The key findings of this study are that treatment of the 3 weeks infarcted rats for a period of 5 weeks with atenolol and propranolol improved cardiac dysfunction and ameliorated depression in myofibrillar Ca2+-stimulated ATPase activity, alterations in α- and β-MHC isoforms and gene expression, as well as decreased cTnI phosphorylation. It is pointed out that, to the best of our knowledge, this study is the first to compare the different effects of high and low doses of selective β1-AR blocker, atenolol and non-selective β-AR blocker, propranolol on cardiac function and myofibrillar remodelling. These observations indicate that both selective and non-selective β-AR antagonists exert beneficial effects in CHF. In this regard, it may be noted that various selective and non-selective β-AR antagonists including atenolol and propranolol have been reported to attenuate LV remodelling, improve cardiac function and reduce sympathetic activity in CHF [23–28]. Moreover, treatment of CHF patients with metoprolol and carvedilol was found to improve myosin ATPase activity and restore cTn1 phosphorylation in the myocardium . Likewise, improvement in cardiac function by treatment with a β-AR blocker, carteolol, attenuated myofibrillar ATPase activity in furazolidine-induced dilated cardiomyopathy in turkey . Furthermore, as decreased sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) content is associated with reduced sarcoplasmic reticulum Ca2+ loading and elevated cytoplasmic Ca2+ levels [31, 32], the restoration of SERCA2a levels is likely to be a critical factor in the normalization of Ca2+ uptake. Therefore, owing to the importance of SERCA2a in restoring the sarcoplasmic reticulum Ca2+ load per excitation–contraction cycle, a study by Sun et al., showed that 6 weeks of treatment of MI rats with metoprolol or carvedilol improved SERCA2a messenger RNA and protein levels. Such effects of β-AR blockade can lead to markedly improved cardiac function as assessed by echocardiography and haemodynamics. This particular investigation extended the results of another study that reported improved SERCA2a levels after propranolol treatment in dilated cardiomyopathy . Thus these support the view that both selective and non-selective β-AR antagonists exert beneficial effects by attenuating alterations in contractile and regulating proteins in CHF.
Attenuation of ventricular remodelling and improvement in cardiac function
β-AR blockade with atenolol and propranolol in infarcted animals reduced cardiac preload, as illustrated by reduction in LVEDP. Interestingly, changes in +dP/dt and –dP/dt in CHF were improved only when propranolol was used in high doses. Atenolol in high doses was also observed to attenuate the depressed +dP/dt without any changes in –dP/dt in infarcted hearts. These effects of propranolol was similar to another study in which carvedilol (another non-selective β-blocker) was shown to have more improvement in haemodynamic parameters compared to metoprolol (a selective β1-blocker) . The mechanisms for this significant haemodynamic improvement by non-selective β-blockade may be due to the reduction of wall stress and oxygen uptake, and an increase in coronary blood flow associated with blockade of excessive sympathetic activation [35, 36], which can protect and enhance the function of remaining surviving myocytes in the LV during CHF. Moreover, the decrease in LVEDP by both atenolol and propranolol suggest a trend towards a reduction in LV preload and afterload, which may further improve LV function.
LVEF was also significantly improved by treatments with these β-AR antagonists. This, together with the haemodynamic effects, as well as the observed reduction in LV end-diastolic and end-systolic diameters and LV dilation, may induce an improvement of LV filling and an increase in cardiac contractility. The increase in lung wet/dry wt. ratio due to CHF was attenuated by treatment with atenolol and propranolol indicating that the drugs were effective in improving pulmonary oedema in animals with CHF. Moreover, treatments with atenolol and propranolol depressed the increase in HR in rats with CHF, which may be due to the depression in the increased sympathetic activity and elevated levels of plasma catecholamines [27, 37].
Modulation of myofibrillar remodelling in CHF
The present study and previous reports from our laboratory have shown a depression in cardiac function and myofibrillar ATPase activity as well as a switch in α-MHC and β-MHC protein content and gene expression in post-MI CHF [19, 38]. The observed decrease in myofibrillar Ca2+-stimulated ATPase activity in failing hearts would result in depression in cardiac function since the magnitude of cardiac contractile force is linearly related to myofibrillar Ca2+-stimulated ATPase activity . On the other hand, myofibrillar ATPase activity is mostly determined by the ratio of the expressed MHC isoforms as α-MHC has a low ATPase activity but produces high cross-bridge force with more economy of energy consumption . The consequence of CHF has been studied in the failing human hearts and was shown that mRNA expression of α-MHC was significantly reduced and β-MHC was significantly increased compared to controls . In rodent models of cardiac hypertrophy and failure, decrease in α-MHC mRNA and increase in β-MHC mRNA as well as corresponding changes in protein content were found to be associated with a reduction in velocity of shortening and other measures of systolic function [42–44]. However, it is pointed out that protein levels do not always correlate with the mRNA levels  as we have observed that 3.8-fold increase in β-MHC mRNA was associated with 15-fold increase in β-MHC protein whereas 0.5-fold decrease in α-MHC mRNA was associated with 4-fold decrease in α-MHC protein in the CHF due to MI in rats.
A study in spontaneously hypertensive rats has shown that atenolol treatment reduced cardiac mass and affected the shift in MHC isozymes in the myocardium . Likewise, we found that atenolol reduced ventricular hypertrophy, increased α-MHC and decreased β-MHC isoform in post-MI rats. However, the observed changes in MHC protein levels did not correspond to similar changes in mRNA levels. This finding suggests that MHC protein levels might be regulated by an altered mRNA turnover or translational activity, which may change the net mRNA level in the myocardium. Thus some caution should be exercised while interpreting the observed changes in α-MHC and β-MHC in terms of quantitative alterations in myofibrillar protein content because we did not determine the absolute values for the MHC isozymes in the failing heart by employing mass spectroscopy , which is more accurate than the semi-quantitative Coomassie staining. Although treatment of hypertensive animals with propranolol induced partial regression in cardiac hypertrophy, it did not improve stroke volume significantly . On the other hand, treatment of infarcted animals with propranolol decreased RV hypertrophy and improved both echocardiographic and haemodynamic parameters. This finding is consistent with a study  in which propranolol was observed to increase the LVEF, increase α-MHC mRNA level and decrease β-MHC mRNA level in patients with dilated cardiomopathy. Furthermore, in another study by Boluyt et al., β-adrenergic stimulation was shown to induce ‘foetal’ pattern of expression of MHC, which was reversed by non-selective β-blocker. Therefore, in view of the high levels of cardiac adrenergic activity present during CHF, we speculate that propranolol by blocking both β1- and β2-mediated adrenergic stimulation can contribute to reversal of induction of elements of the pathological ‘foetal’ gene program more efficiently than the selective β1-blocker atenolol. Thus it is likely that the effectiveness of both selective and non-selective β-AR antagonists on cardiac hypertrophy and myofibrillar remodelling during the development of cardiac dysfunction may depend on the type of CHF.
In CHF, the down-regulation of β-receptors and β-adrenergic signal transduction is mirrored at the molecular level as a decrease in cTnI phosphorylation, which may be responsible for the enhanced myofibrillar Ca2+ sensitivity and lower maximal ATPase activity observed in CHF [49, 50]. The phosphorylation of cTnI by protein kinase A (PKA), which is activated viaβ-receptors, results in a decrease in Ca2+ sensitivity of the contractile machinery . Therefore, although we did not determine the protein levels of PKA, we used the phosphorylation-specific cTnI antibody (specific for the phosphoform of Ser22/Ser23, which is the main phosphorylation site for PKA ) in the present study to determine the phosphorylation of cTnI by PKA. Since treatment with both atenolol and propranolol restored the MI-induced decrease in cTnI phosphorylation without affecting total cTnI content, it appears that PKA-mediated cTnI phosphorylation correlates well with the contractile state of the heart. Moreover, when phosphorylated, cTnI induces conformational changes in the troponin molecules that reduce the Ca2+ affinity of cardiac troponin C and lead to enhanced relaxation . Therefore, it is plausible that the beneficial effects of β-AR blockade on contractile function in CHF may partly rely on their ability to restore cTnI phosphorylation. Besides PKA-mediated cTnI phosphorylation, changes in PKC-mediated phosphorylation of cTnI can also contribute to altered contractile function. However, the importance of PKC-mediated phosphorylation is obscure because different PKC isozymes are not activated uniformly and may subserve distinct biological functions due to their different substrate specificities. Also, PKC-mediated phosphorylation of cTnI has been shown to have negative effects on myofibrillar function, such as inhibition of maximal Mg-ATPase activity, and to decrease myofilament Ca2+ sensitivity, maximal tension development and cross bridge cycling kinetics [54, 55]. Therefore, in this particular study, we studied only the cTnI phosphorylation by PKA, which is considered as more important than phosphorylation by PKC because functional consequences of cTnI phosphorylation by PKC are most probably affected by the phosphorylation status of the PKA phosphorylation sites and also PKC may cross-phosphorylate the PKA phosphorylation sites on cTnI . Nonetheless, PKC has been reported to be up-regulated in secondary cardiac hypertrophy  and CHF . Whether β-blockade affects changes in the PKC activity in the failing heart still needs to be elucidated.
In summary, β-receptor blockade is effective in preventing LV remodelling and cardiac contractile dysfunction in CHF after MI. The molecular mechanisms may be related with normalization of myofibrillar Ca2+-stimulated ATPase activity and MHC protein content. However, only the non-selective β-AR blocker, propranolol at high dose was more effective in preventing the changes in ±dP/dt and gene expression for α-MHC and β-MHC in CHF than the selective β1-AR blocker, atenolol. It should be noted that propranolol being a non-selective β-blocker can inhibit the β-receptor stimulating action of epinephrine and increase the serum concentration of potassium. Potassium is first released from the intracellular space to the extracellular space through the α-adrenergic action of epinephrine and this subsequently stimulates the β-receptor action of epinephrine causing hyperkalemia . Therefore, serum potassium concentration should be closely monitored when patients are on non-selective β-blocker therapy. Nonetheless, from this study, the beneficial effects of atenolol and propranolol on the activity and content of contractile and regulatory proteins may provide further insights regarding the relationship between myofibrillar remodelling and β-AR signalling in the heart.