Advances in myocardiology最新文献

Our awareness of the importance of Na-Ca exchange in cardiac muscle has progressed from early observations of Na-Ca antagonism in the activation of contractile force. This was followed by demonstrations of actual Na-Ca ion countertransport across cell membranes and later functional studies in which manipulation of intracellular and extracellular Na and Ca concentrations has permitted a better characterization of the exchange process and its contribution to contractile force. The recent development of vesicle preparations from cardiac sarcolemmal membranes has, despite some drawbacks, produced useful information on the electrogenicity of the exchange mechanisms and on the relative affinity of the exchange carrier compared to the ATPase-driven Ca pump. These studies confirmed earlier estimates of the approximate exchange ratio of the Na-Ca countertransport system and have demonstrated its large maximum transport rate capabilities. The application of ion-sensitive microelectrodes in recent years has enabled measurements of the actual ion-activity gradients across the sarcolemmal membrane. These activity gradients together with the membrane potential control the rate and direction of the Na-Ca exchange. Despite the wide range of techniques employed to tackle the problem, the exchange ratio of Na to Ca movement is still in some doubt, with most estimates ranging between 5:2 and 4:1.

Using an isolated perfused coronary-artery preparation, we have demonstrated the ability of endothelium inhibit markedly to vasomotion in rabbit coronary arteries. Using a bioassay system, we have shown this effect to be mediated via the release of an unstable humoral agent (t 1/2 approximately equal to 6 sec) from endothelial cells, and we have partially characterized its chemical nature.

The purine nucleotide cycle catalyzes the net reaction: aspartate + GTP + H2O----fumarate + NH3 + GDP + Pi. The cycle leads to regeneration of AMP. In skeletal muscle the cycle's rate of operation increases severalfold in response to a corresponding increase in work load. This results in a net increase in citric-acid-cycle intermediates and in release of ammonia. The same may be expected in heart muscle, which, like skeletal muscle, possesses the enzymes of the purine nucleotide cycle. Isolated and working rat hearts were therefore perfused for 45 min at low or high work load (0.16 vs. 0.42 kg X m/min per g dry wt.) with glucose (5 mM) as substrate. Release of ammonia into the perfusate as well as the content of citric-acid-cycle intermediates (citrate, isocitrate, 2-oxoglutarate, malate, and oxaloacetate), related amino acids (aspartate, glutamate, and glutamine), adenine nucleotides, and creatine phosphate were measured in freeze-clamped tissue. There was no significant change between low and high work load in the sum of the citric-acid-cycle intermediates (1.295 vs. 1.313 mumole/g dry wt.), in aspartate (13.21 vs. 14.32 mumole/g dry wt.), in glutamate (15.58 vs. 15.67 mumole/g dry wt.), ATP (19.06 vs. 19.17 mumole/g dry wt.), ADP (5.00 vs. 4.11 mumole/g dry wt.), AMP (1.45 vs. 1.00 mumole/g dry wt.) or in creatine phosphate (22.58 vs. 25.80 mumole/g dry wt.). Ammonia release was 26 and 22 mumole/hr per g dry wt. at low and high work load, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism by which calmodulin stimulates Ca2+ transport in cardiac microsomal preparations enriched in sarcoplasmic reticulum (SR) was investigated. Under incubation conditions in which the majority of the phosphoprotein formed was Ca2+-dependent and no phospholamban phosphorylation was observed (10 degrees C, 15-sec incubations in the presence of 2 microM ATP), calmodulin was found to have no effect on the steady-state level of the acylphosphate phosphorylation site of Ca2+-ATPase. A significant stimulation of Mg2+, Ca2+-ATPase activity by calmodulin and a 3-fold increase in the turnover of the Ca2+ pump were, however, observed. As the ATP concentration in the incubation media was elevated (20 and 200 microM ATP), a significant degree of phosphoprotein formed was observed to be cyclic AMP (cAMP)-dependent. The degree of Ca2+-dependent phosphorylation remained constant. Under these conditions, calmodulin had no effect on the degree of phosphoprotein formed. However, when the experiments were conducted at 30 degrees C for 5 min in the presence of 500 microM ATP, a significant amount of the phosphoprotein formed was calcium-calmodulin-dependent and was additive to phosphoprotein formation observed in the presence of cAMP-dependent protein kinase. The ratio of calcium-calmodulin-dependent to cAMP-dependent phosphorylation was 1:1. K+ (110 mM) decreased the levels of phosphorylation observed in the presence of calcium and calmodulin, but had less of an effect on the levels observed in the presence of cAMP-dependent protein kinase. Autoradiographic analysis of SR membranes labeled with [32P]-ATP revealed two protein bands (24,500 and 40,000 daltons) phosphorylated in the presence of added calcium and calmodulin that were not observed in the absence of either of these additions to the reaction media. These results suggest that calmodulin stimulates Ca2+ transport by a direct effect on the Mg2+, Ca2+-ATPase. An indirect effect on Ca2+ transport via a calcium-calmodulin-dependent protein kinase, though, cannot be ruled out.

The increase in tissue and coronary effluent adenosine concentration in hearts undergoing net ATP breakdown results from an accelation of adenosine formation and not from an inhibition of adenosine inactivation. Adenosine formation takes place inside intact isolated cells by a pathway distinct from the cell membrane 5'-nucleotidase, which hydrolyzes only extracellular AMP. Both the magnitude and the variation in the rate of adenosine formation in polymorphonuclear leukocytes undergoing ATP catabolism can be accounted for by the properties of a cytosolic 5'-nucleotidase that is also present in heart. This enzyme, which is allosterically activated by ATP-Mg and inhibited by Pi, provides a direct biochemical link between the energy status of the cell and the rate of adenosine formation. The actions of adenosine to dilate coronary arterioles, antagonize the inotropic effect of catecholamines, and reduce sympathetic-nerve firing would ameliorate the original energy imbalance. Adenosine may therefore function in heart and also in brain, skeletal muscle, kidney, and adipose tissue as a "retaliatory metabolite" that protects the cell against excessive external stimulation.

Intracellular Na and pH were measured with recessed-tip ion-selective microelectrodes in voltage-clamped sheep cardiac Purkinje fibers. Intracellular Na activity (aiNa) was elevated by inhibiting the Na/K pump. This produced an increase of twitch tension that had a steep dependence on the increase of aiNa. These effects of aiNa on twitch tension are probably mediated by an Na-Ca exchange. An increase of aiNa also produced a component of tonic tension that appears to be produced directly by the Na-Ca exchange. The dependence of tonic tension and aiNa on membrane potential suggests that this exchange process may be voltage-sensitive. The increase of aiNa is associated with an intracellular acidification that appears to be secondary to an increase of [Ca2+]i produced by Na-Ca exchange. Therefore, as well as affecting [Ca2+]i, Na-Ca exchange can under some circumstances influence pHi indirectly, and this complicates the interpretation of changes in tension, since protons and Ca ions have opposite effects on contractile force.

Although insulin is known to elicit a positive inotropic effect in cardiac-muscle preparations, little is known concerning the mechanism responsible for this action. Because various subcellular organelles such as mitochondria, sarcoplasmic reticulum (SR), sarcolemma, and myofibrils are intimately involved in determining the cardiac contractile function, the effects of insulin (0.1-1000 mU/ml) on some selected enzymatic activities associated with these organelles were investigated. Insulin significantly enhanced Ca2+ uptake and Ca2+-stimulated ATPase activities of SR preparations obtained by two different methods. Insulin had no effect on mitochondrial Ca2+ uptake and ATPase activities or myofibrillar ATPase activities. Sarcolemmal Na+, K+-ATPase activity was stimulated only in the presence of 1 U/ml insulin, whereas sarcolemmal Ca2+ pump activity was increased by all insulin concentrations employed. Sarcolemmal Ca2+-ATPase activity and ATP-independent Ca2+ binding were augmented in the presence of 1 U/ml insulin only. These subcellular effects of insulin, either alone or in concert, may partially explain the positive inotropic action of insulin.

The extracellular matrix of heart muscle contains a considerable variety of structures. We have systematically studied the morphology of these structures using several methods of fixation and microscopy. Endomysial connections between cells are comprised of struts of collagen [1] as well as combinations of elastin fibers, collagen fibers, and microfibrils. The rest of the extracellular matrix is filled with a polyanionic lattice of unit collagen fibrils, microthreads, and granules. In the course of these investigations, we have observed regions of structural continuity across the sarcolemma, from endomysial collagen struts to Z-bands. We have also correlated the mechanical resistance to stretch with orientation of epimysial collagen fibers and sarcomere lengths in living as well as fixed rat papillary muscles. Our observations suggest that the extracellular skeletal framework plays an important role in normal cardiac function.

There is a complex extracellular structural matrix in the heart. This matrix appears to be composed of a variety of fibrils and fibers extending from the cell surface to the basal lamina and from the basal lamina to the matrix. The extensions into the extracellular region interconnect with a system of collagen bundles. The latter are so located that they would tether the myocytes to each other as well as tether the capillaries to the myocetes. There is an extensive weave of collagen analogous to the perimysium of skeletal muscle that separates groups of myocytes. The weave surrounding a group of myocytes is connected to adjacent weave patterns by long, tendonlike structures. The collagen matrix around cells disappears 2-3 hr after coronary-artery occlusion. In the periinfarct region of viable cells, the matrix is similarly lost and is replaced by scarlike collagen. Encephalomyocarditis virus causes a similar loss of the matrix in necrotic as well as some adjacent nonnecrotic regions. Replacement of the lost matrix is by scar tissue. The long-term appearance of the replacement fibrosis closely resembles the appearance of diffuse fibrosis as seen in a variety of conditions. These observations suggest that diffuse fibrosis can occur secondary to loss of the matrix both with and without myocyte necrosis. This may help explain the diffuse left ventricular fibrosis as seen in a variety of human disease.