Postnatal cardiac myocytes respond to stress signals by hypertrophic growth and activation of a fetal gene program. Recently, we showed that class II histone deacetylases (HDACs) suppress cardiac hypertrophy, and mice lacking the class II HDAC, HDAC9, are sensitized to hypertrophic signals. To further define the roles of HDACs in cardiac hypertrophy, we analyzed the effects of HDAC inhibitors on the responsiveness of primary cardiomyocytes to hypertrophic agonists. Paradoxically, HDAC inhibitors imposed a dose-dependent blockade to hypertrophy and fetal gene activation. We conclude that distinct HDACs play positive or negative roles in the control of cardiomyocyte hypertrophy. HDAC inhibitors are currently being tested in clinical trials as anti-cancer agents. Our results suggest that these inhibitors may also hold promising clinical value as therapeutics for cardiac hypertrophy and heart failure.Postnatal cardiac myocytes undergo hypertrophic growth in response to a variety of stress signals (reviewed in Ref. 1). The hypertrophic response is characterized by increases in myocyte size and protein synthesis, assembly and organization of sarcomeres, and activation of a fetal gene program. Although traditionally considered an adaptive response to pathological signaling, chronic expression of fetal cardiac genes in the heart can result in maladaptive changes in cardiac contractility and calcium handling that culminate in dilated cardiomyopathy, heart failure, and sudden death from arrhythmias (2). Moreover, increasing evidence in rodent models indicates that cardiac function is preserved when hypertrophy is inhibited in the face of stress signaling, pointing to the potential importance of therapeutic strategies for modulating the hypertrophic process (3-9).Recent studies have revealed key roles for chromatin-modifying enzymes in the control of cardiac hypertrophy (10 -12). The structure of chromatin is governed by the acetylation state of nucleosomal histones (13,14). Acetylation of histone tails by histone acetyltransferases (HATs) 1 results in relaxation of nucleosomal structure and transcriptional activation. Acetylated histones also serve as targets for binding of bromo-domain proteins that possess HAT activity and act as transcriptional activators. The actions of HATs are opposed by histone deacetylases (HDACs), which deacetylate nucleosomal histones, thereby promoting chromatin condensation and transcriptional repression.Mammalian HDACs can be divided into three classes based on their similarity with three yeast HDACs (reviewed in Refs. 15 and 16). Class I HDACs (HDACs 1, 2, 3, and 8) are expressed ubiquitously and consist mainly of a deacetylase domain. Members of class II (HDACs 4, 5, 7, and 9) are highly expressed in striated muscle and brain and have an extended N terminus in addition to the catalytic domain. Class III HDACs resemble the yeast HDAC Sir2, which is activated by nicotinamide adenine dinucleotide (17).Class II HDACs interact with a variety of positive and negative cofactors as well as other ...

Arterioles and capillaries in the hamster cremaster muscle were observed during electrical stimulation of striated muscle fibers in order to characterize the microcirculatory basis of functional hyperemia. When contraction was restricted to single muscle fibers, responses were variable and frequently transient. Stimulation of either small bundles of muscle fibers or the entire cremaster muscle resulted in reproducible responses typified by: 1) a latency period, 2) an early, often transient phase of dilation, and 3) a second, slower phase of dilation. The latency varied inversely with contraction frequency, and the magnitude of the dilation varied directly with contraction frequency over the range 1--8/s. With stimulation of single fibers and small groups of fibers, arteriolar vasodilation was highly localized to regions of the arterioles that were in close apposition to the stimulated fibers. The number of capillaries with red blood cell flow increased during contraction, and the increase was graded with contraction frequency. The changes observed suggest that the vascular response during functional hyperemia is a two-part process and that the control processes are influenced by contraction frequency.

Small isolated groups of striated muscle cells were stimulated in the hamster cremaster muscle. During and after stimulation, oxygen microelectrodes were employed to determine the relationships among arteriolar vasodilation, tissue Po2, and periarteriolar Po2. Localized contraction produced a biphasic arteriolar vasodilation without associated alteration of Po2 on the surface of the arterioles (vascular smooth muscle Po2). In contrast, muscle contraction produced a decline in muscle tissue Po2 that was proportional to the contraction frequency over the range of 1--4 contractions per second. An increase in contraction frequency also produced a graded increase in arteriolar diameter, the magnitude of which was statistically correlated with the steady-state change in tissue Po2. However, arteriolar diameter changes preceded tissue Po2 changes, both with the initiation of functional dilation and during the recovery period. Tissue Po2 was manipulated at rest and during contraction by increasing the Po2 of the superfusion solution. Increasing the tissue Po2 caused a decrease in vascular diameter under both conditions and a reduction in the magnitude of the arteriolar vasodilation during contraction. Restoration of tissue Po2 to resting levels during muscle contraction produced only partial restoration of vascular diameters. The results are consistent with the hypothesis that at least three components are involved in the vascular control process during muscular activity: an early component independent of tissue oxygen levels, a late component independent of oxygen, and a late component associated with a decrease in muscle Po2, without an effect on vascular smooth muscle Po2. The evidence indicated that Po2 of the smooth muscle of the arterioles had no role in the dilation observed.

To investigate the relative contributions of alterations in blood flow, capillary density, and tissue PO2 to elevated oxygen delivery in working muscle, we conducted experiments on the suffused hamster cremaster muscle, using in vivo microscopic techniques. Muscle PO2 was measured during striated muscle twitch contraction at 1 Hz. Tissue oxygenation was changed by using suffusion solutions equilibrated with 0%, 5%, 10%, 21%, or 50% oxygen. Contraction caused an increase in capillary density (capillary recruitment), whose magnitude was related to the equilibration gas and, thus, to the suffusate PO2. Capillary recruitment first increased as the oxygen content was raised, peaked with 10% oxygen, and then diminished with higher oxygen content. Arteriolar functional dilation was also observed; when oxygen was raised above 21%, dilation was decreased. The data suggest that oxygen supply is increased primarily by arteriolar conductance changes with low suffusion solution oxygen (0% to 5%), and by capillary recruitment and increased PO2 gradients above 10% oxygen. When vasomotor tone was increased by addition of norepinephrine to the suffusion medium, the changes observed were similar to those observed when oxygen was increased. Therefore, we propose that the altered microvascular responses during vasoconstriction are a function of vascular tone rather than the levels of tissue PO2. A model is proposed which may partially explain the relations among vascular tone, functional dilation, and capillary recruitment. Our data also suggest that tissue PO2 may not be precisely regulated about a narrowly defined set point in this striated muscle but that, instead, tissue PO2 is a dependent variable controlled by the integrated effects of capillary recruitment, functional vasodilation, and altered metabolism.

Several short-acting beta-adrenergic receptor blocking agents have been prepared by incorporating ester functions into the aryl portion of certain (aryloxy)propanolamine systems. In particular, methyl 3-[4-[2-hydroxy-3-(isopropylamino)propoxy]phenyl]propionate hydrochloride (ASL-8052) was found to be a moderately potent, cardioselective compound with a short duration of action when determined in in vivo canine models.

In an attempt to produce short-acting beta-adrenergic receptor blocking agents, we prepared several (aryloxy)propanolamines with ester functions incorporated into the nitrogen substituent. Many of these compounds exhibited a short duration of blocking activity after their continuous intravenous infusion for 40 min. However, their durations were found to increase considerably upon longer intravenous infusion.

Esmolol is an ultra-short-acting beta-adrenergic blocking agent that possesses minimal partial agonist activity or direct membrane depressant activity. The short duration of action of esmolol is attributable to rapid enzymatic hydrolysis by red blood cell esterases, forming ASL-8123 and methanol. Experiments in the constant-flow-perfused isolated canine hindlimb indicate that therapeutic (beta blocking) doses of esmolol lack direct vascular effects and alpha-adrenergic blocking activity and that therapeutic doses do not interfere with vascoconstrictor effects of peripheral sympathetic nerve stimulation. Esmolol produces cardiac electrophysiologic and hemodynamic effects consistent with those of beta blockade. Specifically, esmolol decreases heart rate, depresses atrioventricular nodal conduction, and decreases determinants of myocardial oxygen demand. The beneficial antiarrhythmic and infarct-size limiting effects of esmolol have been demonstrated in several experimental models. Whereas beta blockers in general are effective in settings of supraventricular arrhythmias, sinus tachycardia, and myocardial ischemia, esmolol provides the added dimension of "titratability." Thus, the short duration of action of esmolol allows for very rapid titration to a preferred steady-state level of beta blockade; rapid adjustment to different steady-state levels of beta blockade, as may be required by changing status of the patient, and rapid disappearance of beta blockade following discontinuation of esmolol infusion, should this be necessary in the event of deleterious cardiac hemodynamic effects. Thus, esmolol is ideally suited for use in the treatment of patients in whom beta blockade is desirable, but in whom level of beta blockade must be very carefully modulated.

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