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The effects of sympathomimetic amines on Ca 2+ transients and isometric contractions were assessed in isolated rabbit papillary muscles in which multiple superficial cells had been microinjected with the calcium-sensitive bioluminescent protein aequorin. In the presence of /3-adrenoceptor blockade, the a-receptor agonist phenylephrine increased both the amplitude of the aequorin signals and the force of contraction in a concentration-dependent manner. However, the maximum increase i n the aequorin signals was less than 10% of that produced by the /3-receptor agonist isoproterenol, while the maximum increase in force of contraction produced by a-stimulation was about 50% of that elicited via /3-adrenoceptors. For a given increase in the force of contraction, stimulation of a-adrenoceptors produced much less change in the amplitude of the aequorin signals than did elevation of the extracellular Ca 2+ concentration; we interpret this to mean that the positive inotropic effect of a-adrenoceptor stimulation is in large part the result of an increase 1 in myofibrillar sensitivity to Ca 2+ . Stimulation of a-adrenoceptors produced little change or a slight decrease i n the duration of the aequorin signal and an increase in the duration of contraction, while stimulation of /3-adrenoceptors significantly decreased the time to peak and duration of both the aequorin signals and the contractions. For a given level of inotropic effect, high concentrations of isoproterenol often increased the aequorin signals more than did elevations of Ca 2+ , which is consistent with other evidence that the cyclic AMP-dependent phosphorylation of troponin I leads to a decrease in myofibrillar Ca 1+ sensitivity. However, concentrations of isoproterenol that did not produce evidence of this sort of desensitization also abbreviated the contractions much more than they did the aequorin signals. This suggests that the traditionally accepted mechanisms -a decrease in the Ca 2+ affinity of troponin C and an acceleration of Ca uptake by the sarcoplasmic reticulum -may not be sufficient to account for the actions of/3-receptor stimulation on the time course of contraction. In the absence of blocking agents, the naturally occurring catecholamines norepinephrine, epinephrine, and dopamine appear to influence the function of the rabbit papillary muscle through both a-and /3-adrenoceptors. Dopamine has a relatively greater effect on a-adrenoceptors than the other catecholamines. (Circulation Research

Thoracoscopic pulmonary segmentectomy under 3-dimensional multidetector computed tomography simulation is a safe technique.

Background-Diacylglycerol is a lipid second messenger that accumulates in cardiomyocytes when stimulated by Gq␣ protein-coupled receptor (GPCR) agonists such as angiotensin II, phenylephrine, and others. Diacylglycerol functions as a potent activator of protein kinase C (PKC) and is catalyzed by diacylglycerol kinase (DGK) to form phosphatidic acid and inactivated. However, the functional roles of DGK have not been previously examined in the heart. We hypothesized that DGK might prevent GPCR agonist-induced activation of diacylglycerol downstream signaling cascades and subsequent cardiac hypertrophy. Methods and Results-To test this hypothesis, we generated transgenic (DGK-TG) mice with cardiac-specific overexpression of DGK. There were no differences in heart size and heart weight between DGK-TG and wild-type littermate mice. The left ventricular function was normal in DGK-TG mice. Continuous administration of subpressor doses of angiotensin II and phenylephrine caused PKC translocation, gene induction of atrial natriuretic factor, and subsequent cardiac hypertrophy in WT mice. However, in DGK-TG mice, neither translocation of PKC nor upregulation of atrial natriuretic factor gene expression was observed after angiotensin II and phenylephrine infusion. Furthermore, in DGK-TG mice, angiotensin II and phenylephrine failed to increase cross-sectional cardiomyocyte areas and heart to body weight ratios. Phenylephrine-induced increases in myocardial diacylglycerol levels were completely blocked in DGK-TG mouse hearts, suggesting that DGK regulated PKC activity by controlling cellular diacylglycerol levels. Conclusions-These results demonstrated the first evidence that DGK negatively regulated the hypertrophic signaling cascade and resultant cardiac hypertrophy in response to GPCR agonists without detectable adverse effects in in vivo hearts. (Circulation. 2006;113:60-66.)

he heart is able to adjust its pump function to immediately meet the demand of vital organs in the body by increasing cardiac output several fold by means of increases in cardiac contractility and heart rate. Cardiac myocytes constitute a pseudo-syncytium, follow the all-ornone law, and all of them contribute to the maintenance of stroke volume. Therefore, under physiological conditions, alteration of the contractility of individual cardiac myocytes is directly reflected in cardiac pump function.Two of the most important intrinsic characteristics of the contractile regulation of cardiac myocytes are the FrankStarling mechanism and the positive force -frequency relationship. The Frank-Starling mechanism represents the basic cardiac contractility as a length -tension relationship ex vivo and left ventricular (LV) function curve in vivo, in which stretching of myocytes to an optimal length immediately increases the contractile force with little alteration of [Ca 2+ ]i, preceding to a stretch-induced slowly developing increase in contractile force associated with an increase in [Ca 2+ ]i mobilization. The positive force -frequency relationship induced by elevated heart rate increases ventricular contractility by a marked facilitation of [Ca 2+ ]i mobilization, resulting in an increase in stroke volume. 1 In addition to the intrinsic regulatory mechanisms characteristic of cardiac myocytes, numerous external regulatory mechanisms, including receptor activation induced by Circulation Journal Vol.72, December 2008neurotransmitters released by autonomic nerve stimulation, hormones, autacoids and cytokines, contribute to the adaptation of the cardiac pump. 2 In the operation of all of these multiple mechanisms, cardiac Ca 2+ signaling plays a key role in contractile regulation.In 1978, Allen and Blinks developed an experimental procedure for detecting [Ca 2+ ]i simultaneously with contractile activity by applying the Ca 2+ -sensitive bioluminescent protein aequorin to intact cardiac muscle. 3 Since then, it has been directly demonstrated by the use of aequorin and other fluorescent dyes such as fura-2, indo-1, and fluo-3 that Ca 2+ ions play a central role in the regulation of cardiac excitation -contraction (E-C) coupling in the intact myocardium or single cardiac myocytes.Although cardiac contractile regulation is essentially achieved by dynamic modulation of [Ca 2+ ]i mobilization (upstream mechanism), it has been proved that the process subsequent to elevation of [Ca 2+ ]i (central and downstream mechanism) is also an important target of cardiotonic agents and the contractile modulation induced by physiological and pathological interventions on the heart. In the early 1980s there was a strong movement toward the development of cardiotonic agents that act by novel mechanisms to replace the conventional cardiotonic agents, such as digitalis and catecholamines, that were used to treat the cardiac contractile dysfunction in congestive heart failure (CHF). 1,2 Those agents elicit a positive inotropic effect (PIE) by i...

No abstract

Endothelin-1 elicited a positive inotropic effect (PIE) on isolated rabbit, guinea pig, and rat but not on dog ventricular myocardium. Specific high-affinity binding of 125I-labeled endothelin-1 was detected in the ventricular membrane fraction of these species. Maximal binding capacity was the highest in the rabbit, lowest in the dog, and in between in the guinea pig and rat; this rank order corresponds roughly to the effectiveness of endothelin-1 in producing a PIE. There was no difference in the potency or efficacy for the PIE of the endothelin isoforms endothelin-1, -2, and -3 in the rabbit papillary muscle. A tumor-promoting phorbol ester, phorbol 12,13-dibutyrate, inhibited selectively the PIE and the accumulation of [3H]inositol monophosphate induced by endothelin-1 as well as those of myocardial alpha 1-adrenoceptor stimulation in a concentration that did not (10(-8) M) or only slightly (10(-7) M) reduced the PIE of BAY K 8644. Phorbol 12,13-dibutyrate did not affect the specific binding of 125I-labeled endothelin-1 in the ventricular membrane fraction of the rabbit. The present findings indicate that the characteristics of the endothelin-induced PIE in mammalian ventricular myocardium are similar to those of myocardial alpha 1-adrenoceptor activation that may involve phosphoinositide hydrolysis. The receptor density and the PIE of endothelin on mammalian cardiac muscle show a wide range of variation among species.

Abstract-In certain cardiovascular disorders, such as congestive heart failure and ischemic heart disease, several endogenous regulators, including norepinephrine (NE) and endothelin-1 (ET-1), are released from various types of cell. Because plasma levels of these regulators are elevated, it seems likely that cardiac contraction might be regulated by crosstalk among these endogenous regulators. We studied the regulation of cardiac contractile function by crosstalk between ET-1 and NE and its relationship to Ca 2ϩ signaling in canine ventricular myocardium. ET-1 alone did not affect the contractile function. However, in the presence of NE at subthreshold concentrations (0.1 to 1 nmol/L), ET-1 had a positive inotropic effect (PIE). In the presence of NE at higher concentrations (100 to 1000 nmol/L), ET-1 had a negative inotropic effect. ET-1 had a biphasic inotropic effect in the presence of NE at an intermediate concentration (10 nmol/L). The PIE of ET-1 was associated with an increase in myofilament sensitivity to Ca 2ϩ ions and a small increase in Ca 2ϩ transients, which required the simultaneous activation of protein kinase A (PKA) and PKC. ET-1 elicited translocation of PKC⑀ from cytosolic to membranous fraction, which was inhibited by the PKC inhibitor GF 109203X. Whereas the Na ϩ -H ϩ exchange inhibitor Hoe 642 suppressed partially the PIE of ET-1, detectable alteration of pH i did not occur during application of ET-1 and NE. The negative inotropic effect of ET-1 was associated with a pronounced decrease in Ca 2ϩ transients, which was mediated by pertussis toxin-sensitive G proteins, activation of protein kinase G, and phosphatases. When the inhibitory pathway was suppressed, ET-1 had a PIE even in the absence of NE. Our results indicate that the myocardial contractility is regulated either positively or negatively by crosstalk between ET-1 and NE through different signaling pathways whose activation depends on the concentration of NE in the dog. Key Words: endothelin-1 Ⅲ norepinephrine Ⅲ myocardial contractility Ⅲ Ca 2ϩ transients Ⅲ protein kinase C D uring the course of cardiovascular disorders, such as congestive heart failure and ischemic heart disease, plasma levels of both endothelin-1 (ET-1) and norepinephrine (NE) tend to increase. [1][2][3][4] The signal transduction processes that are triggered by the activation of receptors for these endogenous agonists are different, and, thus, it seems likely that crosstalk between ET-1 and NE might play a critical role in the regulation of cardiac function, determining hemodynamic responses to antagonists of ␤-adrenoceptors or endothelin receptors under various pathophysiological conditions. The available evidence implies that these endogenous regulators are engaged in crosstalk at different levels of their respective signaling pathways. For example, the positive feedback mechanism seems to exist at the level of the synthesis of NE by which ET-1 increases the plasma concentration of NE, 5 whereas NE facilitates the expression of mRNA that encodes the prepro-ET...