Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium.

The effects of ventricular myosin heavy chain (MHC) composition on the kinetics of activation and relaxation were examined in both chemically skinned and intact myocardial preparations from adult rats. Thyroid deficiency was induced to alter ventricular MHC isoform expression from ∼80 %α-MHC/20 %β-MHC in euthyroid rats to 100 %β-MHC, without altering the expression of thin-filament-associated regulatory proteins. In single skinned myocytes, increased expression of β-MHC did not significantly affect either maximal Ca2+-activated tension (P0) or the Ca2+ sensitivity of tension (pCa50). However, unloaded shortening velocity (V0) decreased by 80 % due to increased β-MHC expression. The kinetics of activation and relaxation were examined in skinned multicellular preparations using the caged Ca2+ compound DM-nitrophen and caged Ca2+ chelator diazo-2, respectively. Myocardium expressing 100 %β-MHC exhibited apparent rates of submaximal and maximal tension development (kCa) that were 60 % lower than in control myocardium, and a 2-fold increase in the half-time for relaxation from steady-state submaximal force. The time courses of cell shortening and intracellular Ca2+ transients were assessed in living, electrically paced myocytes, both with and without β-adrenergic stimulation (70 nm isoproterenol (isoprenaline)). Thyroid deficiency had no affect on either the extent of myocyte shortening or the resting or peak fura-2 fluorescence ratios. However, induction of β-MHC expression by thyroid deficiency was associated with increased half-times for myocyte shortening and relengthening and increased half-time for the decay of the fura-2 fluorescence ratio. Qualitatively similar results were obtained in both the absence and the presence of β-adrenergic stimulation although the β-agonist accelerated the kinetics of the twitch and the Ca2+ transient. Collectively, these data provide evidence that increased β-MHC expression contributes significantly to the observed depression of contractile function in thyroid deficient myocardium by slowing the rates of both force development and force relaxation. In striated muscle, Ca2+ binding to troponin C (TnC) initiates a series of events that permit strong interaction between myosin and actin (contraction), while dissociation of Ca2+ from TnC leads to reversal of these events and detachment of myosin from actin (relaxation). While Ca2+ binding to TnC initiates contraction, complete activation of the thin filament in terms of tension and the kinetics of tension development most probably involves the synergistic actions of Ca2+ and strong-binding myosin cross-bridges (Swartz & Moss, 1992; Geeves & Lehrer, 1994; Swartz et al. 1996). Furthermore, the kinetics of interaction of myosin with actin are thought to be determined at least in part by the myosin heavy chain (MHC) content of a given muscle. For example, the maximal rate of tension redevelopment is 8-fold faster in fast-twitch skeletal muscle fibres expressing type II B MHC than in slow-twitch fibres expressing type I MHC, i.e. cardiac β-MHC (Metzger & Moss, 1990). Also, muscle-to-muscle variations in maximal shortening velocity, an index of cross-bridge detachment rate, are thought to be determined by MHC content (Harris et al. 1994; VanBuren et al. 1995; Schiaffino & Reggiani, 1996). To date, little information is available as to whether there is MHC isoform-specific modulation of the kinetics of force development or relaxation, especially in heart muscle. Two cardiac MHC isoforms, α and β, have been identified in the adult mammalian ventricle and are products of two closely related genes (Lompre et al. 1984). The phenotypic expression of cardiac MHC isoforms is dynamic and subject to a number of physiological influences, including regulation by hormones such as thyroid hormone (Lompre et al. 1984). Thyroid deficiency results in a complete remodelling of ventricular MHC distribution from ∼80 %α-MHC/20 %β-MHC to 100 %β-MHC (Morkin, 1993). It is interesting to note that while cardiac MHC expression varies with thyroid state, altered levels of triiodothyronine (T3) do not appear to alter the phenotypic expression of other myofibrillar protein isoforms, such as the thin filament proteins troponin I (TnI) and troponin T (TnT) (Averyhart-Fullard et al. 1994; Akella et al. 1997). However, it is well known that thyroid state markedly affects the Ca2+ handling properties of the sarcoplasmic reticulum (SR) (Kiss et al. 1994). Therefore, studies examining modulation of myocardial contraction subsequent to altered thyroid status must account for changes in Ca2+ handling as well as myofilament effects mediated by increased expression of β-MHC. Here, we investigated the influence of thyroid status and altered MHC protein expression on the kinetics of myocardial contraction in both chemically skinned and intact myocardial preparations. The relative proportions of ventricular MHC isoforms in adult rat myocardium were transformed from predominantly α-MHC to exclusively β-MHC by inducing a thyroid deficient state. In skinned myocardium, activation and relaxation kinetics were examined using the photolabile caged Ca2+ compound DM-nitrophen (Kaplan & Ellis-Davies, 1988) and caged Ca2+ chelator diazo-2 (Adams et al. 1989), respectively. Electrically stimulated intact myocytes were used to examine possible interactions between thyroid deficiency-induced changes in MHC content and Ca2+ handling properties of the SR in determining twitch characteristics.