Role of sarcoplasmic reticulum in myocardial contraction of neonatal and adult mice

Age-dependent changes in electrophysiology and calcium handling: implications for pediatric cardiac research

Rodent models are frequently employed in cardiovascular research, yet our understanding of pediatric cardiac physiology has largely been deduced from more simplified two-dimensional cell studies. Previous studies have shown that postnatal development includes an alteration in the expression of genes and proteins involved in cell coupling, ion channels, and intracellular calcium handling. Accordingly, we hypothesized that postnatal cell maturation is likely to lead to dynamic alterations in whole heart electrophysiology and calcium handling. To test this hypothesis, we employed multiparametric imaging and electrophysiological techniques to quantify developmental changes from neonate to adult. In vivo electrocardiograms were collected to assess changes in heart rate, variability, and atrioventricular conduction (Sprague-Dawley rats). Intact, whole hearts were transferred to a Langendorff-perfusion system for multiparametric imaging (voltage, calcium). Optical mapping was performed in conjunction with an electrophysiology study to assess cardiac dynamics throughout development. Postnatal age was associated with an increase in the heart rate (181 ± 34 vs. 429 ± 13 beats/min), faster atrioventricular conduction (94 ± 13 vs. 46 ± 3 ms), shortened action potentials (APD₈₀: 113 ± 18 vs. 60 ± 17 ms), and decreased ventricular refractoriness (VERP: 157 ± 45 vs. 57 ± 14 ms; neonatal vs. adults, means ± SD, P < 0.05). Calcium handling matured with development, resulting in shortened calcium transient durations (168 ± 18 vs. 117 ± 14 ms) and decreased propensity for calcium transient alternans (160 ± 18- vs. 99 ± 11-ms cycle length threshold; neonatal vs. adults, mean ± SD, P < 0.05). Results of this study can serve as a comprehensive baseline for future studies focused on pediatric disease modeling and/or preclinical testing.NEW & NOTEWORTHY This is the first study to assess cardiac electrophysiology and calcium handling throughout postnatal development, using both in vivo and whole heart models.

Interleukin-1β reduces L-type Ca2+ current through protein kinase Cϵ activation in mouse heart

Inflammation is now widely recognized as a key component of heart disease. Patients suffering from arrhythmias and heart failure have increased levels of tumor necrosis factor-α (TNFα) and interleukin-1β (IL-1β). Evidence suggests that these cytokines are important mediators of cardiac remodeling; however, their effects on ion channels and arrhythmogenesis remain incompletely understood. The L-type Ca(2+) current (ICaL) is a major determinant of the plateau phase of cardiac action potential and has a critical excitation-contraction coupling role. Thus, altering its properties could have detrimental effects on cardiac electrical and contractile functions. Accordingly, the objective of this study was to elucidate the effect of TNFα and IL-1β on ICaL, while exploring the underlying regulatory mechanisms. Neonatal mouse ventricular myocytes were treated with a pathophysiological concentration (30 pg/ml) of TNFα and IL-1β for 24 h. Voltage-clamp recordings showed that TNFα had no effect on ICaL, whereas IL-1β decreased the current density by 36%. Although both IL-1β- and TNFα-treated myocytes showed significant increase in reactive oxidative species (ROS), Western blot experiments revealed that only IL-1β increased PKCϵ membrane translocation. The antioxidant N-acetyl-L-cysteine normalized ROS levels and restored ICaL density. Furthermore, the PKCϵ translocation inhibitor ϵ-V1-2 blocked the effect of IL-1β on ICaL. The reduction of ICaL by IL-1β was also seen in cultured adult ventricular myocytes. Overall, chronic IL-1β treatment decreased ICaL density in cardiomyocytes. These effects implicated ROS signaling and PKCϵ activation. These findings could contribute to explain the role of IL-1β in the development of arrhythmia and heart failure.

Age-dependent changes in contractile function and passive elastic properties of myocardium from mice lacking muscle LIM protein (MLP)

AIMS

Muscle LIM protein (MLP) null mice are often used as a model for human dilated cardiomyopathy. So far, little is known about the time course and pathomechanisms leading to the development of the adult phenotype.

METHODS AND RESULTS

We systematically analysed the contractile phenotype, myofilament calcium (Ca(2)(+)) responsiveness, passive myocardial mechanics, histology, and mRNA expression in mice aged 4 and 12 weeks. In 4-week-old animals, there was no significant difference in the force-frequency relationship (FFR) and catecholamine response of intact isolated papillary muscles between wild-type (WT) and MLP null myocardium. In 12-week-old animals, WT myocardium exhibited a significantly positive FFR, while that of MLP null mice was significantly negative, and the inotropic response to catecholamines was significantly reduced in MLP null mice. This time course of decline in contractile function was confirmed in vivo by echocardiography. Whereas at 4 weeks of age MLP null mice and WT littermates showed similar levels of SERCA2a (sarcoplasmic reticulum Ca(2+) ATPase) expression, the expression was significantly lower in 12-week-old MLP null mice compared with littermate controls. Myofilament Ca(2)(+) responsiveness was not affected by the lack of MLP, irrespective of age. Whereas in 4-week-old animals MLP null myocardium showed a trend to an increased compliance compared with the WT, myocardium of 12-week-old MLP null mice was significantly less compliant than WT myocardium. Parallel to the decrease in compliance there was an increase in fibrosis in the MLP null animals.

CONCLUSION

Our data suggest that MLP deficiency does not primarily influence myocardial contractility. A lack of MLP leads to an age-dependent impairment of excitation-contraction coupling with resulting contractile dysfunction and secondary fibrosis.

Phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) potentiates cardiac contractility via activation of the ryanodine receptor

Phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) is the most recently identified phosphoinositide, and its functions have yet to be fully elucidated. Recently, members of our muscle group have shown that PI(3,5)P2 plays an important role in skeletal muscle function by altering Ca(2+) homeostasis. Therefore, we hypothesized that PI(3,5)P2 may also modulate cardiac muscle contractility by altering intracellular Ca(2+) ([Ca(2+)](i)) in cardiac myocytes. We first confirmed that PI(3,5)P2 was present and increased by insulin treatment of cardiomyocytes via immunohistochemistry. To examine the acute effects of PI(3,5)P2 treatment, electrically paced left ventricular muscle strips were incubated with PI(3,5)P2. Treatment with PI(3,5)P2 increased the magnitude of isometric force, the rate of force development, and the area associated with the contractile waveforms. These enhanced contractile responses were also observed in MIP/Mtmr14(-/-) mouse hearts, which we found to have elevated levels of PI(3,5)P2. In cardiac myocytes loaded with fura-2, PI(3,5)P2 produced a robust elevation in [Ca(2+)](i). The PI(3,5)P2-induced elevation of [Ca(2+)](i) was not present in conditions free of extracellular Ca(2+) and was completely blocked by ryanodine. We investigated whether the phosphoinositide acted directly with the Ca(2+) release channels of the sarcoplasmic reticulum (ryanodine receptors; RyR2). PI(3,5)P2 increased [(3)H]ryanodine binding and increased the open probability (P(o)) of single RyR2 channels reconstituted in lipid bilayers. This strongly suggests that the phosphoinositide binds directly to the RyR2 channel. Thus, we provide inaugural evidence that PI(3,5)P2 is a powerful activator of sarcoplasmic reticulum Ca(2+) release and thereby modulates cardiac contractility.

Postnatal development of mouse heart: formation of energetic microdomains

Cardiomyocyte contractile function requires tight control of the ATP/ADP ratio in the vicinity of the myosin-ATPase and sarcoplasmic reticulum ATPase (SERCA). In these cells, the main systems that provide energy are creatine kinase (CK), which catalyses phosphotransfer from phosphocreatine to ADP, and direct adenine nucleotide channelling (DANC) from mitochondria to ATPases. However, it is not known how and when these complex energetic systems are established during postnatal development. We therefore studied the maturation of the efficacy with which DANC and CK maintain ATP/ADP-dependent SR and myofibrillar function (SR Ca(2+) pumping and prevention of rigor tension), as well as the maturation of mitochondrial oxidative capacity. Experiments were performed on saponin-skinned fibres from left ventricles of 3-, 7-, 21-, 42- and 63-day-old mice. Cardiomyocyte and mitochondrial network morphology were characterized using electron microscopy. Our results show an early building-up of energetic microdomains in the developing mouse heart. CK efficacy for myosin-ATPase regulation was already maximal 3 days after birth, while for SERCA regulation it progressively increased until 21 days after birth. Seven days after birth, DANC for these two ATPases was as effective as in adult mice, despite a non-maximal mitochondrial respiration capacity. However, 3 days after birth, DANC between mitochondria and myosin-ATPase was not yet fully efficient. To prevent rigor tension in the presence of working mitochondria, the myosin-ATPase needed more intracellular MgATP in 3-day-old mice than in 7-day-old mice (pMgATP(50) 4.03 +/- 0.02 and 4.36 +/- 0.07, respectively, P < 0.05), whereas the intrinsic sensitivity of myofibrils to ATP (when mitochondria were inhibited) was similar at both ages. This may be due to the significant remodelling of the cytoarchitecture that occurs between these ages (cytosolic space reduction, formation of the mitochondrial network around the myofibrils). These results reveal a link between the maturation of intracellular energy pathways and cell architecture.

Overexpression of diacylglycerol kinase zeta inhibits endothelin-1-induced decreases in Ca2+ transients and cell shortening in mouse ventricular myocytes

Endothelin-1 (ET-1) is released in various cardiovascular disorders including congestive heart failure, and may modulate significantly the disease process by its potent action on vascular and cardiac muscle cell function and gene regulation. In adult mouse ventricular cardiomyocytes loaded with indo-1, ET-1 induced a sustained negative inotropic effect (NIE) in association with decreases in Ca(2+) transients. The ET-1-induced effects on Ca(2+) transients and cell shortening were abolished in diacylglycerol (DAG) kinase zeta-overexpressing mouse ventricular myocytes. A nonselective protein kinase C (PKC) inhibitor, GF109203X, inhibited the ET-1-induced decreases in Ca(2+) transients and cell shortening in concentration-dependent manners, whereas a selective Ca(2+)-dependent PKC inhibitor, Gö6976, did not affect the ET-1-induced effects. A phospholipase Cbeta inhibitor, U73122, and an inhibitor of phospholipase D, C(2)-ceramide, partially, but significantly, attenuated the ET-1-induced effects. Derivatives of the respective inhibitors with no specific effects, U73343 and dihydro-C(2)-ceramide, did not affect the ET-1-induced effects. Taken together, these results indicate that activation of a Ca(2+)-independent PKC isozyme by 1,2-DAG, which is generated by phospholipase Cbeta and phospholipase D activation and inactivated by phosphorylation via DAG kinase, is responsible for the ET-1-induced decreases in Ca(2+) transients and cell shortening in mouse ventricular cardiomyocytes.

Intracellular mechanisms and receptor types for endothelin-1-induced positive and negative inotropy in mouse ventricular myocardium

We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

Three-dimensional distribution of cardiac Na+-Ca2+ exchanger and ryanodine receptor during development

Mechanisms of cardiac excitation-contraction coupling in neonates are still not clearly defined. Previous work in neonates shows reverse-mode Na(+)-Ca(2+) exchange to be the primary route of Ca(2+) entry during systole and the neonatal sarcoplasmic reticulum to have similar capability as that of adult in storing and releasing Ca(2+). We investigated Na(+)-Ca(2+) exchanger (NCX) and ryanodine receptor (RyR) distribution in developing ventricular myocytes using immunofluorescence, confocal microscopy, and digital image analysis. In neonates, both NCX and RyR clusters on the surface of the cell displayed a short longitudinal periodicity of approximately 0.7 microm. However, by adulthood, both proteins were also found in the interior. In the adult, clusters of NCX on the surface of the cell retained the approximately 0.7-microm periodicity whereas clusters of RyR adopted a longer longitudinal periodicity of approximately 2.0 microm. This suggests that neonatal myocytes also have a peri-M-line RyR distribution that is absent in adult myocytes. NCX and RyR colocalized voxel density was maximal in neonates and declined significantly with ontogeny. We conclude in newborns, Ca(2+) influx via NCX could potentially activate the dense network of peripheral Ca(2+) stores via peripheral couplings, evoking Ca(2+)-induced Ca(2+) release.

Mechanisms of endothelin-1-induced decrease in contractility in adult mouse ventricular myocytes

BACKGROUND AND PURPOSE

The potent vasoconstrictor polypeptide endothelin-1 (ET-1) plays an important pathophysiological role in progression of cardiovascular diseases and elicits prominent effects on myocardial contractility. Although ET-1 produces a positive inotropy in cardiac muscle of most mammalian species, it induces a sustained negative inotropy in mice. This study was performed to gain an insight into the cellular mechanisms underlying the negative inotropy in adult mouse ventricular myocytes.

EXPERIMENTAL APPROACH

Cell shortening and Ca(2+) transients were simultaneously recorded from isolated mouse ventricular myocytes loaded with the Ca(2+)-sensitive fluorescent dye indo-1.

KEY RESULTS

ET-1 decreased cell shortening in a concentration-dependent manner (pD(2) value of 10.1). The ET-1-induced decrease in cell shortening was associated with a decrease in Ca(2+) transients. In addition, the Ca(2+) transient/cell-shortening relationship was shifted to the right by ET-1, indicating decreased myofilament Ca(2+) sensitivity. The instantaneous relationship of the rising phase of the Ca(2+) transient and cell shortening was shifted to the right by ET-1. Decreased Ca(2+) transients and cell shortening induced by ET-1 were markedly attenuated by the specific Na(+)/Ca(2+) exchange inhibitor SEA0400.

CONCLUSIONS AND IMPLICATIONS

ET-1-induced negative inotropy in mouse ventricular myocytes was mediated by decreased Ca(2+) transients and myofilament Ca(2+) sensitivity. These data are entirely consistent with the involvement of increased Ca(2+) extrusion via the Na(+)/Ca(2+) exchanger in the ET-1-mediated decrease in Ca(2+) transients. Decreased Ca(2+) sensitivity may be due to retardation of cell shortening in response to a rise in Ca(2+) transients.

Endocardial endothelium-dependent positive inotropy by Ca2+ pump inhibitors: possible involvement of store-operated Ca2+ entry

Positive inotropy by sarcoplasmic/endoplasmic reticulum Ca(2+) pump inhibitors was found and its mechanisms were analyzed pharmacologically. Thapsigargin and cyclopiazonic acid produced positive inotropy in isolated mouse left atria. The responses were inhibited by pretreatment of the endocardial surface with Triton X-100 or by indomethacin, which suggests that the inotropic responses were mediated by prostaglandin(s) released from the endocardial endothelium as well as acetylcholine-induced positive inotropy. The thapsigargin- and acetylcholine-induced positive inotropy was significantly inhibited by Gd(3+), La(3+) and lavendustin A, a tyrosine kinase inhibitor, but not by Ni(2+) and LOE908, a non-selective cation channel inhibitor. Gd(3+) and lavendustin A had no effect on the exogenously applied PGF(2)alpha-induced positive inotropy. In addition, acetylcholine did not induce any positive inotropy when applied after the application of thapsigargin. These results strongly suggest that thapsigargin- as well as acetylcholine-induced prostaglandin release from endocardial endothelium is mediated by store-operated Ca(2+) entry through Gd(3+)-sensitive channels and activation of tyrosine kinase.

Unique excitation–contraction characteristics of mouse myocardium as revealed by SEA0400, a specific inhibitor of Na+–Ca2+ exchanger

The functional role of the sodium-calcium exchanger in mouse ventricular myocardium was evaluated with a newly developed specific inhibitor, SEA0400. Contractile force and action potential configuration were measured in isolated ventricular tissue preparations, and cell shortening and Ca2+ transients were measured in indo-1-loaded isolated ventricular cardiomyocytes. SEA0400 increased the contractile force, cell shortening and Ca2+ transient amplitude, and shortened the late plateau phase of the action potential. alpha-adrenergic stimulation by phenylephrine produced a sustained decrease in contractile force, cell shortening and Ca2+ transient amplitude, which were all inhibited by SEA0400. Increasing the contraction frequency resulted in a decrease in contractile force in the absence of drugs (negative staircase phenomenon). This frequency-dependent decrease was attenuated by SEA0400 and enhanced by phenylephrine. Phenylephrine increased the Ca2+ sensitivity of contractile proteins in isolated ventricular cardiomyocytes, while SEA0400 had no effect. These results provide the first pharmacological evidence in the mouse ventricular myocardium that inward current generated by Ca2+ extrusion through the sodium-calcium exchanger during the Ca2+ transient contributes to the action potential late plateau, that alpha-adrenoceptor-mediated negative inotropy is produced by enhanced Ca2+ extrusion through the sodium-calcium exchanger, and that the negative staircase phenomenon can be explained by increased Ca2+ extrusion through the sodium-calcium exchanger at higher contraction frequencies.

Female mice lacking estrogen receptor beta display prolonged ventricular repolarization and reduced ventricular automaticity after myocardial infarction

BACKGROUND

Major gender-based differences in the incidence of ventricular tachyarrhythmia after myocardial infarction have been shown in humans. Although the underlying mechanisms are unclear, earlier studies suggest that estrogen receptor-mediated effects play a major role in this process.

METHODS AND RESULTS

We examined the effect of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) on the electrophysiological phenotype in female mice with and without chronic anterior myocardial infarction. There was no significant difference in overall mortality, infarct size, and parameters of left ventricular remodeling when we compared infarcted ERalpha-deficient and ERbeta-deficient mice with infarcted wild-type animals. In the 12-hour telemetric ECG recording 6 weeks after myocardial infarction, surface ECG parameters did not show significant differences in comparisons of ERalpha-deficient mice versus wild-type controls, infarcted versus noninfarcted ERalpha-deficient mice, and infarcted ERalpha-deficient versus infarcted wild-type mice. However, infarcted ERbeta-deficient versus noninfarcted ERbeta-deficient mice showed a significant prolongation of the QT (61+/-6 versus 48+/-8 ms; P<0.05) and QTc intervals (61+/-7 versus 51+/-9 ms; P<0.05) and the JT (42+/-6 versus 31+/-4 ms; P<0.05) and JTc intervals (42+/-7 versus 33+/-4 ms; P<0.05). Furthermore, infarcted ERbeta-deficient versus infarcted wild-type mice showed a significant prolongation of the QT (61+/-6 versus 53+/-8 ms; P<0.05) and QTc intervals (61+/-7 versus 53+/-7 ms; P<0.05) and the JT (42+/-6 versus 31+/-5 ms; P<0.05) and JTc intervals (42+/-7 versus 31+/-5 ms; P<0.05), accompanied by a significant decrease of ventricular premature beats (7+/-21/h versus 71+/-110/h; P<0.05). Finally, real-time polymerase chain reaction-based quantitative analysis of mRNA levels showed a significantly lower expression of Kv4.3 (coding for I(to)) in ERbeta-deficient mice (P<0.05).

CONCLUSIONS

Estrogen receptor beta deficiency results in prolonged ventricular repolarization and decreased ventricular automaticity in female mice with chronic myocardial infarction.

Calcium transients in infant human atrial myocytes

Isolated infant human atrial cells have a slower early repolarization than adult human atrial cells. In addition, from room temperature voltage-clamp studies, infant cells have lower basal L-type calcium currents than adult cells. We hypothesized that the slower repolarization increases the calcium transient of infant human atrial cells. Atrial myocytes were enzymatically dissociated from biopsies of human right atrial appendages of infant (3-8 mo) patients who were undergoing open-heart surgery. Intracellular calcium transients were measured with fluorescence microscopy with application of either square waves or action potential waveforms at physiologic temperature. After repetitive application (1 Hz) of 100-ms duration conditioning depolarizations to 10 mV (from -80 mV), a test pulse of varying duration (DeltaT; 2-100 ms) produced smaller transients (expressed as percentage of the last conditioning pulse) at shorter durations (33 +/- 7% for DeltaT = 2 ms, 80 +/- 4% for DeltaT = 25 ms). With repetitive application of either adult or infant prerecorded action potentials to infant cells, the cells had a decreased calcium transient with the adult action potential (F/F(0) 2.2 +/- 0.4 for infant action potential versus 1.6 +/- 0.2 for adult action potential; n = 7; p < 0.05). The delayed early repolarization of infant cells alters the Ca(2+) transient, which may compensate for the lower availability of basal calcium current in infant cells. The steep relationship that we have demonstrated between test-pulse duration and the calcium transient suggests that modulation of the early repolarization phase of the action potential may be of great significance in modulating excitation-contraction coupling.

Cardiac autonomic modulation by estrogen in female mice undergoing ambulatory monitoring and in vivo electrophysiologic testing

INTRODUCTION

Estrogen is an important modulator of cardiovascular risk, but its mechanism of action is not fully understood. We investigated the effect of ovariectomy and its timing on the cardiac electrophysiology in mice.

METHODS

Thirty female mice (age 18.8 +/- 3.1 weeks) underwent in vivo electrophysiologic testing before and after autonomic blockade. Fifteen mice were ovariectomized prepuberty (PRE) and ten postpuberty (POST), 2 weeks prior to electrophysiologic testing. Five age-matched sham-operated female mice (Control) served as controls. A subset of 13 mice (5 PRE, 3 POST, and 5 Controls) underwent 24-hour ambulatory monitoring.

RESULTS

With ambulatory monitoring, the average (668 +/- 28 vs 769 +/- 52 b/min, P = 0.008) and minimum (485 +/- 47 vs 587 +/- 53 b/min, P = 0.02) heart rates were significantly slower in the ovariectomized mice (PRE and POST groups) compared to the Control group. At baseline electrophysiologic testing, there were no significant differences among the ovariectomized and intact mice in any of the measured parameters. With autonomic blockade, the Control group had a significantly larger change (delta) in the atrioventricular (AV) nodal Wenckebach (AVW) periodicity (deltaAVW = 11.3 +/- 2.9 vs 2.1 +/- 7.3 ms, P = 0.05) and functional refractory period (deltaFRP = 11.3 +/- 2.1 vs 1.25 +/- 6.8 ms, P = 0.02) compared to the ovariectomized mice. These results were not altered by the time of ovariectomy (PRE vs POST groups).

CONCLUSION

Our results suggest that estrogen modulates the autonomic inputs into the murine sinus and AV nodes. These findings, if replicated in humans, might underlie the observed clustering of certain arrhythmias around menstruation and explain the higher incidence of arrhythmias in men and postmenopausal women.

Pharmacological evidence for involvement of phospholipase D, protein kinase C, and sodium-calcium exchanger in alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle

The intracellular signalling pathway for alpha-adrenoceptor-mediated negative inotropy was studied pharmacologically in isolated adult mouse ventricle. The negative inotropy was inhibited by GF-109203X, a nonselective protein kinase C inhibitor. Phorbol 12-myristate 13-acetate also produced sustained negative inotropy, which was inhibited by KB-R7943, a Na(+)/Ca(2+) exchanger inhibitor. The alpha-adrenoceptor-mediated negative inotropy was augmented by RHC-80267, a diacylglycerol lipase inhibitor, but was inhibited either by C(2)-ceramide, a phospholipase D inhibitor, and high concentration of propranolol (50 micro M), which inhibits phosphatidate phosphohydrolase. The inotropy was not affected by U-73122, a phospholipase C inhibitor. Lavendustin-A, a tyrosine kinase inhibitor, also inhibited the negative inotropy. These findings suggest that alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle is mediated by activation of tyrosine kinase, the phospholipase D-phosphatidate phosphohydrolase pathway, and protein kinase C.

alpha-Adrenoceptor stimulation-mediated negative inotropism and enhanced Na(+)/Ca(2+) exchange in mouse ventricle

Mechanisms underlying the negative inotropic response to alpha-adrenoceptor stimulation in adult mouse ventricular myocardium were studied. In isolated ventricular tissue, phenylephrine (PE), in the presence of propranolol, decreased contractile force by approximately 40% of basal value. The negative inotropic response was similarly observed under low extracellular Ca(2+) concentration ([Ca(2+)](o)) conditions but was significantly smaller under high-[Ca(2+)](o) conditions and was not observed under low-[Na(+)](o) conditions. The negative inotropic response was not affected by nicardipine, ryanodine, ouabain, or dimethylamiloride (DMA), inhibitors of L-type Ca(2+) channel, Ca(2+) release channel, Na(+)-K(+) pump, or Na(+)/H(+) exchanger, respectively. KB-R7943, an inhibitor of Na(+)/Ca(2+) exchanger, suppressed the negative inotropic response mediated by PE. PE reduced the magnitude of postrest contractions. PE caused a decrease in duration of the late plateau phase of action potential and a slight increase in resting membrane potential; time courses of these effects were similar to that of the negative inotropic effect. In whole cell voltage-clamped myocytes, PE increased the L-type Ca(2+) and Na(+)/Ca(2+) exchanger currents but had no effect on the inwardly rectifying K(+), transient outward K(+), or Na(+)-K(+)-pump currents. These results suggest that the sustained negative inotropic response to alpha-adrenoceptor stimulation of adult mouse ventricular myocardium is mediated by enhancement of Ca(2+) efflux through the Na(+)/Ca(2+) exchanger.

Positive and negative inotropic effects of muscarinic receptor stimulation in mouse left atria

In isolated mouse left atria, acetylcholine (ACh) produced a biphasic inotropic response; a transient decrease in developed tension was followed by an increase. Both negative and positive responses were concentration dependent and were inhibited by atropine. The negative and positive inotropic responses were also observed with a nonselective muscarinic stimulant, oxotremorine-M, but not with an M1-receptor selective stimulant, McN-A343. Pirenzepine, an M1-receptor antagonist, inhibited both negative and positive inotropic responses at high concentrations. Gallamine, an M2-receptor antagonist, inhibited the negative response. Hexahydro-siladifenidol hydrochloride, p-fluoro analog (p-F-HHSiD), an M3-receptor antagonist, inhibited the positive response with no effect on the negative phase. In pertussis toxin (PTX) treated preparations, negative inotropic response to ACh was not observed. These results suggest that the negative and positive inotropic responses to acetylcholine in mouse atria are mediated by M2 and M3 receptors, respectively. The negative phase, but not the positive phase, was mediated by a PTX-sensitive G protein.

Developmental conversion of inotropism by endothelin I and angiotensin II from positive to negative in mice

Inotropic effects on isolated neonatal and adult mouse myocardium of endothelin I and angiotensin II were examined. Endothelin I produced a sustained positive inotropic response in the neonate but a sustained negative response in the adult. Both were concentration-dependent and were inhibited by the endothelin ETA receptor antagonist, BQ-123 (Cyclo(D-a-aspartyl-L-prolyl-D-valyl-L-leucyl-D-tryptophyl)). Angiotensin II produced a sustained positive inotropic response in the neonate while a sustained negative response in the adult. Both were concentration-dependent and were inhibited by the angiotensin AT1 receptor antagonist, YM358 (2,7-diethyl-5-((2'-(1 H-tetrazol-5-yl)biphenyl-4-yl)methyl-5H-pyrazolo(1,5-b)(1,2,4)tria zole potassium salt monohydrate). These results indicate that inotropic responses of the mouse heart to cardioactive peptides are unique among experimental animal species and may be reversed during development.