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Sex Differences in Renal and Cardiovascular Function: Physiology and Pathophysiology

Myofilament response to Ca²⁺ and Na⁺/H⁺ exchanger activity in sex hormone-related protection of cardiac myocytes from deactivation in hypercapnic acidosis

¹Department of Physiology and Biophysics, Medicine and Center for Cardiovascular Research, College of Medicine, University of Illinois at Chicago, Chicago, Illinois; and ²Department of Physiology, Faculty of Science, Mahidol University, Bangkok, Thailand

Submitted 31 May 2006 ; accepted in final form 3 October 2006

	ABSTRACT

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Compared to sham-operated controls, myofilaments from hearts of ovariectomized (OVX) rats demonstrate an increase in Ca²⁺ sensitivity with no change in maximum tension (Wattanapermpool J and Reiser PJ. Am J Physiol 277: H467–H473, 1999). To test the significance of this modification in intact cells, we compared intracellular Ca²⁺ transients and shortening of ventricular myocytes isolated from sham and 10-wk OVX rats. There was a decrease in the peak Ca²⁺ transient with prolonged 50% decay time in OVX cardiac myocytes without changes in the resting intracellular Ca²⁺ concentration. Percent cell shortening was also depressed, and relaxation was prolonged in cardiac myocytes from OVX rats compared with shams. Ovariectomy induced a sensitization of the myofilaments to Ca²⁺. Hypercapnic acidosis suppressed the shortening of OVX myocytes to a lesser extent than that detected in shams. Moreover, a larger compensatory increase in %cell shortening was obtained in OVX myocytes during prolonged acidosis. The elevated compensation in cell shortening was related to a higher amount of increase in the amplitude of the Ca²⁺ transient in OVX myocytes. However, these differences in Ca²⁺ transients and %cell shortening were no longer evident in the presence of 1 µM cariporide, a specific inhibitor of Na⁺/H⁺ exchanger type 1 (NHE₁). Our results indicate that deprivation of female sex hormones modulates the intracellular Ca²⁺ concentration in cardiac myocytes, possibly via an increased NHE₁ activity, which may act in concert with Ca²⁺ hypersensitivity of myofilament activation as a determinant of sex differences in cardiac function.

heart; estrogen; sarcomeric proteins; sodium; proton exchanger

Although, antiapoptotic effects of estrogen have been reported (18), depression in Ca²⁺ fluxes to and from the myofilaments appears to account in part for the depression of contractility in hearts of ovariectomized (OVX) rats. Ren et al. (21) reported that myocytes from 6-wk OVX rat hearts demonstrated slowing of relaxation and contraction kinetics associated with a slower decay and depressed peak amplitude of Ca²⁺ transients. Bupha-Intr and Wattanapermpool (3, 4) reported a depression in Ca²⁺ uptake rate and ATPase activity by vesicles of the cardiac sarcoplasmic reticulum (SR) derived from hearts of rats 10 wk following ovariectomy, compared with sham controls. Depression in expression of sarco(endo)plasmic reticulum Ca²⁺-ATPase at the protein and mRNA levels appeared to account for this depression in myocyte function and Ca²⁺ fluxes (4). There is also evidence of sex-related differences in fluxes through the Na⁺/Ca²⁺ exchanger (NCX). Sugishita et al. (28) reported that metabolic inhibition induced a relatively large increase in intracellular Ca²⁺ in myocytes of transgenic male vs. female mouse hearts overexpressing NCX. Treatment of the males with 17-estradiol (E₂) significantly attenuated this difference. Whether these effects on intracellular Ca²⁺ are due directly to NCE activity or indirectly through modification of the activity of the Na⁺/H⁺ exchanger (NHE₁) remains unclear. There is, however, evidence of acute effects of E₂ on regulation of NHE₁ activity. In demonstrating that acute treatment with E₂ improved recovery of hearts stressed by normoxic acidification, Anderson et al. (1) proposed a mechanism involving, in part, promotion of NO synthesis that increased NHE₁ activity.

Alterations in myofilament response to Ca²⁺ also appear to play an important role in effects of estrogen on cardiac function. There is a reduction in ATPase rate in myofilaments isolated from hearts of OVX rats, which is attributable to a shift in myosin isoform population from one predominant in the alpha-myosin heavy chain to one containing significant amounts of beta-myosin heavy chain (32, 35). Moreover, compared with preparations from shams, ATPase activity (33) and tension (32) of myofilaments from OVX rat hearts were more sensitive to Ca²⁺. These changes in myofilament response to Ca²⁺ were reversed in preparations from OVX rats supplemented with E₂ (35) or in animals subjected to exercise training (3).

In experiments reported here, we have extended our studies of the relationship between sex hormones and myofilament response to Ca²⁺ with the objective of understanding the relevance of this effect in isolated cardiac myocytes. We think this understanding is important in view of evidence that an increase in myofilament response to Ca²⁺ is a common functional consequence of mutations in sarcomeric proteins that are linked to familial hypertrophic myopathies (9). Moreover, the role of myofilament response to Ca²⁺ has not been generally considered in the cardio-protective effects of E₂ in acidosis (1) and metabolic inhibition (28), even though there is evidence that altered myofilament response to Ca²⁺ is a dominant mechanism responsible for the fall in tension associated with acidosis (26). We, therefore, compared the effects of hypercapnic acidosis on shortening and intracellular Ca²⁺ of myocytes isolated from sham, OVX, and OVX + E₂ rat hearts. We also compared tension-pCa relations in detergent extracted fiber bundles from these hearts. To probe for changes in myofilament response to Ca²⁺, we compared responses at physiological pH to responses at acidic pH. Our results confirm that cardiac myofilaments functioning in intact cardiac myocytes from OVX rat hearts are more sensitive to Ca²⁺ than in control myocytes. Our data also indicate that chronic deprivation of female sex hormones modulates the intracellular Ca²⁺ concentration in cardiac myocytes via an increase in NHE₁ activity, which may act in concert with the hypersensitivity of myofilament activation as a significant determinant of sex differences in cardiac function.

	MATERIALS AND METHODS

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Animals. Our studies used female Sprague-Dawley rats that were OVX or sham operated at 8 wk of age by the supplier (Harlan, Indianapolis, IN) and killed at 18 wk of age. We randomly divided the OVX rats into two groups, and injected them subcutaneously with 0.1 ml of corn oil with or without 5 µg of 17-estradiol. We did these injections three times per week for 10 wk, as previously described (4). We verified deficiency of ovarian sex hormones by the absence of ovarian tissues and by the shrinkage of uterine mass (4).

Cardiac myocyte isolation. At the 10-wk time point, we heparinized (1,000 unit/kg body wt) the rats, and after 30 min, we anesthetized them with an intraperitoneal injection of pentobarbital sodium (25 mg/kg body wt). Left ventricular myocytes were isolated as previously described (30, 39, 40). The heart was rapidly removed, and the aorta was cannulated and perfused with nominally oxygenated Ca²⁺-free Tyrode solution until complete cessation of beating. The perfusion solution was then switched to 25 µM CaCl₂ Tyrode solution containing 1 mg/ml BSA, 160 unit/ml collagenase type II (Boehringer Mannheim), and 0.1 mg/ml protease type XIV (Sigma, St. Louis, MO). The hearts were then perfused for 16.5 min/g wet wt with the above solution. At the end of the perfusion period, the hearts were removed from the cannula and placed into a petri dish containing 50 µM CaCl₂ Tyrode-BSA buffer. Ventricles were cut into small pieces and gently triturated with a pipette and filtered through a cell collector. The cell suspension was placed into centrifuge tubes and gradually increased the extracellular CaCl₂ concentration to a final level of 0.5 mM CaCl₂. Myocytes were studied within 6 h after isolation.

Intracellular Ca²⁺ transients, pH, and cell shortening. Myocytes were loaded with 5 µM Fura-2 acetoxymethyl ester (Fura-2-AM) for 15 min in 0.5 mM CaCl₂ Tyrode buffer with 1 mg/ml BSA and 5% FBS for 15 min. After washing off the excess Fura-2-AM, the myocytes were electrically stimulated at a frequency of 0.5 Hz for 10 to 15 min or until the shortening was stable with perfusion of 2 mM CaCl₂ Tyrode buffer at room temperature. Fura-2 fluorescence (340:380 ratio) and cell shortening were monitored simultaneously, as described in detail by Wolska et al. (38).

To test the effect of acidic pH on intracellular Ca²⁺ transients and cell shortening, we perfused the myocytes with control Krebs-Henseleit buffer (KH) saturated with 95% O₂-5% CO₂ for 15 min, with KH saturated with 85% O₂-15% CO₂ for 20 min, and then with control KH. In these experiments, we also determined intracellular pH with the aid of 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, as previously described (37), and we made the measurements in the presence and absence of 1.0 µM cariporide, an inhibitor of the NHE₁ (11).

Force measurements on skinned fiber bundles. We determined the relationship between pCa (–log of the molar-free Ca²⁺) and tension (force/cross-sectional area) in detergent (Triton X-100) extracted fiber bundles prepared from left ventricular papillary muscles, as previously described (34, 38, 39). The fiber bundles (150–200 µm in diameter and 4–5 mm long) were cut in ice-cold high relaxing (HR) buffer: 10 mM EGTA, 2 mM free Mg²⁺, 5 mM MgATP^2–, 79.2 mM KCl, 12 mM creatine phosphate, 20 mM MOPS, pH 7.0, ionic strength 0.15 M, 2.5 µg/ml pepstatin A, 1 µg/ml leupeptin, and 50 µM PMSF. After mounting the fiber bundles in an apparatus for determination of force, sarcomere length was set at 2.2 µm, as determined from a laser diffraction pattern in HR solution and cross-sectional area determined. Isometric force was recorded on a chart recorder at different pCa at room temperature. CaCl₂ concentrations in HR were varied to achieve a range of Ca²⁺ concentrations as computed using a computer program and binding constants summarized by Godt and Lindley (8). Only fiber bundles that developed 85–90% of their initial maximum tension at the end of experiment were retained for analysis. We plotted isometric tension as a function of pCa, fitted the curves to the A. V. Hill equation, as previously described (41), and expressed the relations as a percent of maximum tension. pCa₅₀ (half maximally activating pCa values) was computed from individual Hill fits of each pCa-tension relation and then averaged.

Expression of NHE₁. To determine the level of expression of the NHE₁ gene, real-time, one-step quantitative RT-PCR with SYBR Green detection (Qiagen,Valencia, CA) was performed using a LightCycler thermocycler (Roche Diagnostics), as previously described (1). Briefly, total RNA was isolated from ventricular tissue with the RNeasy Fibrous Tissue Mini Kit (Qiagen, Valencia, CA). The RNA was reverse transcribed, and the cDNA used to derive a PCR product for NHE₁ and ribosomal protein L7. The PCR product was cloned into the vector, pBlue-Topo (Invitrogen, Carlsbad, CA). The plasmid underwent amplification, DNA purification, and in vitro transcription. The resulting RNA was recovered, quantified, and used as standards in the RT-PCR reaction. Gene expression was normalized to ribosomal protein L7. The following primers were used: NHE₁ (forward) 5'-TTTCCCCgATTTCCTTCTCT-3'; NHE₁ (reverse) 5'-gCgTgTAAgACCTgggACAT-3'; Ribosomal protein L7 (forward) 5'-gAAgCTCATCTATgAgAAggC-3'; Ribosomal protein L7 (reverse) 5'-CAgACggAgCAgCTgCAgCAC-3'. We also determined the levels of NHE₁ by Western blot analysis of left ventricular proteins separated by 8% SDS-PAGE with anti-NHE₁ antibody (Chemicon, 1:1,000), using an HRP-labeled secondary antibody and visualization using ECL (Amersham Pharmacia, Piscataway, NJ).

Statistical analysis. Data are presented as means ± SE. All curve fittings were achieved using GraphPad Prism (San Diego, CA). One-way ANOVA was used for statistical analysis and Student-Newman-Kuels test for multiple comparisons. P < 0.05 was considered the significant difference among groups.

	RESULTS

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Data in Table 1 summarize heart, body, and uterine weights of sham, OVX, and OVX +E2 animals. Compared with the shams, OVX rats demonstrated a significant increase in both heart and body weight but no change in heart weight/body wt ratio. As expected, uterine weight was significantly lower in the OVX rats compared with shams and increased significantly upon E₂ supplementation. However, uterine weight did not return fully to control levels with E₂ supplementation. We think this is likely due to a requirement for other hormones in the regrowth of the uterus.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Effects of ovariectomy on relationship between the percent maximum shortening and amplitude of calcium transient (reported as the Fura-2 ratio) of cardiac myocytes isolated from hearts of sham-operated (SHAM), ovariectomized (OVX) rats, and OVX rats supplemented with E2. Data are expressed as means ± SE from 10 myocytes. *Significantly different from shams, P < 0.05. #Significantly different from OVX, P < 0.05.

Figure 2A shows the relations between pCa and relative tension at pH 7.0, pH 6.8, and pH 6.5 for skinned fiber bundles from the three groups of rat hearts. These results demonstrate the well-known effect of acidic pH to desensitize cardiac myofilaments to Ca²⁺. Data in Fig. 2B summarize the half-maximal activating pCa values (pCa₅₀). In agreement with previous results (33), the skinned fibers from OVX rat hearts demonstrated a significant increase in sensitivity to Ca²⁺ at pH 7.0 (pCa₅₀ = 5.84 ± 0.02) compared with fibers from shams (pCa₅₀ = 5.77 ± 0.02). Skinned fibers from hearts of OVX rats supplemented with E₂ had a sensitivity to Ca²⁺ essentially the same as controls (pCa₅₀ = 5.75 ± 0.03). However, at pH 6.8 and 6.5, there were no significant differences in the pCa₅₀ among the fiber bundles from the three groups. In all cases, there were no significant differences in the Hill coefficient among the fiber groups.

View larger version (34K): [in this window] [in a new window]   Fig. 2. A: pCa-tension relations of skinned fiber bundles from sham, OVX, and OVX + E2 rats at three different pH values. B: comparison of pC50 values. Data are expressed as the means ± SE from 10 preparations. *Significantly different from shams, P < 0.05. #Significantly different from OVX, P < 0.05.

View larger version (37K): [in this window] [in a new window]   Fig. 3. A: representative tracings showing effects of acidosis on shortening during twitch contractions with and without the Na+/H+ exchanger type 1 (NHE1) inhibitor cariporide of myocytes isolated from hearts of shams, OVX rats, and OVX rats supplemented with E2. B: data summarizing extent of shortening in the absence (top) and presence (bottom) of cariporide at 5 and 10 min of acidosis and following recovery in myocytes from hearts of shams, OVX rats, and OVX rats supplemented with E2. Percent shortening under control conditions was taken as 100%. Data are means ± SE from 10 myocytes. *P < 0.05 = significantly different from sham.

View larger version (33K): [in this window] [in a new window]   Fig. 4. Data summarizing of Ca2+ transient amplitude in the absence (top) and presence (bottom) of cariporide at 5 and 10 min of acidosis and following recovery in myocytes from hearts of shams, OVX rats, and OVX rats supplemented with E2. Percent shortening under control conditions was taken as 100%. Data are means ± SE from 10 myocytes. *P < 0.05 = significantly different from sham.

View larger version (31K): [in this window] [in a new window]   Fig. 5. Data demonstrating no change in expression of Na+/H+ exchanger 1 (NHE1). A: NHE1 expression of studied in seven separate hearts from SHAM, seven from OVX, and nine from OVX with estrogen supplementation (OVX+E2). Expression was normalized to ribosomal protein L7, and the data are means ± SE. B: representative immunoblot image of proteins and calsequestrin (CQ) of SHAM, OVX, and OVX+E2.

	DISCUSSION

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There is now general agreement that altered myofilament response to Ca²⁺ is a significant aspect of the integrated biology of cardiac function (27). A mechanism involving sarcomere length-dependent alterations in myofilament response to Ca²⁺ remains the dominant theory for the cellular basis of Starling's Law of the heart (22). Moreover, there is substantial evidence for the functional significance of myofilament protein phosphorylation as a significant factor controlling cardiac dynamics (25). As illustrated by the data presented here, depressed cardiac function in acidosis also reflects an influence of intracellular environment to depress myofilament response to Ca²⁺, a mechanism that is dominant in determining cellular function. For example, Figs. 4 and 5 summarized data showing a severe depression in cell shortening with acidosis in the presence or absence of cariporide with little or no change in the peak amplitude of the Ca²⁺ transient. Chronic increases in cardiac myofilament response to Ca²⁺, which our previous (32, 33) and present data indicate occur with ovariectomy, are now also recognized to be of major significance as a cause of hypertrophy associated with mutations in sarcomeric proteins (9, 10). The increased Ca²⁺ sensitivity of OVX cardiac myofilaments is also similar to that reported in cardiomyopathic and heart failure models (10, 36). The elevated myofilament response to Ca²⁺ may induce diastolic defects leading to cell stretch and elaboration of signals promoting hypertrophic growth (7).

The effects of chronic increases in myofilament response to Ca²⁺ are, however, complex inasmuch as there is evidence of the beneficial effects of therapeutic agents, which induce myofilament Ca²⁺ sensitization (12). Moreover, hearts of transgenic mouse models with constitutive myofilament Ca²⁺-sensitization associated with modification of troponin I demonstrate remarkable resistance to acidosis (32, 35), sepsis (15), and ischemia/reperfusion (2, 6). An important distinction is that compared with controls, the myofilaments containing modified troponin I or treated with Ca²⁺-sensitizing agents are not only more sensitive to Ca²⁺ they are resistant to deactivation by acidic pH (26, 39), whereas the myofilaments from OVX hearts are not.

To more clearly understand whether the increase in myofilament response to Ca²⁺ associated with ovariectomy is an adaptive or a maladaptive response, we stressed myocytes with acidosis. We chose this approach because of the dominance of depressed myofilament Ca²⁺ responsiveness in acidosis, and because of its relevance to ischemia and ischemia/reperfusion injury (2). We found that the acidosis-induced depression of shortening of myocytes isolated from hearts of OVX rats was blunted compared with the controls. Although this blunting may have been due, in part, to the enhanced sensitivity of the myofilaments to Ca²⁺, data shown in Fig. 2 indicate that the sensitization is apparent only at pH 7.0.

It is likely that this increase in Ca²⁺ during acidosis in myocytes from OVX hearts and not in controls occurred as a result of increased Na exchange associated with stimulation of the NHE₁ and a resultant decrease in the efflux of Ca²⁺ through NCX. This could be due either to a reduced Ca²⁺ efflux or increased influx via reverse mode activity of the NCX. It is also possible that the elevation of intracellular Ca²⁺ is due, in part, to a concerted depression in Ca²⁺ uptake by the SR, which was demonstrated in earlier studies to occur in OVX rat hearts (4). Yet, inhibition of the differential effects of acidosis on myocytes from OVX and sham hearts by cariporide demonstrated the critical role of NHE₁ in this response. We did not find a difference in expression of NHE1 among the three groups studied. We therefore conclude that compared with controls, the loss of ovarian sex hormones may induce a sensitization of NHE₁ activity to pH. This conclusion fits with studies investigating acute effects of E₂, which reported administration of E₂ to hearts during ischemia, and reperfusion limited the rise in Ca²⁺ occurring with ischemia and reperfusion (1). By use of inhibitors, these studies provided evidence that acute administration of E₂ increased nitric oxide synthase (NOS) activity and nitric oxide (NO) release and thereby depressed the activity of the NHE₁, resulting in a decreased intracellular Na and a limited rise in Ca²⁺ during ischemia/reperfusion. A role for NOS has been documented as a basis for the difference in control of NCX activity between hearts of male and female rats (5, 17). Our data provide evidence that the protective effect of E₂ in ischemia and reperfusion occurs during chronic depressions in levels of ovarian sex hormones. In this case, the withdrawal of E₂ would release NHE₁ from inhibition by NO, resulting in an enhanced activity, which in concert with altered NCX activity, led to elevated intracellular Ca²⁺. Morimoto et al. (16) reported an increase in mesenteric endothelial NOS expression associated with E₂ treatment of OVX rats. An effect of chronic deprivation of ovarian sex hormones on NOS activity is an attractive mechanism, but whether this occurs with ovariectomy awaits further investigation.

	GRANTS

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This work was supported by National Institutes of Health Grants RO1 HL 64035 (R. J. Solaro), PO1 HL 62426 (R. J. Solaro), and RO1 HL 64209 and 79032 (B. M. Wolska). Dr. Bupha-Intr also received support as a Golden Jubilee predoctoral fellow of Mahidol University, Bangkok, Thailand.

	FOOTNOTES

 

Address for reprint requests and other correspondence: R. J. Solaro, Dept. of Physiology and Biophysics, College of Medicine, Univ. of Illinois at Chicago, 835 S. Wolcott Ave., Chicago, IL 60612–7342 (e-mail: solarorj{at}uic.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

	REFERENCES

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1. Anderson SE, Kirkland DM, Beyschau A, Cala PM. Acute effects of 17-estradiol on myocardial pH, Na, and Ca²⁺ and ischemia-reperfusion injury. Am J Physiol Cell Physiol 288: C57–C64, 2005.[Abstract/Free Full Text]
2. Arteaga GM, Warren CM, Milutinovic S, Martin AF, Solaro RJ. Specific enhancement of sarcomeric response to Ca²⁺ protects murine myocardium against ischemia/reperfusion dysfunction. Am J Physiol Heart Circ Physiol 289: H2183–H2192, 2005.[Abstract/Free Full Text]
3. Bupha-Intr T, Wattanapermpool J. Cardioprotective effects of exercise training on myofilament calcium activation in ovariectomized rats. J Appl Physiol 96: 1755–1760, 2004.[Abstract/Free Full Text]
4. Bupha-Intr T, Wattanapermpool J. Regulatory role of ovarian sex hormones in the calcium uptake activity of cardiac sarcoplasmic reticulum. Am J Physiol Heart Circ Physiol 291: H1101–H1108, 2006.[Abstract/Free Full Text]
5. Cross HR, Murphy E, Steenbergen C. Ca(2+) loading and adrenergic stimulation reveal male/female differences in susceptibility to ischemia-reperfusion injury. Am J Physiol Heart Circ Physiol 283: H481–H489, 2002.[Abstract/Free Full Text]
6. Day SM, Westfall MV, Fomicheva EV, Hoyer K, Yasuda S, La Cross NC, D'Alecy LG, Ingwall JS, Metzger JM. Histidine button engineered into cardiac troponin I protects the ischemic and failing heart. Nat Med 12: 181–189, 2006.[CrossRef][Web of Science][Medline]
7. Frey N, Olson EN. Cardiac hypertrophy: the good, the bad, and the ugly. Annu Rev Physiol 65: 45–79, 2003.[CrossRef][Web of Science][Medline]
8. Godt RE, Lindley B. Influence of temperature upon contractile activation and isometric force production in mechanically skinned muscle fibers of the frog. J Gen Physiol 80: 279–297, 1982.[Abstract/Free Full Text]
9. Gomes AV, Potter JD. Molecular and cellular aspects of troponin cardiomyopathies. Ann NY Acad Sci 1015: 214–224, 2004.[CrossRef][Web of Science][Medline]
10. Heyder S, Malhotra A, Ruegg JC. Myofibrillar Ca²⁺ sensitivity of cardiomyopathic hamster hearts. Pflügers Arch 429: 539–545, 1995.[CrossRef][Web of Science][Medline]
11. Karmazyn M. The role of the myocardial sodium-hydrogen exchanger in mediating ischemic and reperfusion injury. From amiloride to cariporide. Ann NY Acad Sci 874: 326–334, 1999.[CrossRef][Web of Science][Medline]
12. Kass D, Solaro RJ. Mechanisms and use of calcium-sensitizing agents. Circulation 113: 305–315, 2006.
13. Kuhn FE, Rackley CE. Coronary artery disease in women. Risk factors, evaluation, treatment, and prevention. Arch Intern Med 13: 2626–2636, 1993.
14. Layland J, Cave AC, Grieve DJ, Sparks E, Kentish JC, Warren C, Solaro RJ, Shah AM. Protection against endotoxaemia induced contractile dysfunction in mice with cardiac-specific expression of slow skeletal troponin I. FASEB J 19: 1137–1139, 2005.[Abstract/Free Full Text]
15. Lindenfeld J, Ghali JK, Krause-Steinrauf HJ, Khan S, Adams K, Goldman S, Peberdy MA, Yancy C, Thaneemit-Chen S, Larsen RL, Young J, Lowes B, Rosenberg YD, Investigators BEST. Hormone replacement therapy is associated with improved survival in women with advanced heart failure. J Am Coll Cardiol 42: 1238–1245, 2003.[Abstract/Free Full Text]
16. Morimoto K, Kurahashi Y, Shintani-Ishida K, Kawamura N, Miyashita M, Uji Tan N M, Yoshida K. Estrogen replacement suppresses stress-induced cardiovascular responses in ovariectomized rats. Am J Physiol Heart Circ Physiol 287: H1950–H1956, 2004.[Abstract/Free Full Text]
17. Murphy E, Cross HR, and Steenbergen C. Is Na/Ca²⁺ exchange during ischemia and reperfusion beneficial or detrimental? Ann NY Acad Sci 976: 421–430, 2002.[Web of Science][Medline]
18. Patten RD, Pourati I, Aronovitz MJ, Baur J, Celestin F, Chen X, Michael A, Haq S, Nuedling S, Grohe C, Force T, Mendelsohn ME, Karas RH. 17-estradiol reduces cardiomyocyte apoptosis in vivo and in vitro via activation of phospho-inositide-3 kinase/Akt signaling. Circ Res 95: 692–629, 2004.[Abstract/Free Full Text]
19. Pines A, Fisman EZ, Shemesh J, Levo Y, Ayalon D, Kellermann JJ, Motro M, Drory Y. Menopause-related changes in left ventricular function in healthy women. Cardiology 80: 413–416, 1992.[Web of Science][Medline]
20. Reis SE, Holubkov R, Young JB, White BG, Cohn JN, Fledman AM. Estrogen is associated with improved survival in aging women with congestive heart failure: analysis of the vesnarinone studies. J Am Coll Cardiol 36: 529–533, 2000.[Abstract/Free Full Text]
21. Ren J, Hintz KK, Roughead ZK, Duan J, Colligan PB, Ren BH, Lee KJ, Zeng H. Impact of estrogen replacement on ventricular myocyte contractile function and protein kinase B/Akt activation. Am J Physiol Heart Circ Physiol 284: H1800–H1807, 2003.[Abstract/Free Full Text]
22. Rice JJ, de Tombe PP. Approaches to modeling crossbridges and calcium-dependent activation in cardiac muscle. Prog Biophys Mol Biol 85: 179–195, 2004.[CrossRef][Web of Science][Medline]
23. Schaible TF, Scheuer J. Comparison of heart function in male and female rats. Basic Res Cardiol 79: 402–412, 1984.[CrossRef][Web of Science][Medline]
24. Schaible TF, Malhotra A, Ciambrone G, Scheuer J. The effects of gonadectomy on left ventricular function and cardiac contractile proteins in male and female rats. Circ Res 54: 38–49, 1984.[Abstract/Free Full Text]
25. Solaro RJ. Modulation of cardiac myofilament activity by protein phosphorylation. Handbook of Physiology: The Cardiovascular System. Bethesda, MD: Am. Physiol. Soc., 2001, sect. 2, vol. 1, chapt. 7, 2001, p. 264–300.
26. Solaro RJ, Lee JA, Kentish JC, Allen DG. Effects of acidosis on ventricular muscle from adult and neonatal rats. Circ Res 63: 779–787, 1988.[Abstract/Free Full Text]
27. Solaro RJ, Hasenfuss G. Control of cardiac function in health and disease by mechanisms at the level of the sarcomere. In: Molecular Mechanisms of Cardiac Hypertrophy and Failure, edited by Walsh RA. London: Taylor and Francis, 2005, p. 265–279.
28. Sugishita K, Su Z, Li F, Philipson KD, Barry WH. Gender influence [Ca²⁺]I during metabolic inhibition in myocytes over expressing the Na⁺-Ca²⁺ exchanger. Circulation 104: 2102–2106, 2001.
29. Turgeon JL, McDonnell DP, Martin KA, Wise PM. Hormone therapy: physiological complexity belies therapeutic simplicity. Science 304: 1269–1273, 2004.[Abstract/Free Full Text]
30. Tytgat J. How to isolate cardiac myocytes. Cardiovasc Res 28: 280–283, 1994.[Free Full Text]
31. Urboniene D, Dias F, Peña JR, Walker LA, Solaro RJ, Wolska BM. Expression of slow skeletal troponin I in adult mouse heart helps to maintain the left ventricular systolic function during respiratory hypercapnia. Circ Res 97: 70–77, 2005.[Abstract/Free Full Text]
32. Wattanapermpool J. Increase in calcium responsiveness of cardiac myofilament activation in ovariectomized rats. Life Sci 63: 955–964, 1998.[CrossRef][Web of Science][Medline]
33. Wattanapermpool J, Reiser PJ. Differential effects of ovariectomy on calcium activation of cardiac and soleus myofilaments. Am J Physiol Heart Circ Physiol 277: H467–H473, 1999.[Abstract/Free Full Text]
34. Wattanapermpool J, Reiser PJ, Solaro RJ. Troponin I isoforms and differential effects of acidic pH on soleus and cardiac myofilaments. Am J Physiol Cell Physiol 268: C323–C330, 1995.[Abstract/Free Full Text]
35. Wattanapermpool J, Riabroy T, Preawnim S. Estrogen suppement prevents the calcium hypersensitivity of cardiac myofilaments in ovariectomized rats. Life Sci 66: 533–543, 2000.[CrossRef][Web of Science][Medline]
36. Wolff MR, Buck SH, Stoker SW, Greaser ML, Mantzer RM. Myofibrillar calcium sensitivity of isometric tension is increased in human dilated cardiomyopathies: role of altered beta-adrenergically mediated protein phosphorylation. J Clin Invest 98: 167–176, 1996.[Web of Science][Medline]
37. Wolska BM, Averyhart-Fullard V, Omachi A, Stojanovic MO, Kallen RG, Solaro RJ. Changes in thyroid state affect pH_i and Na_i⁺ homeostasis in rat ventricular myocytes. J Mol Cell Cardiol 29: 2653–2663, 1997.[CrossRef][Web of Science][Medline]
38. Wolska BM, Kitada Y, Palmiter KA, Westfall MA, Johnson MD, and Solaro RJ. CGP48506, a novel benzodiazocine derivative, increases contractility of ventricular myocytes and myofilaments by direct effects on the actin-myosin reaction. Am J Physiol Heart Circ Physiol 270: H24–H32, 1996.[Abstract/Free Full Text]
39. Wolska BM, Vijayan K, Arteaga GM, Konhilas JP, Phillips RM, Kim R, Naya T, Leiden JM, Martin AF, de Tombe PP, Solaro RJ. Expression of slow skeletal troponin I in adult transgenic mouse heart muscle reduces the force decline observed during acidic conditions. J Physiol 536: 863–870, 2001.[Abstract/Free Full Text]
40. Wolska BM, Stojanovic MO, Luo W, Kranias EG, Solaro RJ. Effect of ablation of phospholamban on dynamics of cardiac myocyte contraction and intracellular Ca²⁺. Am J Physiol Cell Physiol 271: C391–C397, 1996.[Abstract/Free Full Text]

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