Relaxation properties of mammalian atrial muscle.

Abstract

SUMMARYThe properties of relaxation, in particular the sensitivity of relaxation to load, were analyzed in isolated intact atrial muscle and in manually dissected, detergent-treated cellular preparations from cat, dog, and rat atria. Force and length traces under increasing afterloads and following load clamps were obtained using an electromagnetic lever-force transducer system for the intact muscles and a capacitance transducer system for the cellular preparations. In both types of preparations, the time course of relaxation was hardly affected by the load or by alterations in load (load clamps), unlike intact mammalian ventricular muscle. This load independence of relaxation, which was hardly influenced by variations of initial muscle length, resembled relaxation in intact frog ventricular muscle and in detergent-treated mammalian ventricular single cells. As relaxation of these ventricular preparations with poorly developed (frog) or absent (detergent-treated single cells) calcium-sequestering systems was shown to be governed by the dissipation of activation, these results suggest a similar control mechanism for relaxation in mammalian atrial muscle. Furthermore, load independence of relaxation of mammalian atrial muscle in late diastole may promote optimal filling of the ventricle. Circ Res 48: 352-356, 1981ALTHOUGH the contractile properties of the contraction phase of mammalian atrial muscle have been well studied and compared with the contractile performance of ventricular muscle (Fabiato and Fabiato, 1972;Tarr et al., 1979;Urthaler et al., 1975), its properties during the relaxation phase are less well understood. Because of the unquestionable contribution of atrial contraction to the pump function of the ventricle, and because of the significant differences between the contractile properties of atrial and ventricular muscles (Blinks and KochWeser, 1963;Koch-Weser and Blinks, 1963), we have analyzed, in the present study, the mechanical behavior of mammalian atrial muscle during relaxation and in particular, its sensitivity to load or load alterations and its possible role in the functioning of the heart. Two kinds of preparation were used: (1) intact atrial muscle strips from cat and dog and (2) single atrial cells of rat, obtained by manual dissection after treatment with detergent to destroy the membranous systems. MethodsAtrial muscle strips were dissected from cat (n = 9) and dog (n = 2). The basic characteristics of these preparations are summarized in Table 1.Long thin strips of longitudinally oriented bundles of muscle fibers were cut from the free left atrial wall. The muscle strips were mounted vertically, the lower end being held by a force transducer (compliance, 0.3 |iim/mN; resonant frequency in aqueous solution, 250 Hz) and the upper end being tied (7.0 braided thread, Deknatel, Surgical Tevdek, Code 103-T) to an electromagnetic lever system (compliance 0.2 jum/mN, equivalent moving mass 155 mg, step response 3 msec). The current through the coil of the electromagnet determined the load on...

Differences in Effects of Length-Dependent Regulation of Force and Ca2+ Transient in the Myocardial Trabeculae of the Rat Right Atrium and Ventricle

“…In atrial myocytes, they are not expressed or lacked, which may reduce the mechanical synchronization of adjacent cells [38]. As a result, the relative contractile ability, i.e., force per cell cross-section, is lower in atrial muscles, as shown in previous studies [28,32,37] and in the present study.…”

“…In the human atrial myocardium, preload has been shown to affect both contractility and diastolic/systolic cytosolic Ca 2+ levels [32], while, in isolated atrial cardiomyocytes, no considerable effect of preload on Ca 2+ transient characteristics was observed [36]. Some other mammals showed weaker effects of preload compared to ventricular myocardium [37]. This may be explained by the prominent differences in the expression of ion channels, regulatory and contractile proteins of myofilaments, and by more subtle differences in the structure of the sarcoplasmic membrane between ventricular and atrial myocytes [4,7,9,10].…”

Contractile Behavior of Right Atrial Myocardium of Healthy Rats and Rats with the Experimental Model of Pulmonary Hypertension

“…The following differences in the characteristics of the contractility and calcium kinetics between right atrial and right ventricular myocardium of rats can be highlighted: (i) the atrial myocardium develops lower active isometric tension (FSM is less effective); (ii) the mechanical tension develops and declines faster in the atrial myocardium; (iii) the maximal rate of cytosolic Ca 2+ elevation is lower and the time to peak of Ca transient is longer in the atrial myocardium; and (iv) the Ca transient decline does not show a "bump" phase, which is typical in the ventricular myocardium [21,24]. Compared to the ventricular myocardium, the length-dependent modulation of atrial myocardial contractility is much weaker in many mammals [13]. In addition, in rats with monocrotaline-induced right heart remodeling, SFR is increased in the right atrial myocardium (the present results) while it is decreased in the right ventricular myocardium, as we previously reported [20].…”

“…On atrial strips from human healthy donor hearts, it has been shown that length-dependent changes do exist and they relate not only to the mechanical characteristics of isometric twitch (FSM) but also to the kinetic characteristics of cytosolic calcium [12]. Compared to the ventricular myocardium, a weaker length-dependent modulation of atrial myocardial contractility has been shown in different mammals [13]. The contemporary changes in the mechanical activity and Ca transient in the atrial myocardium have been analyzed in a few studies [12,14], but these studies are limited to mechanically unloaded conditions or, at best, to different preloads.…”