISO stimulation, the time of relaxation was unchanged in PLBKO/ssTnI muscles. Our data directly demonstrate that phosphorylation of both PLB and cTnI contributes to increased rate of relaxation during ␤-adrenergic stimulation. Key Words: troponin I Ⅲ phospholamban Ⅲ ␤-adrenergic stimulation Ⅲ relaxation T he dynamics of cardiac relaxation are controlled by complex processes involving Ca 2ϩ removal from the sarcoplasm, release of Ca 2ϩ from the myofilaments, the relation between bound Ca 2ϩ and myofilament activation, and the rate of turnover of force-generating crossbridges. 1,2 The relative role of each of these processes during relaxation in the basal state and during altered autonomic activity remains unclear. Understanding these processes is critical to our understanding of the alterations that occur physiologically as well as in hypertrophy and failure of the heart, which commonly demonstrate impaired relaxation. 3 There is ample evidence that phospholamban (PLB) of the sarcoplasmic reticulum (SR) and the myofilament regulatory protein cardiac troponin I (cTnI) are not only critical as proteins determining relaxation dynamics but also as sites of modulation by signaling cascades. For example, ␤-adrenergic stimulation of the heart is known to result in an enhanced relaxation rate in association with phosphorylation of both cTnI and PLB. 4 -6 In turn, in vitro studies demonstrated that phosphorylation of PLB releases the SR Ca 2ϩ pump from a prevailing inhibition, and that phosphorylation of cTnI enhances the off rate for Ca 2ϩ exchange with cTnC 7 and increases the rate ...