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ANESTHETIC PHARMACOLOGY

Modulation of Myofilament Ca²⁺ Sensitivity by - and -Opioid Agonists in Intact Guinea Pig Hearts

Yuri Nakae, MD PhD^*, Satoshi Fujita, MD PhD, and Akiyoshi Namiki, MD PhD^*

*Department of Anesthesiology, Sapporo Medical University School of Medicine; and Departments of Anesthesiology and Critical Care Medicine, Asahikawa Medical College, Japan

Address correspondence and reprint requests to Yuri Nakae, MD, PhD, Department of Anesthesiology, Sapporo Medical University School of Medicine, West-16, South-1, Chuo-ku, Sapporo 060-8543, Japan. Address e-mail to yurinaka{at}mac.com

	Abstract

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We investigated whether - and -opioid agonists alter myocardial function, intracellular Ca²⁺ concentration ([Ca²⁺]_i), and myofilament Ca²⁺ sensitivity in intact guinea pig beating hearts and whether these effects are mediated by an opioid receptor. Intact guinea pig hearts were perfused with modified Krebs Ringer solution containing - (TAN-67) and - (ICI-199441) opioid agonists in the absence and presence of - (BNTX) and - (nor-BNI) opioid antagonists, respectively, while functional variables and [Ca²⁺]_i were recorded. TAN-67 (1 µM) and ICI-199441 (1 µM) decreased heart rate (P < 0.05). TAN-67 (1 µM) and ICI-199441 (1 µM) decreased available [Ca²⁺]_i without changing developed left ventricular pressure (LVP) (P < 0.05). TAN-67 (1 µM) and ICI-199441 (1 µM) also caused a leftward shift in the curve of developed LVP as a function of available [Ca²⁺]_i (P < 0.05). ICI-199441 (1 µM) produced a steeper slope in the relation curve compared with baseline (P < 0.05). BNTX (1 µM) and nor-BNI (1 µM) blocked the effects of TAN-67 and ICI-199441, respectively. - and -opioid agonists enhance myofilament Ca²⁺ sensitivity despite decreasing available [Ca²⁺]_i in intact isolated guinea pig hearts, and these effects are mediated by - and -opioid receptor stimulation.

IMPLICATIONS: Our results indicate that - and -opioid agonists enhance myofilament Ca²⁺ sensitivity despite decreasing available intracellular Ca²⁺ concentrations in intact isolated guinea pig beating hearts, and these effects are mediated by - and -opioid receptor stimulation.

	Introduction

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Opioid decreases the sympathetic and somatic responses to noxious stimulation and can provide hemodynamic stability during opioid-based general anesthesia, even in patients with impaired cardiac function. Since opioid peptide receptors have been discovered (1) on cardiac myocytes, experimental evidence has indicated a direct effect of opioids on cardiac contractility and intracellular Ca²⁺ concentration ([Ca²⁺]_i) transients (2,3). Previous reports demonstrate that - and -opioid agonists have a negative inotropic action in adult rat left ventricular myocytes because of a decrease of [Ca²⁺]_i (4,5). However, other investigators have shown that -opioid agonists increase [Ca²⁺]_i in isolated rat adult ventricular cells and cultured neonatal rat cardiac myocytes and that -opioid agonists do not change [Ca²⁺]_i transients in cultured neonatal rat cardiac myocytes (6–8). One recent report demonstrated that the ₂-opioid agonist deltorphin enhances cardiac performance by increasing the responsiveness to cytosolic Ca²⁺ in an intact guinea pig heart (9). However, the direct effects of - and -opioid agonists on cardiac contractility, [Ca²⁺]_i transients, and myofilament Ca²⁺ sensitivity are controversial. We previously reported that morphine enhances myofilament Ca²⁺ sensitivity despite decreasing available [Ca²⁺]_i in intact guinea pig beating heart, and this effect of morphine is mediated by -opioid stimulation (10).

Accordingly, we investigated whether - and -opioid agonists alter myocardial function, [Ca²⁺]_i transients, and myofilament Ca²⁺ sensitivity in intact guinea pig beating hearts and whether these effects are mediated by - and -opioid stimulation. We selected an intact guinea pig spontaneously beating heart model because we could evaluate myofilament Ca²⁺ sensitivity by measuring [Ca²⁺]_i and left ventricular pressure (LVP) simultaneously under conditions that excluded neural and endocrine effects.

	Materials and Methods

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Approval from the Institutional Animal Care Committee of Sapporo Medical University was obtained before initiation of this study. English shorthaired guinea pigs weighing 250–300 g were intraperitoneally injected with 20 mg of ketamine (Sigma-Aldrich, Tokyo, Japan) and 1000 U of heparin (Sigma-Aldrich) and were decapitated after they became unresponsive to noxious stimulation. We confirmed that ketamine for anesthetizing the guinea pigs before decapitation does not have any effects on the results of the following experiments. The heart was isolated and perfused in a retrograde manner through the aorta at 55 mm Hg by the method previously reported (10). The perfusate, a modified Krebs Ringer solution (in mM: Na 137, K⁺ 5, Mg²⁺ 1.2, Ca²⁺ 2.5, Cl^- 134, HCO₃^- 15.5, H₂PO₄^- 1.2, glucose 11.5, pyruvate 2, mannitol 1.6, and EDTA 0.05, with 5 U/L of insulin), was equilibrated with a gas mixture of 95% oxygen and 5% carbon dioxide (CO₂) at a flow rate of 1 L/min, resulting in a pH value of 7.4. Perfusate and bath temperatures were regulated at 37°C ± 0.1°C by a thermostatically controlled water circulator (Thermo Pump^TM; Taitec, Koshiya, Japan).

Isovolemic developed (systolic - diastolic [s - d]) LVP, maximum rates of increase and decrease in LVP, spontaneous atrial heart rate (HR), and aortic inflow, which indicates coronary inflow, were measured continuously. LVP was measured by a pressure transducer (DTX Plus Press^TM; Ohmeda, Madison, WI) connected to a thin, saline-filled latex balloon (LB-3^TM; MARS, Sapporo, Japan) inserted into the left ventricle (LV) through the left atrium. The balloon volume was adjusted to maintain a diastolic LVP of 0 mm Hg during the initial control period. Coronary inflow was measured at constant temperature and at a constant perfusion pressure (55 mm Hg) by using a self-calibrating, in-line, ultrasonic flowmeter (T106; Transonic, Tokyo, Japan) placed directly into the aortic inflow line. Peak coronary responsiveness was tested with adenosine to temporarily arrest the hearts. A bolus of adenosine (0.2 mL of 200-µM stock solution) was injected directly into the aortic root cannula during the initial control period and after the last control reading. HR pacing was not applied to examine the effect of drugs during spontaneous beating.

Transmural LV phasic [Ca²⁺]_i was measured by the fluorescence ratio of indo-1 emission at 385 nm and 456 nm (F₃₈₅/F₄₅₆) by using a fiber bundle (FB-50; Japan Spectroscopic, Tokyo, Japan) connected to a luminescence spectrometer (CAF-110; Japan Spectroscopic). Our methods have been previously described in detail (10). Briefly, the excitation light was derived from a 75-W xenon arc lamp through a 350-nm filter, an interference filter (bandwidth, 12 nm), and a visible light-blocking filter, and it penetrated transmurally (5 mm). The light emitted from the heart was collected by the outgoing fiber bundle and filtered at 385 nm and 456 nm. The end of the fiberoptic cable (tip surface area, 38.5 mm²) was placed against the LV epicardial surface through a hole in the bath. To reduce cardiac motion artifacts, the heart was fastened to the fiberoptic tip with nylon mesh.

After perfusion for 15 min using the standard perfusate, the heart was loaded in an ambient light-free room at 25°C ± 0.1°C for 20–30 min with 166 mL of recirculating standard perfusate containing free Ca²⁺ fluorescent indicator indo-1 AM (6 µM, cell-permeable ester form of indo-1; Sigma-Aldrich). Loading of indo-1 was stopped when F₃₈₅/F₄₅₆ intensities had increased by approximately 10-fold. Residual indo-1 was washed out by the perfusion of Krebs Ringer solution for at least 20 min. To avoid blanching of indo-1, the arc lamp shutter was opened only for 2.5-s recording intervals for a total exposure of <125 s for each indo-1 loaded heart. Fluorescence of the indo-1 dye is independent of the tissue oxygenation state at these two isobestic wavelengths (11).

After correcting for autofluorescence and noncytosolic (primarily mitochondria) uptake, F₃₈₅/F₄₅₆ was calibrated to [Ca²⁺]_i transients (in nM) by the standard equation for fluorescent indicators: [Ca²⁺]_i = S₄₅₆ x K_d([R-R_min]/[R_max-R]), where K_d is the indo-1 dissociation constant for Ca²⁺, S₄₅₆ is the ratio between the fluorescence intensities measured at 456 nm in the absence of Ca²⁺ and in the presence of saturating Ca²⁺, R is the ratio of detected indo-1 fluorescence intensities, R_max is the fluorescence ratio in the presence of saturating Ca²⁺ (R_max = Sr/bH, for >100 µM of Ca²⁺, where Sr = [1-S₄₅₆]/[1-S₃₈₅], and bH is the ratio of F₄₅₆ as a function of F₃₈₅), and R_min is the fluorescence ratio in the absence of Ca²⁺ (R_min = R_max x S₃₈₅/S₄₅₆, for 0 Ca²⁺), respectively. Five additional hearts (1.4 ± 0.3 g) were used for calculation of K_d. K_d of indo-1 for Ca²⁺ was calculated to be 220 ± 37 nM at 37°C, and the average value of K_d was used in the equation for calculation of [Ca²⁺]_i.

To determine the fraction of noncytosolic (primarily mitochondria) fluorescence in whole hearts loaded with indo-1 AM, 17.5 µM of MnCl₂ was used to quench cytosolic fluorescence at the end of each experiment (12). Mn²⁺ crosses the sarcolemma and enters the cytosol, most likely via L-type Ca²⁺ channels (13). Once Mn²⁺ gains access to the cytosol, it binds to indo-1 with a 20-fold greater affinity than that of Ca²⁺ (14). Therefore, the residual fluorescence was inferred to be from indo-1 in noncytosolic compartments, whereas the loss of fluorescence transients during Mn²⁺ infusion was taken as evidence that cytosolic fluorescence was quenched. For each of 22 hearts measured [Ca²⁺]_i, background autofluorescence was corrected at each wavelength at 37°C after the initial perfusion and equilibration and before indo-1 loading. In pilot studies (n = 4), the drugs used did not alter basal autofluorescence and Mn²⁺-corrected noncytosolic Ca²⁺, and autofluorescence was unaffected over the 3-h testing period with normal oxygenation. Also, the drugs used did not interact with the fluorescence of indo-1 AM. The [Ca²⁺]_i transients during each contraction were defined as available [Ca²⁺]_i (s - d [Ca²⁺]_i). The [Ca²⁺]_i transients can be measured reliably for at least 3 h after washout of extracellular indo-1 AM. The data were digitized by a MacLab^TM system (AD Instruments, NSW, Australia) and stored in a personal computer.

Thirty hearts were used during the following protocols. At first, we studied the dose-dependent effects of a -opioid agonist (TAN-67) and a -opioid agonist (ICI-199441) on cardiac functional variables to determine the administration dose for [Ca²⁺]_i study at 2.5 mM of extracellular Ca²⁺ ([CaCl₂]_e). Initial control measurements were obtained during perfusion of Krebs Ringer solution (CaCl₂ 2.5 mM) after a 30-min stabilization period. Four hearts were perfused with Krebs Ringer solution (vehicle, CaCl₂ 2.5 mM) containing TAN-67 (0.01, 0.1, and 1 µM) for 4 min. The other four hearts were perfused with Krebs Ringer solution (vehicle, CaCl₂ 2.5 mM) containing ICI-199441 (0.01, 0.1, and 1 µM) for 4 min. All cardiac variables were measured after a 4-min equilibration period. After control measurements, different concentrations of drugs were tested in random fashion in the same heart with a 20-min washout period.

Next, we studied the effects of TAN-67 and ICI-199441 on developed LVP and [Ca²⁺]_i. After loading and residual washout of indo-1 AM, six hearts were perfused with Krebs Ringer solution (vehicle) and Krebs Ringer solution containing TAN-67 (0.1 and 1 µM). Similarly, another six hearts were perfused with Krebs Ringer solution (vehicle) and Krebs Ringer solution containing ICI-199441 (0.1 and 1 µM). At the same time, the concentration of [CaCl₂]_e was increased incrementally from 0.3 to 4.5 mM for an 11-min period, during which all cardiac variables and [Ca²⁺]_i were measured at 1-min intervals until, under any drug protocol, LVP reached its maximum value. Different concentrations were tested in random fashion in the same heart with a 20-min washout period. Each heart underwent a change in [CaCl₂]_e three times. Postcontrol values (n = 4) were measured in the same heart at the end of all interventions. Normalized, developed LVP was plotted as a function of s - d [Ca²⁺]_i. Normalized, developed LVP was expressed as a percentage of maximum developed LVP.

Finally, we examined the effects of TAN-67 and ICI-199441 on [Ca²⁺]_i and developed LVP in the presence of each opioid antagonist. Five hearts were perfused with Krebs Ringer solution (vehicle), Krebs Ringer solution containing TAN-67 (1 µM), a -opioid antagonist (BNTX; 1 µM), and TAN-67 (1 µM) with BNTX (1 µM). We determined the concentrations of opioid antagonists that equivalently inhibited the effects of each agonist on cardiac functional variables. Similarly, another five hearts were perfused with Krebs Ringer solution (vehicle), Krebs Ringer solution containing ICI-199441 (1 µM), a -opioid antagonist (nor-BNI; 1 µM), and ICI-199441 (1 µM) with nor-BNI (1 µM). At the same time, [CaCl₂]_e was incrementally increased, as described in the previous protocol. Each drug was given in the same heart with a 20-min washout period. Each heart underwent a change in [CaCl₂]_e four times, including once in the absence of drugs (baseline) so that each heart served as its own control. We confirmed that the drug effect was maximal within 5 min and stable during drug perfusion and that 20 min after discontinuation of the drug, baseline conditions had returned. Postcontrol values (n = 4) were measured in the same heart at the end of all interventions. Altering [CaCl₂]_e did not significantly alter noncytosolic compartment Ca²⁺ for a 20-min period (data not shown). All opioid agonists and antagonists were kindly supplied by H. Nagase and T. Endoh (Basic Research Laboratories, Toray Industries, Kamakura, Japan).

All data are expressed as mean ± SD except when otherwise indicated. Cardiac functional data were compared using Tukey comparison of means test after analysis of variance for repeated measures. Curves of normalized, developed LVP as a function of s - d [Ca²⁺]_i were compared using nonlinear regression analyses with Boltzmann sigmoidal equations (Prism^TM; GraphPad Software, San Diego, CA), where all correlation coefficients (R) were >0.98. Slopes, 95% confidence intervals (CI), 50% maximum values of normalized, developed LVP (ED₅₀), and slope comparisons were determined for each curve. Differences between means were considered significant when P < 0.05.

	Results

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TAN-67 (1 µM) decreased HR by 41 ± 3 bpm at 2.5 mM of [CaCl₂]_e (P < 0.05), whereas it did not change other variables (Table 1). TAN-67 (0.01 and 0.1 µM) did not change any functional variables. Similarly, ICI-199441 (1 µM) decreased HR by 33 ± 7 bpm at 2.5 mM of [CaCl₂]_e (P < 0.05), whereas it did not change other variables (Table 2). ICI-199441 (0.01 and 0.1 µM) did not change any functional variables. Although the half-maximal inhibitory concentration of TAN-67 and ICI-199441 for selective - and -opioid receptors were 3.65 nM and 25 nM in their analgesic activity, respectively (15,16), the present study showed that the opioid agonists change the cardiac variables at the concentration of 1 µM. Therefore, we selected the opioid concentrations of 0.1 and 1 µM in the following protocols.

View larger version (24K): [in this window] [in a new window]   Figure 1. (A) Changes in normalized, developed left ventricular pressure (LVP [%]) with increases in systolic - diastolic (s - d) [Ca2+]i (nM) while extracellular Ca2+ ([CaCl2]e) was incrementally increased from 0.3 to 4.5 mM in the perfusate of vehicle (baseline) and -agonist (TAN-67) (see text for values). Data are presented as mean ± SD (n = 6). (B) Changes in normalized, developed LVP with increases in s - d [Ca2+]i (nM) while [CaCl2]e was incrementally increased from 0.3 to 4.5 mM in the perfusate of vehicle (baseline) and -agonist (ICI-199441) (see text for values). Data are presented as mean ± SD (n = 6).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
Our most important finding is that - and -opioid agonists enhance myofilament Ca²⁺ sensitivity despite decreasing available [Ca²⁺]_i. We assessed myofilament Ca²⁺ sensitivity by evaluating the relationship between available [Ca²⁺]_i and developed LVP. The - and -opioid agonists caused a leftward shift in the curve relating to developed LVP and available [Ca²⁺]_i. On the basis of previously published work on isolated tissue and cells (17,18), these findings suggest that - and -opioid agonists enhance myofilament Ca²⁺ sensitivity, indicating that they may increase the affinity for Ca²⁺ binding to the contractile proteins, especially troponin C. Moreover, the -opioid agonist produced an increase in the slope of the curve, indicating that it may modulate myofilament Ca²⁺ sensitivity by increasing efficacy or rate of individual cross-bridge cycle formation.

Mechanisms of inotropic effect are explained by the alterations of available [Ca²⁺]_i, time course of Ca²⁺ transients, myofilament Ca²⁺ sensitivity, and efficacy or rate of individual cross-bridge cycle formation (17,18). The decrease of available [Ca²⁺]_i is caused by the decrease of systolic [Ca²⁺]_i or diastolic [Ca²⁺]_i. The decrease of systolic [Ca²⁺]_i reflects the decrease of transsarcolemmal Ca²⁺ influx through Ca²⁺ channels and other ion channels and Ca²⁺ release from the sarcoplasmic reticulum (19). Also, the decrease of diastolic [Ca²⁺]_i reflects the increase of Ca²⁺ uptake rate and Ca²⁺-adenosine triphosphate activity in the sarcoplasmic reticulum (20) and enhanced Ca²⁺ extrusion from the myocytes via sarcolemmal Ca²⁺-adenosine triphosphatase and Na⁺-Ca²⁺-exchange activity. Because the - and -opioid agonists did not have a negative inotropic effect in the present study, the decrease of available [Ca²⁺]_i could be compensated for by the enhancement of myofilament Ca²⁺ sensitivity.

In the present study, - and -opioid agonists caused a negative chronotropic effect in isolated beating hearts. This finding is in agreement with other studies using different experimental preparations (21,22). However, the effects of opioids on inotropy and [Ca²⁺]_i transients are still controversial. Previous studies have reported that - and -opioid agonists have a negative inotropic action, with a decrease of [Ca²⁺]_i transients in adult rat LV myocytes (4). However, ₂-opioid agonists enhance cardiac performance despite decreasing [Ca²⁺]_i transients in intact guinea pig hearts (9), and -opioid agonists increase the contraction with an increase of [Ca²⁺]_i transients in neonatal rat cultured cardiac myocytes (8). There are differences in animal species, experimental preparations and conditions, and opioid receptor subtypes used between previous studies and the present study. It has been reported that there are differences in opioid receptor subtypes and the Na⁺/Ca²⁺ exchange mechanism between rats and guinea pigs (23). In addition, neonatal and adult ventricular myocytes differ in the developmental stage of the sarcoplasmic reticulum (24). Furthermore, the experimental conditions such as low temperature or a low frequency of stimulus may have an effect on [Ca²⁺]_i transients (25). The heart is able to maintain a good contraction at rates slower than those physiologically encountered. Because TAN-67 and ICI-199441 decreased HR in the present study, this chronotropic effect may modify the relationship between LVP and available [Ca²⁺]_i.

It remains controversial how opioid agonists modify the cardiac effects. A previous study has demonstrated that - and -opioid receptor stimulation may modulate ß-adrenergic effects (22) and that -opioid agonists may exert an indirect effect to enhance the sensitivity to Ca²⁺ by increasing cytosolic pH values (6). The present study showed that the effects of opioids on chronotropy, [Ca²⁺]_i transients, and myofilament Ca²⁺ sensitivity were eliminated in the presence of each specific opioid antagonist. These findings indicate that opioid receptor stimulation may directly modulate cardiac function and [Ca²⁺]_i homeostasis because the neural and endocrine effects are excluded in our intact guinea pig heart model.

In conclusion, our results indicate that - and -opioid agonists enhance myofilament Ca²⁺ sensitivity despite decreasing available [Ca²⁺]_i in intact isolated guinea pig beating hearts and that these effects are mediated by - and -opioid receptor stimulation.

	Acknowledgments

 
Supported, in part, by a Grant-in-Aid for Scientific Research (No. 10671434) from the Ministry of Education, Japan.

	Footnotes

 
Presented, in part, at the annual meeting of the American Society of Anesthesiologists, October 9–13, 1999, Dallas, TX.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

1. Ventura C, Bastagli L, Bernardi P, et al. Opioid receptors in rat cardiac sarcolemma: effect of phenylephrine and isoproterenol. Biochim Biophys Acta 1989; 987: 69–74.[Medline]
2. Huang XD, Wong TM. Morphine and (D-Ala², NMe-Phe4, Gly-0l)-enkephalin increase the intracellular free calcium in isolated rat myocytes: effect of naloxone or pretreatment with morphine. Life Sci 1991; 48: 1101–7.[Web of Science][Medline]
3. Laurent S, Marsh JD, Smith TW. Enkephalins have a direct positive inotropic effect on cultured cardiac myocytes. Proc Natl Acad Sci USA 1985; 82: 5930–4.[Abstract/Free Full Text]
4. Ventura C, Spurgeon H, Lakatta EG, et al. and opioid receptor stimulation affects cardiac myocyte function and Ca²⁺ release from an intracellular pool in myocytes and neurons. Circ Res 1992; 70: 66–81.[Abstract/Free Full Text]
5. Xiao RP, Pepe S, Spurgeon HA, et al. Opioid peptide receptor stimulation reverses ß-adrenergic effects in rat heart cells. Am J Physiol 1997; 272: H797–805.[Abstract/Free Full Text]
6. Ventura C, Capogrossi MC, Spurgeon HA, Lakatta EG. -Opioid peptide receptor stimulation increases cytosolic pH and myofilament responsiveness to Ca²⁺ in cardiac myocytes. Am J Physiol 1991; 261: H1671–4.[Abstract/Free Full Text]
7. Tai KK, Bian CF, Wong TM. -Opioid receptor stimulation increases intracellular free calcium in isolated rat ventricular myocytes. Life Sci 1992; 51: 909–13.[Web of Science][Medline]
8. Ela C, Barg J, Vogel Z, et al. Distinct components of morphine effects on cardiac myocytes are mediated by and opioid receptors. J Mol Cell Cardiol 1997; 29: 711–20.[Web of Science][Medline]
9. Fujita S, Smart SC, Stowe DF. Enhanced contractile responsiveness to cytosolic Ca²⁺ by delta-2 opioid agonist deltorphin in intact guinea pig hearts. J Mol Cell Cardiol 2000; 32: 1647–59.[Web of Science][Medline]
10. Nakae Y, Fujita S, Namiki A. Morphine enhances myofilament Ca²⁺ sensitivity in intact guinea pig beating hearts. Anesth Analg 2001; 92: 602–8.[Abstract/Free Full Text]
11. Brandes R, Figueredo VM, Camacho SA, et al. II. Investigation of factors affecting fluorometric quantitation of cytosolic [Ca²⁺] in perfused hearts. Biophys J 1993; 65: 1983–93.[Web of Science][Medline]
12. Schreur JH, Figueredo VM, Miyamae M, et al. Cytosolic and mitochondrial [Ca²⁺] in whole hearts using indo-1 acetoxymethyl ester: effects of high extracellular Ca²⁺. Biophys J 1996; 70: 2571–80.[Web of Science][Medline]
13. Payet MD, Schanne OF, Ruiz CE. Competition for slow channel of Ca²⁺, Mn²⁺, verapamil, and D-600 in rat ventricular muscle? J Mol Cell Cardiol 1980; 12: 635–8.[Web of Science][Medline]
14. Miyata H, Silverman HS, Sollott SJ, et al. Measurement of mitochondrial free Ca²⁺ concentration in living single rat cardiac myocytes. Am J Physiol 1991; 261: H1123–34.[Abstract/Free Full Text]
15. Fujii H, Kawai K, Kawamura K, et al. Synthesis of optically active TAN-67, a highly selective opioid receptor agonist, and investigation of its pharmacological properties. Drug Des Discov 2001; 17: 325–30.[Medline]
16. Kumar V, Marella MA, Cortes-Burgos L, et al. Arylacetamides as peripherally restricted kappa opioid receptor agonists. Bioorg Med Chem Lett 2000; 10: 2567–70.[Medline]
17. Blinks JR. Analysis of the effects of drugs on myofibrillar Ca²⁺ sensitivity in intact cardiac muscle. In: Lee JA, Allen DG, eds. Modulation of cardiac calcium sensitivity. Oxford, England: Oxford University Press, 1993: 242–82.
18. Brenner B. Changes in calcium sensitivity at the cross-bridge level. In: Lee JA, Allen DG, eds. Modulation of cardiac calcium sensitivity. Oxford, England: Oxford University Press, 1993: 197–214.
19. Shattock MJ, Bers DM. Inotropic response to hypothermia and the temperature-dependence of ryanodine action in isolated rabbit and rat ventricular muscle: implications for excitation-contraction coupling. Circ Res 1987; 61: 761–71.[Abstract/Free Full Text]
20. Suko J. The effect of temperature on Ca²⁺ uptake and Ca²⁺-activated ATP hydrolysis by cardiac sarcoplasmic reticulum. Experientia 1973; 29: 396–8.[Web of Science][Medline]
21. Hall ED, Wolf DL, McCall RB. Cardiovascular depressant effects of the kappa opioid receptor agonists U50488H and spiradoline mesylate. Circ Shock 1988; 26: 409–17.[Web of Science][Medline]
22. Micol JA, Laorden ML. Effects of µ-, -, and -agonists on isolated right atria of the rat. Neuropeptides 1994; 26: 365–70.[Web of Science][Medline]
23. Lewartowski B, Zdanowski K. Net Ca²⁺ influx and sarcoplasmic reticulum Ca²⁺ uptake in resting single myocytes of rat heart: comparison with guinea pig. J Mol Cell Cardiol 1990; 22: 1221–9.[Web of Science][Medline]
24. Tanaka H, Shigenobu K. Effect of ryanodine on neonatal and adult rat heart: developmental increase in sarcoplasmic reticulum function. J Mol Cell Cardiol 1989; 21: 1305–13.[Web of Science][Medline]
25. Nakae Y, Fujita S, Namiki A. Isoproterenol enhances myofilament Ca²⁺ sensitivity during hypothermia in isolated guinea pig beating hearts. Anesth Analg 2001; 93: 846–52.[Abstract/Free Full Text]

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