Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin.

Of the ions involved in myocardial function, Ca2+ is the most important. Ca2+ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation-contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca2+ concentration is regulated to elicit excitation-contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca2+ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca2+ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation-contraction coupling.

Alpha1-adrenenoceptor stimulation inhibits cardiac excitation-contraction coupling through tyrosine phosphorylation of beta1-adrenoceptor.

Adrenoceptor stimulation is a key determinant of cardiac excitation-contraction coupling mainly through the activation of serine/threonine kinases. However, little is known about the role of protein tyrosine kinases (PTKs) activated by adrenergic signaling on cardiac excitation-contraction coupling. A cytoplasmic tyrosine residue in ß1-adrenoceptor is estimated to regulate Gs-protein binding affinity from crystal structure studies, but the signaling pathway leading to the phosphorylation of these residues is unknown. Here we show α1-adrenergic signaling inhibits ß-adrenergically activated Ca(2+) current, Ca(2+) transients and contractile force through phosphorylation of tyrosine residues in ß1-adrenoceptor by PTK. Our results indicate that inhibition of ß-adrenoceptor-mediated Ca(2+) elevation by α1-adrenoceptor-PTK signaling serves as an important regulatory feedback mechanism when the catecholamine level increases to protect cardiomyocytes from cytosolic Ca(2+) overload.

Greater potency of darbepoetin-α than erythropoietin in suppression of serum hepcidin-25 and utilization of iron for erythropoiesis in hemodialysis patients.

BACKGROUND: The potency of darbepoetin-α (DPO-α) to improve anemia in hemodialysis (HD) patients is greater than that of recombinant human erythropoietin (rHuEPO). DESIGN AND METHODS: To assess the potency of DPO-α to mobilize iron from body stores in comparison with rHuEPO in HD patients without apparent inflammation or infection, serum iron, transferrin saturation (TSAT), ferritin, and hepcidin-25 were measured serially. This study included (i) a long-term crossover study for 3 yr to compare the effects of the two erythropoiesis-stimulating agents (ESA) on serum iron, TSAT, and ferritin, and (ii) a short-term crossover study for 8 wk to examine their effects on serum hepcidin-25 in HD patients. RESULTS: The long-term crossover study demonstrated that the change of ESA from rHuEPO to DPO-α significantly decreased serum ferritin while serum iron and TSAT remained unchanged, while DPO-α as well as rHuEPO maintained hemoglobin level in the target range between 10.0 and 11.0 g/dL. Furthermore, in the short-term crossover study, area under the percent suppression of serum hepcidin-25 time curve for the first 7 d during the DPO-α treatment period was significantly greater than that during the rHuEPO period (348.0 ± 92.4 vs. 178.4 ± 131.5%.day P = 0.030). The greater suppression of hepcidin-25 by DPO-α may facilitate iron mobilization, resulting in diminution of body iron stores without any significant effect on serum iron utilizable for erythropoiesis. CONCLUSION: This study demonstrated that DPO-α has a greater advantage than rHuEPO in that it facilitates iron mobilization from body stores into bone marrow to induce effective erythropoiesis and thus could protect against possible harmful effects caused by excessive iron stores in the body.

Sarcomere imaging by quantum dots for the study of cardiac muscle physiology.

We here review the use of quantum dots (QDs) for the imaging of sarcomeric movements in cardiac muscle. QDs are fluorescence substances (CdSe) that absorb photons and reemit photons at a different wavelength (depending on the size of the particle); they are efficient in generating long-lasting, narrow symmetric emission profiles, and hence useful in various types of imaging studies. Recently, we developed a novel system in which the length of a particular, single sarcomere in cardiomyocytes can be measured at ~30 nm precision. Moreover, our system enables accurate measurement of sarcomere length in the isolated heart. We propose that QDs are the ideal tool for the study of sarcomere dynamics during excitation-contraction coupling in healthy and diseased cardiac muscle.

Real-time measurement of the length of a single sarcomere in rat ventricular myocytes: a novel analysis with quantum dots.

As the dynamic properties of cardiac sarcomeres are markedly changed in response to a length change of even ∼0.1 µm, it is imperative to quantitatively measure sarcomere length (SL). Here we show a novel system using quantum dots (QDs) that enables a real-time measurement of the length of a single sarcomere in cardiomyocytes. First, QDs were conjugated with anti-α-actinin antibody and applied to the sarcomeric Z disks in isolated skinned cardiomyocytes of the rat. At partial activation, spontaneous sarcomeric oscillations (SPOC) occurred, and QDs provided a quantitative measurement of the length of a single sarcomere over the broad range (i.e., from ∼1.7 to ∼2.3 µm). It was found that the SPOC amplitude was inversely related to SL, but the period showed no correlation with SL. We then treated intact cardiomyocytes with the mixture of the antibody-QDs and FuGENE HD, and visualized the movement of the Z lines/T tubules. At a low frequency of 1 Hz, the cycle of the motion of a single sarcomere consisted of fast shortening followed by slow relengthening. However, an increase in stimulation frequency to 3-5 Hz caused a phase shift of shortening and relengthening due to acceleration of relengthening, and the waveform became similar to that observed during SPOC. Finally, the anti-α-actinin antibody-QDs were transfected from the surface of the beating heart in vivo. The striated patterns with ∼1.96-µm intervals were observed after perfusion under fluorescence microscopy, and an electron microscopic observation confirmed the presence of QDs in and around the T tubules and Z disks, but primarily in the T tubules, within the first layer of cardiomyocytes of the left ventricular wall. Therefore, QDs are a useful tool to quantitatively analyze the movement of single sarcomeres in cardiomyocytes, under various experimental settings.

Thin filament-reconstituted skinned muscle fibers for the study of muscle physiology.

We review the use of thin filament-reconstituted muscle fibers in the study of muscle physiology. Thin filament extraction and reconstitution protocol is a powerful technique to study the role of each component of the thin filament. It is also useful for studying the properties of genetically modified molecules such as actin and tropomyosin. We also review the combination of this protocol with sinusoidal analysis, which will provide a solid technique for determining the effect of regulatory proteins on actomyosin interaction and concomitant cross-bridge kinetics. We suggest that thin filament-reconstituted muscle fibers are an ideal system for studying muscle physiology especially when gene modifications of actin or tropomyosin are involved.

Sequential semi-supervised segmentation for serial electron microscopy image with small number of labels.

BACKGROUND: Segmentation of electron microscopic continuous section images by deep learning has attracted attention as a technique to reduce the cost of annotation for researchers attempting to make observations using 3D reconstruction methods. However, when the observed samples are rare, or scanning circumstances are unstable, pursuing generalization performance for newly obtained samples is not appropriate. NEW METHODS: We assume a transductive setting that predicts all labels in a dataset from only partially obtained labels while avoiding the pursuit of generalization performance for unknown data. Then, we propose sequential semi-supervised segmentation (4S), which semi-automatically extracts neural regions from electron microscopy image stacks. This method focuses on the fact that adjacent images have a strong correlation in serial images. Our 4S repeats training, inference, and pseudo-labeling using a minimal number of teacher labels and performs segmentation on all slices. RESULT: Our experiments using two types of serial section images showed effectiveness in terms of both quality and quantity. In addition, we experimentally clarified the effect of the number and position of teacher labels on performance. COMPARISON WITH EXISTING METHODS: Compared with supervised learning when a small number of labeled data was obtained, the performance of the proposed method was shown to be superior. CONCLUSION: Our 4S leverages a limited number of labeled data and a large amount of unlabeled data to extract neural regions from serial image stacks in a transductive setting. We plan to develop this method as a core module of a general-purpose annotation tool in our future work.

Prevalence of COVID-19 Mimics in the Emergency Department.

Objective Due to the lack of specific clinical manifestations and symptoms, it is difficult to distinguish COVID-19 from mimics. A common pitfall is to rush to make a diagnosis when encountering a patient with COVID-19-like symptoms. The present study describes a series of COVID-19 mimics using an outpatient database collected from a designated COVID-19 healthcare facility in Tokyo, Japan. Methods We established an emergency room (ER) tailored specifically for patients with suspected or confirmed COVID-19 called the "COVID-ER." In this single-center retrospective cohort study, we enrolled patients who visited the COVID-ER from February 1 to September 5, 2020. The outcomes included the prevalence of COVID-19, admission, potentially fatal diseases and final diagnosis. Results We identified 2,555 eligible patients. The median age was 38 (interquartile range, 26-57) years old. During the study period, the prevalence of COVID-19 was 17.9% (457/2,555). Non-COVID-19 diagnoses accounted for 82.1% of all cases. The common cold had the highest prevalence and accounted for 33.0% of all final diagnoses, followed by gastroenteritis (9.4%), urinary tract infections (3.8%), tonsillitis (2.9%), heat stroke (2.6%) and bacterial pneumonia (2.1%). The prevalence of potentially fatal diseases was 14.2% (298/2,098) among non-COVID-19 patients. Conclusion Several potentially fatal diseases remain masked among the wave of COVID-19 mimics. It is imperative that a thorough differential diagnostic panel be considered prior to the rendering of a COVID-19 diagnosis.

Titin-based regulations of diastolic and systolic functions of mammalian cardiac muscle.

Titin is the largest protein in mammals; it forms an elastic filament along the myofibril of cardiac and skeletal muscles. Novel studies employing the recently available varied technologies have revealed the molecular mechanisms by which titin generates passive force in the sarcomere in response to external stretch. Changes in titin stiffness occur during heart disease via a shift in the expression ratio of the two main titin isoforms, called N2B (stiff type) and N2BA (compliant type) titins. Protein kinase (PK)A, PKG and PKC phosphorylate the cardiac specific I-band titin segment, resulting in an acute decrease (by PKA and PKG) or increase (by PKC) in passive force. It has also been discovered that titin performs roles that go beyond passive force generation, by enhancing or terminating active force production, thereby adjusting the Frank-Starling mechanism of the heart. Therefore, titin is a self-adjustable and multi-functional spring that is indispensable for proper heart functions. Here, we discuss how titin regulates the passive and active properties of cardiac muscle in normal physiological conditions as well as in chronic heart disease.

Role of Ca(2+)/calmodulin-dependent protein kinase II in the regulation of the cardiac L-type Ca(2+) current during endothelin-1 stimulation.

Endothelin-1 (ET-1) shows a positive inotropic effect on cardiac muscle. Although the L-type Ca(2+) current (I(Ca)) is one of the important determinants of cardiac excitation-contraction coupling, the effect of ET-1 on the I(Ca) is not always clear. The controversial results appear to be due to different patch-clamp methods. The present study measured the effect of ET-1 on the I(Ca) of rat ventricular myocytes using the perforated patch-clamp technique. The holding potential was set to -40 mV, and depolarization was applied every 10 s. ET-1 (10 nM) increased the I(Ca) in a monophasic manner. The current reached a steady state 15 min after the application of ET-1, when the measurement was done. Endothelin receptor subtype expression was also investigated using Western immunoblotting. ET(A)-receptor protein was expressed, but ET(B)-receptor protein was not expressed, in the cell membranes of rat ventricular myocytes. The effect of ET-1 on the I(Ca) was inhibited by a selective ET(A)-receptor antagonist, BQ-123, but not by a selective ET(B)-receptor antagonist, BQ-788. The effect was inhibited by protein kinase C (PKC) inhibitor chelerythrine and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93, but not by its inactive analog KN-92. The effect of ET-1 was also blocked by another CaMKII inhibitor, autocamtide-2-related inhibitory peptide. These results suggest that ET-1 increases the I(Ca) via the ET(A)-receptor-PKC-CaMKII pathway.

Interaction of alpha1-adrenoceptor subtypes with different G proteins induces opposite effects on cardiac L-type Ca2+ channel.

We examined the effect of alpha(1)-adrenoceptor subtype-specific stimulation on L-type Ca2+ current (I(Ca)) and elucidated the subtype-specific intracellular mechanisms for the regulation of L-type Ca2+ channels in isolated rat ventricular myocytes. We confirmed the protein expression of alpha(1A)- and alpha(1B)-adrenoceptor subtypes at the transverse tubules (T-tubules) and found that simultaneous stimulation of these 2 receptor subtypes by nonsubtype selective agonist, phenylephrine, showed 2 opposite effects on I(Ca) (transient decrease followed by sustained increase). However, selective alpha(1A)-adrenoceptor stimulation (> or =0.1 micromol/L A61603) only potentiated I(Ca), and selective alpha(1B)-adrenoceptor stimulation (10 mumol/L phenylephrine with 2 micromol/L WB4101) only decreased I(Ca). The positive effect by alpha(1A)-adrenoceptor stimulation was blocked by the inhibition of phospholipase C (PLC), protein kinase C (PKC), or Ca2+/calmodulin-dependent protein kinase II (CaMKII). The negative effect by alpha(1B)-adrenoceptor stimulation disappeared after the treatment of pertussis toxin or by the prepulse depolarization, but was not attributable to the inhibition of cAMP-dependent pathway. The translocation of PKCdelta and epsilon to the T-tubules was observed only after alpha(1A)-adrenoceptor stimulation, but not after alpha(1B)-adrenoceptor stimulation. Immunoprecipitation analysis revealed that alpha(1A)-adrenoceptor was associated with G(q/11), but alpha(1B)-adrenoceptor interacted with one of the pertussis toxin-sensitive G proteins, G(o). These findings demonstrated that the interactions of alpha(1)-adrenoceptor subtypes with different G proteins elicit the formation of separate signaling cascades, which produce the opposite effects on I(Ca). The coupling of alpha(1A)-adrenoceptor with G(q/11)-PLC-PKC-CaMKII pathway potentiates I(Ca). In contrast, alpha(1B)-adrenoceptor interacts with G(o), of which the betagamma-complex might directly inhibit the channel activity at T-tubules.

[Evaluation of cardiac and myocardial inotropism and lusitropism using half-logistic curve-fitting].

Half-logistic (h-L) function, which is half of the sigmoidal, logistic function with a boundary at the inflection point, curve-fits the isovolumic relaxation left ventricular (LV) pressure curve from the minimum of the first order time derivative of pressure (dP/dt(min)), and the myocardial isometric relaxation tension curve from the minimum of the first order time derivative of tension (dF/dt(min)) superior to the conventional mono-exponential function. Recently, we found that h-L function could curve-fit the other partial curves. The isovolumic LV pressure curve in the excised, cross-circulated canine heart, was divided into four distinct phases with boundaries set at the maximum of dP/dt (dP/dt(max)), peak LV pressure, dP/dt(min), and LV end-diastolic pressure (LVEDP) resulting in the first phase from the point corresponding to QR on the electrocardiogram to dP/dt(max); the second phase from dP/dt(max) to the peak LV pressure; the third phase from the peak LV pressure to dP/dt(min); and the fourth phase from dP/dt(min) to LVEDP. Similarly, the isometric twitch tension curves in the mouse LV and rabbit right ventricular (RV) papillary muscles were divided into four distinct phases with boundaries set at the maximum of dF/dt (dF/dt(max)), peak tension, dF/dt(min), and resting tension resulting in the first phase from the point corresponding to twitch stimulation to dF/dt(max); the second phase from dF/dt(max) to the peak tension; the third phase from the peak tension to dF/dt(min); and the fourth phase from dF/dt(min) to resting tension. The h-L correlation coefficient (r) values for the sequential curves were larger than the m-E r values, respectively, and the h-L residual mean squares (RMS) were smaller than the m-E RMS values, respectively. The h-L time constants are indices which quantify cardiac and myocardial inotropism and lusitropism more accurately. We consider that the h-L approach also applies for evaluation of the isovolumic contraction phase and the isovolumic relaxation phase in the beating hearts.

Warfarin-induced impairment of bone material quality in a patient undergoing maintenance hemodialysis: A case report.

INTRODUCTION: The use of warfarin in patients undergoing hemodialysis is associated with decreased bone mineral density and an increased incidence of bone fracture. However, no studies to date have directly estimated bone quality with bone histomorphometry in patients with bone abnormalities who are taking warfarin and undergoing hemodialysis. PATIENT CONCERNS: A 47-year-old female with Noonan syndrome presented with progressive bilateral lower extremity pain on walking, and skin sclerosis. She had been undergoing maintenance hemodialysis for 25 years following 2 years of peritoneal dialysis for chronic glomerulonephritis. She had been taking warfarin as an anticoagulant agent for 13 years after she underwent an aortic valve replacement. DIAGNOSIS: Warfarin-induced impairment of bone material quality. INTERVENTIONS AND OUTCOMES: Histomorphometric analysis of the bone biopsy specimens showed impairment in bone calcification processes, a high turnover of bone remodeling, low bone volume, and mild fibrosis. The bone abnormality could not be categorized into any type of representative bone disease classification such as osteitis fibrosa, osteomalacia, adynamic bone disease, uremic osteodystrophy, or hyperparathyroidism, but was consistent with warfarin-induced impairment of bone material quality. CONCLUSION: Warfarin can induce impairment of bone material quality in a patient undergoing hemodialysis.

Disuse-induced preferential loss of the giant protein titin depresses muscle performance via abnormal sarcomeric organization.

Persistent muscle weakness due to disuse-associated skeletal muscle atrophy limits the quality of life for patients with various diseases and individuals who are confined to bed. Fibers from disused muscle exhibit a marked reduction in active force production, which can exacerbate motor function, coupled with the well-known loss of muscle quantity. Despite recent understanding of the signaling pathways leading to the quantity loss, the molecular mechanisms of the depressed qualitative performance still remain elusive. Here we show that long-term disuse causes preferential loss of the giant sarcomere protein titin, associated with changes in physiologic muscle function. Ca(2+) sensitivity of active force decreased following 6 wk of hindlimb immobilization in the soleus muscle of the rat, accompanied by a shift in the length-active force relationship to the shorter length side. Our analyses revealed marked changes in the disused sarcomere, with shortening of thick and thin filaments responsible for altered length dependence and expansion of interfilament lattice spacing leading to a reduction in Ca(2+) sensitivity. These results provide a novel view that disuse-induced preferential titin loss results in altered muscle function via abnormal sarcomeric organization.

Prognostic significance of epidermal growth factor receptor phosphorylation and mutation in head and neck squamous cell carcinoma.

The molecular status of the epidermal growth factor receptor (EGFR) has not been as well studied in head and neck squamous cell carcinoma (HNSCC) as in lung cancer. We examined the frequencies of EGFR mutations as well as the expression/phosphorylation status of the EGFR protein in HNSCC patients. Moreover, we tried to elucidate associations between EGFR molecular status and patient characteristics and disease-free survival. In this prospective cohort study, clinical data and samples were obtained from 82 consecutive patients who had not been treated with EGFR molecular targeting therapy. Full-length EGFR was sequenced, and expression and phosphorylation of the EGFR protein were measured by Western blotting. Four novel mutations (E709K, V765G, Ins770G, and G1022S) and one mutation well-known in lung cancer (L858R) were identified in six HNSCC samples (7%), but we could not find any mutations in the extracellular domain of EGFR, such as EGFRvIII, in this study. E709K and Ins770G as well as L858R appear to be functional mutations based on the use of Ba/F3 cells. In terms of patient characteristics, the number of metastatic lymph nodes and node stage were associated with phosphorylation of EGFR. No patients with EGFR mutations relapsed during the study period. Excluding mutated cases, patients whose tumor samples showed phosphorylated EGFR relapsed significantly earlier than those without phosphorylated EGFR. This finding was still significant after adjusting for mutation and overexpression of EGFR protein using the Cox proportional hazard model. In conclusion, phosphorylated EGFR without mutations may be a marker of poor prognosis in patients with HNSCC.

Protein kinase A-dependent phosphorylation of ryanodine receptors increases Ca2+ leak in mouse heart.

In heart failure, chronic catecholaminergic stimulation increases diastolic Ca(2+) leak from ryanodine receptors (RyRs) of sarcoplasmic reticulum (SR), possibly due to the phosphorylation of RyRs through the activation of protein kinase A (PKA) or Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). In the present study, we attempted to identify which activated kinase is responsible for the enhanced Ca(2+) leak caused by beta-adrenergic stimulation. Trabeculae obtained from the hearts of adult male C57BL/6J mice were treated with isoproterenol and then permeabilized with saponin. To examine SR functions, Ca(2+) in SR was released with caffeine and measured with fluo-3. The Ca(2+) leak in isoproterenol-treated preparations was significantly increased when the PKA-dependent phosphorylation of RyR was increased without the involvement of CaMKII-dependent phosphorylation. Both the increase in Ca(2+) leak and the phosphorylation of RyR were blocked by a PKA inhibitor. Our results show that beta-adrenergic stimulation increases Ca(2+) leak from SR through PKA-dependent phosphorylation of RyR.

[Curve-fit with hybrid logistic function for intracellular calcium transient].

As the left ventricular (LV) pressure curve and myocardial tension curve in heart are composed of contraction and relaxation processes, we have found that hybrid logistic (HL) function calculated as the difference between two logistic functions curve-fits better the isovolumic LV pressure curve and the isometric twitch tension curve than the conventional polynomial exponential and sinusoidal functions. Increase and decrease in intracellular Ca2+ concentration regulate myocardial contraction and relaxation. Recently, we reported that intracellular Ca2+ transient (CaT) curves measured using the calcium-sensitive bioluminescent protein, aequorin, were better curve-fitted by HL function compared to the polynomial exponential function in the isolated rabbit RV and mouse LV papillary muscles. We speculate that the first logistic component curve of HL fit represents the concentration of the Ca2+ inflow into the cytoplasmic space, the concentration of Ca2+ released from sarcoplasmic reticulum (SR), the concentration of Ca2+ binding to troponin C (TnC), and the attached number of cross-bridge (CB) and their time courses, and that the second logistic component curve of HL fit represents the concentration of Ca2+ sequestered into SR, the concentration of Ca2+ removal from the cytoplasmic space, the concentration of Ca2+ released from TnC, and the detached number of CB and their time courses. This HL approach for CaT curve may provide a more useful model for investigating Ca2+ handling, Ca(2+) -TnC interaction, and CB cycling.

Spontaneous platelet aggregation in normal subject assessed by a laser light scattering method: an attempt at standardization.

Platelet aggregometry by the laser light scattering (LS) method is sufficiently sensitive to detect small platelet aggregates that form spontaneously in vitro in the absence of agonists. Platelet aggregation without agonists is named spontaneous platelet aggregation (SPA). Since SPA has been suggested to be associated with various thrombotic diseases, it is essential to measure SPA and to establish a standard range of SPA values. In this study, we measured SPA in 167 healthy subjects by the LS method and attempted to clarify various factors influencing SPA, including the blood collection procedure. We also attempted to establish a tentative standard range of SPA values. SPA was quantitatively measured in terms of the maximum total LS intensity, which reflects small aggregates formed over 10 minutes (SMAX) and the area under the total LS intensity curve of small aggregates (SAUC). Since both the values of SMAX and SAUC were skewed and the log SMAX and log AUC values showed a normal distribution, the statistical analyses were performed using log SMAX and log SAUC. The log SMAX and log SAUC were significantly higher in the samples collected using a tourniquet and/or a 21 G needle, than in those collected without a tourniquet and/or with an 18 G needle. The log SAUC values were significantly lower in samples obtained with a syringe and/or 3.8% sodium citrate than in those obtained in vacuum sampling tubes and/or 3.13% or 3.14% sodium citrate. The Ht and plasma glucose concentration influenced the log SMAX values. We propose that to standardize SPA measurements, the measurements should be completed within two hours of blood sample collection and collected using the regular concentration of citrate. The standard range of SMAX values measured in samples obtained using a tourniquet and a 21 G needle was 2.0-23.99 (*10(3) mV*count). The standard range of SAUC values measured under same conditions was 0.58-9.12 (*10(6) mV*count*min). The standard range of SMAX values measured in samples obtained using a tourniquet, 21 G needle and a vacuum tube was 1.7-29.51 (*10(3) mV*count). The standard range of SAUC values measured under same conditions was 0.59-9.33 (*10(6) mV*count*min).

[Novel assessment of intracellular calcium transient decay in cardiac muscle by curve-fitting with half-logistic function].

A decrease in intracellular calcium (Ca2+) concentration in the cardiac muscle is one of the important factors to induce myocardial relaxation. A mono-exponential (m-E) function has been used for assessing myocardial relaxation curve of isometric tension and intracellular calcium transient (CaT) decay, and the m-E time constants for the relaxation curve of isometric tension (F tau E) and CaT decay (Ca tau E) have been recognised as lusitropic indices. However, we found that a half-logistic (h-L) function fits the relaxation curve of isometric tension much more precisely than the conventional m-E function in the ferret right ventricular (RV) papillary muscle. Moreover, we demonstrated that the goodness of the h-L fits for CaT decays was superior to the goodness of the m-E fits in the rabbit RV and murine left ventricular papillary muscles. The changes in the h-L time constants for the relaxation curves of isometric tension (F tau L) and CaT decays (Ca tau L) with the different onsets were significantly smaller than the changes in F tau E and Ca tau E, respectively. The differences in the h-L non-zero asymptotes for the relaxation curves of isometric tension and CaT decays with the different onsets were smaller than the changes in the m-E non-zero asymptotes. The h-L function model characterises the amplitudes and time courses of the relaxation curve of isometric tension and CaT decay more precisely than the m-E function model, and thus F tau L and Ca tau L serve as more novel and reliable lusitropic indices. Simultaneous analysis of myocardial relaxation curve of isometric tension and CaT decay using h-L functions can become a useful method for assessment of myocardial calcium handling.