Ca(2+)-channels and adrenoceptors in diabetic skeletal muscle.

The status of Ca(2+)-channels and adrenoceptors in the hind leg skeletal muscle was examined in rats 8 weeks after inducing diabetes by an intravenous injection of streptozotocin (65 mg/kg). Scatchard plot analysis of the data on specific binding of 3H-nitrendipine with crude membranes from diabetic muscle revealed an increase in the density of Ca(2+)-channels without any significant change in their affinity for the ligand. An increase in the density of beta-adrenoceptors without any alteration in their affinity, as measured by 3H-dihydroalprenolol binding, was also evident in the diabetic muscle. The observed increase in the number of Ca2+ channels or beta-adrenoceptors seems specific since no change in the alpha-adrenoceptor density or affinity, as measured by 3H-prazosin binding, was seen in the diabetic membranes. These results support the view that higher activities of Ca2+ transport systems or regulatory mechanisms may be associated with hyperfunction of the diabetic skeletal muscle.     

Characterization of rat heart plasma membrane Ca2+/Mg2+ ATPase

The Ca2+/Mg2+ ATPase of rat heart plasma membrane was activated by millimolar concentrations of Ca2+ or Mg2+; other divalent cations also activated the enzyme but to a lesser extent. Sodium azide at high concentrations inhibited the enzyme by about 20%; oligomycin at high concentrations also inhibited the enzyme slightly. Trifluoperazine at high concentrations was found inhibitory whereas trypsin treatment had no significant influence on the enzyme. The rate of ATP hydrolysis by the Ca2+/Mg2+ ATPase decayed exponentially; the first-order rate constants were 0.14-0.18 min-1 for Ca2+ ATPase activity and 0.15-0.30 min-1 for Mg2+ ATPase at 37 degrees C. The inactivation of the enzyme depended upon the presence of ATP or other high energy nucleotides but was not due to the accumulation of products of ATP hydrolysis. Furthermore, the inactivation of the enzyme was independent of temperature below 37 degrees C. Con A when added into the incubation medium before ATP blocked the ATP-dependent inactivation; this effect was prevented by alpha-methylmannoside. In the presence of low concentrations of detergent, the rate of ATP hydrolysis was reduced while the ATP-dependent inactivation was accelerated markedly. Both Con A and glutaraldehyde decreased the susceptibility of Ca2+/Mg2+ ATPase to the detergent. These results suggest that the Ca2+/Mg2+ ATPase is an intrinsic membrane protein which may be regulated by ATP.     

Renaissance of cytochemical localization of membrane ATPases in the myocardium

ATPases of cardiac cells are known to be among the most important enzymes to maintain the fluxes of vital cations by hydrolysis of the terminal high-energy phosphate of ATP. Biochemically the activities of Ca²⁺-pump ATPase, Ca²⁺/Mg²⁺-ecto ATPase, Na⁺,K⁺-ATPase and Mg²⁺-ATPase are determined in homogenates and isolated membranes as well as in myofibrillar and mitochondrial fractions of various purities. Such techniques permit estimation of enzyme activitiesin vitro under optimal conditions without precise enzyme topography. On the other hand, cytochemical methods demonstrate enzyme activityin situ, but not under optimal conditions. Until recently several cytochemical methods have been employed for each enzyme in order to protect its specific activity and precise localization but the results are difficult to interpret. To obtain more consistent data from biochemical and cytochemical point of view, we modified cytochemical methods in which unified conditions for each ATPase were used. The fixative solution (1% paraformaldehyde –0.2% glutaraldehyde in 0.1 M Tris Base buffer, pH 7.4), the same cationic concentrations of basic components in the incubation medium (0.1 M Tris Base, 2mM Pb(NO₂)₃, 5 mM MgSO₄, 5 mM ATP) and selective stimulators or inhibitors were employed. The results reveal improved localization of Ca²⁺-pump ATPase, Na⁺–K⁺ ATPase and Ca²⁺/Mg²⁺-ecto ATPase in the cardiac membrane.     

Decreased expression of cardiac sarcoplasmic reticulum Ca2+-pump ATPase in congestive heart failure due to myocardial infarction

Myocardial infarction in rats induced by occluding the left coronary artery for 4, 8 and 16 weeks has been shown to result in congestive heart failure (CHF) characterized by hypertrophy of the viable ventricular myocardial tissue. We have previously demonstrated a decreased calcium transport activity in the sarcoplasmic reticulum (SR) of post-myocardial infarction failing rat hearts. In this study we have measured the steady state levels of the cardiac SR Ca²⁺-pump ATPase (SERCA2) mRNA using Northern blot and slot blot analyses. The relative amounts of SERCA2 mRNA were decreased with respect to GAPDH mRNA and 28 S rRNA in experimental failing hearts at 4 and 8 weeks post myocardial infarction by about 20% whereas those at 16 weeks declined by about 35% of control values. The results obtained by Western blot analysis, revealed that the immunodetectable levels of SERCA2 protein in 8 and 16 weeks postinfarcted animals were decreased by about 20% and 30%, respectively. The left ventricular SR Ca²⁺-pump ATPase specific activity was depressed in the SR preparations of failing hearts as early as 4 weeks post myocardial infarction and declined by about 65% at 16 weeks compared to control. These results indicate that the depressed SR Ca²⁺-pump ATPase activity in CHF may partly be due to decreased steady state amounts of SERCA2 mRNA and SERCA2 protein in the failing myocardium.     

β-Adrenoceptor mediated signal transduction in congestive heart failure in cardiomyopathic (UMX7.1) hamsters

In view of the lack of information regarding the status of -adrenoceptor mediated signal transduction mechanisms at severe stages of congestive heart failure, the status of -adrenoceptors, G-proteins and adenylyl cyclase activities was examined in 2202–275 day old cardiomyopathic hamster hearts. Although no changes in the Kd values for ₁- and ₂-adrenoceptors were seen, the number of ₁-adrenoceptors, unlike that of R2-adrenoceptors, was markedly decreased in cardiac membranes from failing hearts. The activation of adenylyl cyclase in the failing hearts by different concentrations of isoproterenol was also attenuated in comparison to the control preparations. The basal adenylyl cyclase activity in cardiac membranes from the failing hearts was not altered; however, the stimulated enzyme activities, when measured in the presence of forskolin, NaF or Gpp(NH)p were depressed significantly. The functional activity of Gs-proteins (measured by cholera toxin stimulation of adenylyl cyclase) was depressed whereas that of Gi-proteins (measured by pertussis toxin stimulation of adenylyl cyclase) was increased in the failing hearts. Not only were the Gs- and Gi-protein contents (measured by immunoblotting) increased, the bioactivities of these proteins as determined by ADP-ribosylations in the presence of cholera toxin and pertussis toxin, respectively, were also higher in failing hearts in comparison to the control values. Northern blot analysis revealed that the signals for Gs- and Gi-protein mRNAs were augmented at this stage of heart failure. These results indicate that the loss of adrenergic support at severe stages of congestive heart failure in cardiomyopathic hamsters may involve a reduction in the number of ₁-adrenoceptors, and an increase in Gi-protein contents as well as bioactivities in addition to an uncoupling of Gs-proteins from the catalytic site of adenylyl cyclase in cardiac membrane.     

Alterations in cardiac lysosomal hydrolases following induction of the calcium paradox

Although perfusion of the heart with calcium-free medium for a brief period followed by reperfusion with calcium-containing medium results in marked structural derangements (calcium paradox), the mechanisms for this cell damage are far from clear. Since activation of lysosomal enzymes has been associated with pathological damage, it was the purpose of this study to examine alterations in the activities of several lysosomal enzymes in rat hearts subjected to calcium paradox. No significant changes in the activities of beta-acetylglucosaminidase, beta-galactosidase, alpha-mannosidase, or acid phosphatase were seen in the homogenates of hearts exposed to the calcium paradox. However, there were dramatic alterations in the lysosomal enzyme activities in the sedimentable and nonsedimentable fractions during calcium paradox. The lysosomal enzyme activities were also detected in the perfusate collected during reperfusion with calcium-containing medium. These changes occurred during the reperfusion period since no alterations were apparent after calcium-free perfusion and were dependent upon the time of reperfusion with medium containing Ca2+ as well as the time of perfusion with Ca2+ -free medium before inducing Ca2+ paradox. These data indicate that alterations in lysosomal enzymes owing to reinstitution of calcium in Ca2+-deprived hearts may occur as a part of cardiac damage and general cellular disintegration.     

Stimulation of Ca2+-pump in rat heart sarcolemma by phosphatidylethanolamine N-methylation

Incubation of purified cardiac sarcolemmal vesicles (SL) in the presence of S-adenosyl-L-methionine, a methyl donor for the enzymatic N-methylation of phosphatidylethanolamine (PE), increased the Ca2+-stimulated ATPase and ATP-dependent Ca2+ accumulation activities. Quantitative analysis of the methylated phospholipids revealed that maximal increase of Ca2+-pump activities was associated with predominant synthesis and intramembranal accumulation of phosphatidyl-N,N-dimethylethanolamine. The stimulation of SL Ca2+-pump activities was prevented by inhibitors of PE N-methylation such as S-adenosyl-L-homocysteine and methyl acetimidate hydrochloride. The results suggest a possible role of PE N-methylation in the regulation of Ca2+-transport across the heart SL membrane.     

Properties of 5'-nucleotidase in rat heart sarcolemma

The activity of 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) was examined in membrane fractions isolated by hypotonic shock-LiBr treatment (fraction HL) and sucrose gradient separation (fraction S) of rat ventricle homogenate. The enzyme activity in these two fractions differed significantly in several respects. In fraction HL, 5'-nucleotidase had a high affinity for AMP (Km 35 microM), and ATP was a potent competitive inhibitor. In contrast, the 5'-nucleotidase displayed by fraction S showed a low substrate affinity (Km 130 microM) and less sensitivity to ATP. Treatment of membranes with trypsin and neuraminidase markedly stimulated 5'-nucleotidase in fraction HL, whereas only a modest effect was observed in fraction S. Exposure of the membranes to Triton X-100 resulted in a 60% and 10% increase in the enzyme activity in fractions HL and S, respectively. The characteristic activity ratios of 5'-nucleotidase at 200 microM relative to 50 microM AMP in fractions HL and S were modified by alamethicin in an opposite way and became identical. Although concanavalin A almost completely inhibited the 5'-nucleotidase activity in both membrane preparations at a concentration of 2 microM, Hill plots of the data on concanavalin A inhibition revealed a coefficient of 2.2 for fraction S and 1.1 for fraction HL. The differences in 5'-nucleotidase activity of the two membrane fractions are considered to be due to differences in the orientation of the vesicles of the sarcolemmal preparations. These results suggest that two distinct catalytic sites for 5'-nucleotidase are present at the intra- and extracellular surface of the rat heart sarcolemma.