Studies on Adenosine Triphosphate-supported Calcium Accumulation by Cardiac Subcellular Particles

The role of microtubules and inositol triphosphate induced Ca2+ release in the tyrosine phosphorylation of mitogen-activated protein kinase in extracts of Xenopus laevis oocytes

Microsomal fractions of Xenopus oocytes release preloaded 45 Ca2+ when treated with inositol triphosphate (InsP3). The effective concentration of InsP3 required for half-maximal release (EC50) is 59 nM and maximal release occurs at approximately 2 microM InsP3. Uptake and release of 45 Ca2+ are not altered by the catalytic subunit of protein kinase A, dibutyrl cyclic adenosine monophosphate, protein kinase A peptide inhibitor or nocodazole. In contrast, taxol decreases the sensitivity of the microsomal fraction to InsP3, shifting the EC50 for InsP3-induced Ca2+ release from 59 to 259 nM. In lysates of oocytes, InsP3-induced Ca2+ release causes the tyrosine phosphorylation of a 42,000 (M(r) 42k) protein identified as 42k mitogen-activated protein (MAP) kinase. InsP3-induced tyrosine phosphorylation of MAP kinase is prevented by BAPTA and taxol, but not by nocodazole. Thus, microtubule polymerisation modifies InsP3-induced Ca2+ release, thereby inhibiting phosphorylation of MAP kinase.

Inhibition of dicarboxylic anion transport by fluorescein isothiocyanate in skeletal sarcoplasmic reticulum

It has been demonstrated previously that dicarboxylic anions are cotransported during ATP-dependent Ca2+ transport by skeletal muscle sarcoplasmic reticulum (SR) membranes, and that anion cotransport stimulates Ca2+ transport. In the current study, we present evidence that dicarboxylic anion cotransport and Ca2+ transport are kinetically distinct in SR, but both functions are mediated by the CaATPase protein. Preincubation of SR with 40 microM fluorescein isothiocyanate (FITC) (pH 7.0) inhibited essentially all of the Ca2+ ATPase activity, as well as active oxalate-supported and oxalate-independent 45Ca2+ accumulation. The addition of 1 mM beta, gamma-methyleneadenosine 5'-triphosphate (AMP-PCP) to the preincubation media fully protected the dicarboxylic anion-independent Ca2+ ATPase activity and the oxalate-independent active 45Ca2+ accumulation from the inhibitory effects of FITC; however, the ATP-associated [14C]oxalate accumulation, the oxalate-dependent 45Ca2+ accumulation, and the oxalate- and maleate-dependent stimulation of Ca2+ ATPase activity were not protected by AMP-PCP. Thus, the dicarboxylic anion accumulation and the stimulation of Ca2+ uptake by dicarboxylic anions could be functionally separated from the ATP-dependent, anion-independent Ca2+ translocation. FITC bound exclusively to the 100-kDa (CaATPase) and 92-kDa (phosphorylase) proteins in the SR membranes and to purified CaATPase in sodium dodecyl sulfate-polyacrylamide gel electrophoresis; 1 mM AMP-PCP inhibited 50-55% of the FITC fluorescence on the 100-kDa protein, but did not significantly alter fluorescence on the 92-kDa protein. Two-dimensional gel analysis demonstrated a single 100-kDa protein in longitudinal SR membranes. FITC appears to inhibit ATP-dependent Ca2+ transport, and dicarboxylic anion translocation through interaction at separate domains of the CaATPase protein.

Analysis of azide-insensitive Ca2+-dependent ATPase activity in atrial specimens from patients with coronary or valvular heart disease

The activity of the azide-insensitive Ca2+-dependent ATPase (highly enriched in myofibrillar ATPase activity) was studied in specimens of both right and left atria which were taken from patients with ischemic and/or valvular heart disease during coronary by pass and/or valvular substitution. A significantly lower enzymatic activity was found in atrial specimens from patients with left ventricular heart failure in comparison to the atrial fragments obtained from the patients with normal heart function. Such an inhibition reflected a significant increase in the Km of the enzyme for ATP and was associated with a concomitant reduction in Vmax, both more evident in the left atrial fragments. Moreover, tissue homogenates of atrial specimens from failing hearts exhibited a lower protein SH group content when compared to the atrial homogenates from the heart with normal left ventricular heart function.

Regulation of myocardial contractility 1958-1983: an odyssey

The past 25 years have seen an unprecedented growth in our understanding of the mechanisms responsible for the regulation of myocardial contractility. Beginning with a demonstration that myocardial contractility represents an important determinant of cardiac function, this quarter century has witnessed a series of attempts to explain length-independent changes in myocardial contractile function. Alterations in the properties of the cardiac contractile proteins and changes in the amount of calcium (Ca2+) available for binding to the contractile proteins during excitation-contraction coupling are now recognized as important biochemical bases for physiologic, pharmacologic and pathologic changes in myocardial contractility. Identification of the central role of Ca2+ in the initiation of the cardiac contractile process has made possible the analysis of the mechanisms by which several drugs alter myocardial contractile function, including the biochemical processes that allow beta-adrenergic agonists to enhance contractility. The rapid developments in this field since 1958 promise an even greater flow of new knowledge regarding the causes, prevention and treatment of heart failure provided that there is given adequate support for such research activities.

Characterization of contractions in mechanically disaggregated myocardial cells from the rat

Myocardial cell groups, mechanically disaggregated from the rat heart, beat spontaneously in a medium which resembles intracellular fluid. The cells, initially functioning in an uncoordinated way, i.e. each cell in its own rhythm, gradually become coordinated as a function of time. After 90 min of incubation at 15 degrees C all the cell groups were functioning in a coordinated way. The initiation of coordination was temperature-dependent. Lowering the temperature from 15 degrees C to 10 degrees C significantly delays the coordination, only 75% of the cell groups being coordinated after 90 min. The contraction wave in coordinated cell groups was very slow; 90 +/- 7 micrometers/s at 20 degrees C, 100 +/- 6 micrometers/s at 30 degrees C and 100 +/- 7 micrometers/s at 35 degrees C. The velocity of contraction waves was not significantly temperature-dependent, suggesting a neonenzymatical type of propagation. It also seems too slow to be an electrical event. Apparently the contraction waves are propagated by diffusion of Ca2+. The addition of caffeine to the medium hastened the coordination of contractions, but Na-azide strongly inhibited it. The results indicate that the sarcoplasmic reticulum functions in the coordination of contractions, probably by releasing Ca2+ for the activation of the myofilaments.

A comparison of vesicles derived from terminal cisternae and longitudinal tubules of sarcoplasmic reticulum isolated from rabbit skeletal muscle

Sarcoplasmic reticulum vesicles were separated into heavy (derived from terminal cisternae) and light (derived from longitudinal tubules) fractions, according to Meissner [Biochim. Biophys. Acta, 389, 51-68 (1975)]. The similar Ca2+ sensitivities of phosphoprotein formation, ATPase activity and calcium uptake, and the similar phosphoprotein turnover rates (ATPase/phosphoprotein formation) of both fractions indicate that the same ATPase enzyme is present in the terminal cisternae and longitudinal sarcoplaxmic reticulum. The higher V for Ca2+-activated ATPase activity and calcium uptake in the light fraction correlated with the higher concentration of ATPase enzyme per mg of membrane protein in this fraction. In both the presence and absence of calcium-precipitating anions, the light fraction stored more calcium than the heavy. The Ca2+ dependence of calcium release after addition of EGTA appeared similar in both fractions, but the rate of calcium release was more rapid in the light fraction. These findings suggest that calcium release may occur more rapidly from longitudinal than terminal cisternae portions of the sarcoplasmic reticulum and that calcium release, like calcium uptake, may be mediated by the ATPase enzyme in the sarcoplasmic reticulum membrane. Although the activation energies for Ca2+-activated ATPase activity above and below the transition temperature were significantly different for the heavy and light fractions, their transition temperatures were similar. Partial purification of the ATpase enzyme by deoxycholate treatment modified the activation energies of the light but not the heavy fraction and caused the activation energies to become similar. The phosphoprotein levels of heavy and light vesicles did not become similar after deoxycholate treatment, although gel electrophoretograms indicated both samples contained > 90% ATPase protein. These results indicate the protein-lipid associations in these two fractions may be different.

Exercise and mitochondrial calcium transport in the BIO 14.6 hamster

To examine the effects of exhaustive swimming in normal and myopathic hamsters on muscle mitochondrial Ca2+ metabolism, sedentary normal and BIO 14.6 dystrophic Syrian hamsters swam individually in 35 degree C water until exhaustion occurred. Although the normal hamsters swam three times longer than did the BIO 14.6 hamsters, both swimming groups had a comparable two-fold increase in blood lactate. Contrary to exhaustive running, exhaustive swimming did not significantly affect mitochondrial Ca2+ uptake in either cardiac or skeletal muscle, regardless of the disease state. However, in general, the coefficients of variation for mitochondrial Ca2+ metabolism increased as a function of exercise, with the BIO 14.6 swimmers more variable than the normal swimmers. This suggests that the mitochondrial Ca2+ uptake process may be affected in some manner by exhaustive swimming, so that deviations from the norm are more apparent. The results provide further evidence that mitochondrial Ca2+ metabolism adapts to the specific type of exercise utilized to produce exhaustion.

A comparison of the intrinsic protein kinase activities of membrane preparations from various tissues

The ability of membrane preparations from different tissues to catalyse the phosphorylation of their endogenous protein (intrinsic protein kinase activity) was determined. It was found that membrane fragments prepared from a large variety of tissues contain this activity although the actual level varies quite widely. Preparations from vas deferens and brain have nearly ten times more activity than preparations from heart, kidney, or erythrocytes. Plasma membranes from skeletal muscle have no detectable activity. The intrinsic protein kinase activity of membrane fragments from most tissues is stimulated by cyclic AMP although the phosphorylation of proteins in preparations of kidney microsomes or heart plasma membranes, is not affected. cyclic GMP (10 micronM) has no effect on the intrinsic protein kinase activity of any membrane preparation examined. A specific inhibitor of soluble, cyclic AMP-stimulated, protein kinase has no effect on the intrinsic protein kinase activity of any of the membrane preparations examined. This suggests that the intrinsic protein kinase activity of membrane preparations may be due to the presence of a specific protein kinase. It is suggested that an examination of the distribution of membrane-bound intrinsic protein kinase activity among different tissues may be helpful in determining the function of the reaction.

Renal handling of oxalate. A micropuncture study in the rat

Clearance and micropuncture experiments were performed in rats to study the renal handling of oxalate. The 14C-oxalate to 3H-inulin clearance ratio (Cox/Cin) was 1.36 +/- 0.04 and was lowered by probenecid (200 mg/kg) to 1.11 +/- 0.03 (+/- S.E., n = 6, P less than 0.005). An attempt was made to localize the assumed secretion of oxalate in three different micropuncture protocols. In free flow micropuncture experiments single nephron clearance of oxalate was not different when obtained from proximal or distal tubular puncture sites. The fractional delivery of oxalate averaged 0.84 +/- 0.03 regardless of the puncture site from mid-proximal to late distal. This finding indicates a net outflux of oxalate in an early proximal loop since oxalate is freely ultrafilterable. In microperfusion experiments the mean recovery of oxalate ranged from 79--90%. The outflux of oxalate correlated linearly with the tubular load (r = 0.95). The results suggest that no net secretion occurs in superficial nephron segments accessible for micropuncture. Since whole kidney clearances of oxalate always exceeded glomerular filtration rate, it is concluded that net addition of oxalate into the tubular fluid can occur at sites beyond the superficial late distal tubules or is due to higher delivery of oxalate by deep cortical nephrons.

Enzymic analysis of endomyocardial biopsy specimens from patients with cardiomyopathies

Myocardial biopsies have been obtained from patients with hypertrophic or congestive cardiomyopathies. Marker enzymes for the principal subcellular organelles of the myocardium were estimated using highly sensitive assay procedures. The results were compared with those obtained in tissue from patients with valvular heart disease with good or poor left ventricular function. Left ventricular myocardial tissue from patients with hypertrophic cardiomyopathy showed essentially normal levels of enzymic activities. In congestive cardiomyopathy, right ventricular tissue showed reduced levels of mitochondrial enzymes with increased levels of lactate dehydrogenase. Left ventricular tissue from patients with congestive cardiomyopathy showed reduced levels of mitochondrial and myofibril enzymes but high levels of lactate dehydrogenase. The reduced levels of myofibril Ca++-activated ATP in congestive cardiomyopathy is similar to that found in patients with impaired left ventricular function secondary to valvular disease. It is suggested that defective mitochondrial function is a characteristic feature of congestive cardiomyopathy and that the increased levels of lactate dehydrogenase reflect a compensatory response.

Enzymic analysis of cardiac biopsy material from patients with valvular heart-disease

Cardiac tissue obtained by left-ventricular endomyocardial biopsy from patients with valvular heart-disease was assayed for marker enzyme activities of subcellular organelles and these were correlated with left ventricular function as assessed by haemodynamic studies. In patients with poor left ventricular function, calcium-dependent adenosine-triphosphatase (A.T.P.ase) activity, predominantly localised to the myofibrils, was strikingly reduced. Activity of lactate dehydrongenase, a cytosol enzyme, was significantly increased in tissue from patients with poor left ventricular function. The activity of enzymes associated with sarcolemma (5'-nucleotidase), mitochondria (glutamate dehydrogenase and monoamine oxidase), microsomes (neutral alpha-glucosidase), and lysosomes (acid phosphatase, N-acetyl-beta-glucosaminidase) was no different in patients with good or poor left ventricular function. It is suggested that reduced myofibrillary A.T.P.ase concentration is the biochemical basis for the impaired ventricular function.

Calcium-accumulating properties of subcellular fractions of bovine vascular smooth muscle

This study examined the calcium transport properties of two subcellular fractions of bovine aorta obtained by differential centrifugation. The vesicular fraction had a high-affinity (Km = 1.05 X 10(-6)M) calcium transport mechanism which could be potentiated by using the calcium-precipitating anion, oxalate. The mitochondria-enriched fraction's calcium transport system had a lower affinity for calcium than did that of the vesicular fraction. The calcium capacity of the vesicular fraction was determined by comparing the steady-state calcium uptake in the presence of oxalate in the whole homogenate with the same uptake by the vesicular fraction alone. A calcium capacity of about 100 mumoles Ca2+/kg aorta was obtained. It is concluded that the vesicular fraction has all of the properties of a major site of subcellular calcium regulation in vascular smooth muscle.

Effects of angiotensin, catecholamines and cyclic AMP on calcium storage in aortic microsomes

1. The binding of calcium observed in microsomal membranes derived from the intimal-medial layer of rabbit aorta was specifically increased by the magnesium salt of adenosine triphosphate (Mg-ATP). This calcium uptake was inhibited by potassium and sodium.2. Angiotensin II, both in the presence and in the absence of Mg-ATP, reduced the binding of calcium and increased the release of membrane-incorporated calcium. These effects were dose-dependent. Analogues of angiotensin II devoid of intrinsic activity, failed to alter the release of calcium.3. Dibutyryl-cyclic AMP increased the binding of calcium and reduced the rate of its release. The maximal effect was observed at the concentration of 10(-5)M.4. (-)-Noradrenaline reduced the binding of calcium; this effect was inhibited by phenoxybenzamine. Conversely, adrenaline like cyclic AMP, increased the calcium binding; the effect of adrenaline was suppressed by a beta-adrenoceptor blocking agent.5. These observations demonstrate the existence of membranes in rabbit aorta capable of storing calcium. Excitatory drugs seem to affect directly and specifically the binding and release of calcium in these membranes.

Incorporation of the terminal phosphate of ATP into membranal protein of rabbit cardiac sarcoplasmic reticulum. Correlation with active calcium transport and study of the effects of cyclic AMP

The transfer of the terminal phosphate of ATP to a material from the sarcoplasmic reticulum of cardiac muscle which can be precipitated by trichloroacetic acid was studied, and the relationship of this biochemical event to active Ca 2+ transport was examined. A component of the phosphoryl transfer reaction was stimulated specifically by Ca 2+ . Both the phosphorylation of the reticulum and the active sequestering of Ca 2+ were proportional to the Ca 2+ concentration between 10 -7 and 10 -5 M. The time course of both phenomena was similar. These and other observations relating Ca 2+ -dependent phosphorylation of the membrane of the sarcoplasmic reticulum with the activesequestering of Ca 2+ suggest that the phosphoryl transfer reaction may represent the formation of a carrier system which facilitates the inward flux of Ca 2+ against a concentration gradient. Cyclic AMP, theophylline, or the combination of these two agents did not influence the rate or extent of the phosphorylation of the sarcoplasmic reticulum. These results have led us to question the hypothesis that the cardiac sarcoplasmic reticulum is the site of action of cyclic AMP in its role as a mediator of cardiac inotropic effects.