Marina Lin

Dantrolene sodium can increase or attenuate activity of skeletal muscle ryanodine receptor calcium release channel : Clinical implications

Background Dantrolene sodium (DS) is a direct‐acting skeletal muscle relaxant whose only known action is to block calcium release from intracellular storage sites. The exact site of action for DS is unknown, but its efficacy in treating and preventing anesthetic‐induced malignant hyperthermia (MH) is well established. Methods Single ryanodine (Ry1) receptor calcium release channels were incorporated into a planar lipid bilayer for electrophysiologic recording and for subsequent analysis of the channels gating and conductance properties. The cellular effects of low DS concentrations were investigated by isometric contracture tension responses in biopsied MH human and dog muscle fascicles and in normal, single fibers from human vastus lateralis muscle. Results Two concentration‐dependent DS effects on the isolated Ry sub 1 receptor were discovered, suggesting at least two different binding sites. At nanomolar concentrations, DS activated the channel by causing three‐ to fivefold increases in open‐state probability and dwell times. At micromolar concentrations, DS first increased then reduced activity in the channels; with the dominant effect being reduced activity. A 20 nM concentration of DS produced significant contracture tension in human muscle from one MH subject and caused potentiation of twitch in muscle from another MH patient. Halothane contracture in MH dog muscle was followed by an additional increase in tension when treated with 20 nM DS. Other investigations on chemically skinned, human fibers showed that calcium loaded in the sarcoplasmic reticulum was partially released by nM DS. Conclusions The study results suggest that at least two binding sites for DS exist on the Ry1 receptor calcium channel. A low‐affinity (micro Meter) site is associated with reduced channel gating and open‐state dwell time and may relate to the established pharmacologic muscle relaxant effect of DS. The proposed high‐affinity (nM) DS binding site activates the channel, producing Calcium2+ release to the myoplasm, which, under environmentally adverse conditions, could damage genetically predisposed MH muscle. Such a phenomenon, if it occurs in DS‐treated MH patients, could generate a recrudescence of the syndrome.

Characterization of the cardiac renin angiotensin system in oophorectomized and estrogen-replete mRen2.Lewis rats.

The cardioprotective effects of estrogen are well recognized, but the mechanisms remain poorly understood. Accumulating evidence suggests that the local cardiac renin-angiotensin system (RAS) is involved in the development and progression of cardiac hypertrophy, remodeling, and heart failure. Estrogen attenuates the effects of an activated circulating RAS; however, its role in regulating the cardiac RAS is unclear. Bilateral oophorectomy (OVX; n = 17) or sham-operation (Sham; n = 13) was performed in 4-week-old, female mRen2.Lewis rats. At 11 weeks of age, the rats were randomized and received either 17 β-estradiol (E2, 36 µg/pellet, 60-day release, n = 8) or vehicle (OVX-V, n = 9) for 4 weeks. The rats were sacrificed, and blood and hearts were used to determine protein and/or gene expression of circulating and tissue RAS components. E2 treatment minimized the rise in circulating angiotensin (Ang) II and aldosterone produced by loss of ovarian estrogens. Chronic E2 also attenuated OVX-associated increases in cardiac Ang II, Ang-(1–7) content, chymase gene expression, and mast cell number. Neither OVX nor OVX+E2 altered cardiac expression or activity of renin, angiotensinogen, angiotensin-converting enzyme (ACE), and Ang II type 1 receptor (AT1R). E2 treatment in OVX rats significantly decreased gene expression of MMP-9, ACE2, and Ang-(1–7) mas receptor, in comparison to sham-operated and OVX littermates. E2 treatment appears to inhibit upsurges in cardiac Ang II expression in the OVX-mRen2 rat, possibly by reducing chymase-dependent Ang II formation. Further studies are warranted to determine whether an E2-mediated reduction in cardiac chymase directly contributes to this response in OVX rats.

Effects of Moderate and Deep Hypothermia on Ca2+ Signaling in Rat Ventricular Myocytes

Background/Aims: We investigated whether the degree of hypothermia determines the impairment in cardiac muscle function upon rewarming and whether the sarcoplasmic reticulum Ca2+ release channel, RyR2, contributes to hypothermia-induced changes in myoplasmic [Ca2+].Methods: Tension measurements using rat papillary muscle and calcium transients (Fluorescent Ca2+ indicator Fura 2-AM) in rat ventricular myocytes were compared during deep (10°C-16°C) and moderate hypothermic (28°C) myocardial temperatures. In a second experiment, myocytes were pretreated with dantrolene, an RyR2 antagonist; calcium transients were determined at control temperatures (32°C), 16°C, and upon rewarming (32°C).Results: Papillary muscle contractility and myocyte calcium transients were significantly reduced during and after rewarming from 16°C. At 28°C, papillary muscle isometric tension was potentiated and calcium transients were unaffected. After rewarming from 28°C, excitation-contraction coupling was maintained as isometric tension returned to 90% of control values. After rewarming from 16°C, myocytes pretreated with dantrolene had return of calcium transients to 89% of control values while myocytes not treated with dantrolene recovered to only 50% of their control values.Conclusion: We conclude that deep hypothermia, as opposed to moderate hypothermia of the myocardium, disrupts excitation-contraction coupling at cellular and tissue levels. Our finding of preserved calcium transients in dantrolene-pretreated myocytes exposed to deep hypothermia suggests a potential role for the RyR2 channel in post-hypothermia reductions in cardiac function.

Abnormal Function of Porcine Malignant Hyperthermia Calcium Release Channel in the Absence and Presence of Halothane

Malignant hyperthermia (MH) is an anesthetic-induced, life-threatening hypermetabolic state occurring in genetically predisposed pigs, dogs and humans. It is widely believed that MH is caused by an ab

Effect of Age, Estrogen Status, and Late-Life GPER Activation on Cardiac Structure and Function in the Fischer344×Brown Norway Female Rat

Age-associated changes in cardiac structure and function, together with estrogen loss, contribute to the progression of heart failure with preserved ejection fraction in older women. To investigate the effects of aging and estrogen loss on the development of its precursor, asymptomatic left ventricular diastolic dysfunction, echocardiograms were performed in 10 middle-aged (20 months) and 30 old-aged (30 months) female Fischer344×Brown-Norway rats, 4 and 8 weeks after ovariectomy (OVX) and sham procedures (gonads left intact). The cardioprotective potential of administering chronic G1, the selective agonist to the new G-protein-coupled estrogen receptor (GPER), was further evaluated in old rats (Old-OVX+G1) versus age-matched, vehicle-treated OVX and gonadal intact rats. Advanced age and estrogen loss led to decreases in myocardial relaxation and elevations in filling pressure, in part, due to reductions in phosphorylated phospholamban and increases in cardiac collagen deposition. Eight weeks of G-protein-coupled estrogen receptor activation in Old-OVX+G1 rats reversed the adverse effects of age and estrogen loss on myocardial relaxation through increases in sarcoplasmic reticulum Ca2+ ATPase expression and reductions in interstitial fibrosis. These findings may explain the preponderance of heart failure with preserved ejection fraction in older postmenopausal women and provide a promising, late-life therapeutic target to reverse or halt the progression of left ventricular diastolic dysfunction.

Calcium-Sensitizing Function for the Dipeptide Carnosine in Skeletal Muscle Contractility

Carnosine (α-alanyl- l -histidine), a dipeptide, the exact functions of which are unknown, is found at millimolar concentrations in muscle cells. In skinned skeletal muscle fibers, ca

Cardiomyocyte-specific deletion of the G protein-coupled estrogen receptor (GPER) leads to left ventricular dysfunction and adverse remodeling: A sex-specific gene profiling analysis

Activation of G protein-coupled estrogen receptor (GPER) by its agonist, G1, protects the heart from stressors such as pressure-overload, ischemia, a high-salt diet, estrogen loss, and aging, in various male and female animal models. Due to nonspecific effects of G1, the exact functions of cardiac GPER cannot be concluded from studies using systemic G1 administration. Moreover, global knockdown of GPER affects glucose homeostasis, blood pressure, and many other cardiovascular-related systems, thereby confounding interpretation of its direct cardiac actions. We generated a cardiomyocyte-specific GPER knockout (KO) mouse model to specifically investigate the functions of GPER in cardiomyocytes. Compared to wild type mice, cardiomyocyte-specific GPER KO mice exhibited adverse alterations in cardiac structure and impaired systolic and diastolic function, as measured by echocardiography. Gene deletion effects on left ventricular dimensions were more profound in male KO mice compared to female KO mice. Analysis of DNA microarray data from isolated cardiomyocytes of wild type and KO mice revealed sex-based differences in gene expression profiles affecting multiple transcriptional networks. Gene Set Enrichment Analysis (GSEA) revealed that mitochondrial genes are enriched in GPER KO females, whereas inflammatory response genes are enriched in GPER KO males, compared to their wild type counterparts of the same sex. The cardiomyocyte-specific GPER KO mouse model provides us with a powerful tool to study the functions of GPER in cardiomyocytes. The gene expression profiles of the GPER KO mice provide foundational information for further study of the mechanisms underlying sex-specific cardioprotection by GPER.

GPR30 decreases cardiac chymase/angiotensin II by inhibiting local mast cell number.

Chronic activation of the novel estrogen receptor GPR30 by its agonist G1 mitigates the adverse effects of estrogen (E2) loss on cardiac structure and function. Using the ovariectomized (OVX) mRen2.Lewis rat, an E2-sensitive model of diastolic dysfunction, we found that E2 status is inversely correlated with local cardiac angiotensin II (Ang II) levels, likely via Ang I/chymase-mediated production. Since chymase is released from cardiac mast cells during stress (e.g., volume/pressure overload, inflammation), we hypothesized that GPR30-related cardioprotection after E2 loss might occur through its opposing actions on cardiac mast cell proliferation and chymase production. Using real-time quantitative PCR, immunohistochemistry, and immunoblot analysis, we found mast cell number, chymase expression, and cardiac Ang II levels were significantly increased in the hearts of OVX-compared to ovary-intact mRen2.Lewis rats and the GPR30 agonist G1 (50 mg/kg/day, s.c.) administered for 2 weeks limited the adverse effects of estrogen loss. In vitro studies revealed that GPR30 receptors are expressed in the RBL-2H3 mast cell line and G1 inhibits serum-induced cell proliferation in a dose-dependent manner, as determined by cell counting, BrdU incorporation assay, and Ki-67 staining. Using specific antagonists to estrogen receptors, blockage of GPR30, but not ERα or ERβ, attenuated the inhibitory effects of estrogen on BrdU incorporation in RBL-2H3 cells. Further study of the mechanism underlying the effect on cell proliferation showed that G1 inhibits cyclin-dependent kinase 1 (CDK1) mRNA and protein expression in RBL-2H3 cells in a dose-dependent manner.

Blunting of estrogen modulation of cardiac cellular chymase/RAS activity and function in SHR

The relatively low efficacy of ACE‐inhibitors in the treatment of heart failure in women after estrogen loss may be due to their inability to reach the intracellular sites at which angiotensin (Ang) II is generated and/or the existence of cell‐specific mechanisms in which ACE is not the essential processing pathway for Ang II formation. We compared the metabolic pathway for Ang II formation in freshly isolated myocytes (CMs) and non‐myocytes (NCMs) in cardiac membranes extracted from hearts of gonadal‐intact and ovariectomized (OVX) adult WKY and SHR rats. Plasma Ang II levels were higher in WKY vs. SHR (strain effect: WKY: 62 ± 6 pg/ml vs. SHR: 42 ± 9 pg/ml; p < 0.01), independent of OVX. The enzymatic activities of chymase, ACE, and ACE2 were higher in NCMs versus CMs, irrespective of whether assays were performed in cardiac membranes from WKY or SHR or in the presence or absence of OVX. E2 depletion increased chymase activity, but not ACE activity, in both CMs and NCMs. Moreover, cardiac myocyte chymase activity associated with diastolic function in WKYs and cardiac structure in SHRs while no relevant functional and structural relationships between the classic enzymatic pathway of Ang II formation by ACE or the counter‐regulatory Ang‐(1‐7) forming path from Ang II via ACE2 were apparent. The significance of these novel findings is that targeted cell‐specific chymase rather than ACE inhibition may have a greater benefit in the management of HF in women after menopause.

Sevoflurane modulation of Ca2+ regulation in skeletal muscle sarcoplasmic reticulum vesicles from young and mature rabbits

Introduction:  Developmental differences in splice variants of the two key sarcoplasmic reticulum (SR) calcium regulatory proteins, ryanodine (RyR1), and sarcoendoplasmic reticulum calcium pump (SERCA1) have been linked to various neuromuscular disorders, but not malignant hyperthermia (MH). However, it is unclear whether an age‐related difference in volatile anesthetic‐mediated SR calcium function exists that could add to our current understanding of the clinical presentation of MH syndrome and provide insight into molecular mechanisms for general anesthesia that may have other physiologic and/or pathophysiologic significance. Therefore, the effects of sevoflurane on intracellular calcium regulation in isolated SR membrane vesicles from the skeletal muscle of healthy young rabbits were compared to their adult counterpart using an established in vitro model with the assumption that exposure to sevoflurane would elicit a weaker response in the young SR.