Recent advances in studies on cardiac structure and metabolism最新文献

Halothane, methoxyflurane, and enflurane produce dose-dependent depression in ventricular function in the dog. Myocardial blood flow and oxygen consumption are decreased accordingly without evidence of myocardial tissue hypoxia. Low-dose fluoxene does not depress the heart, while there is less depression with high-dose fluroxene than with the other anesthetics. In spite of this depression, myocardial blood flow was unchanged, and the decreased oxygen consumption during high-dose fluroxene was a result of decreased oxygen extraction by the heart. Sympathetic nervous system stimulation produced by fluroxene anesthesia is probably responsible for these effects, but further work is necessary for confirmation of this hypothesis.

The tolerance to ischemic cardiac arrest during open-heart surgery depends on the degree of hypertrophy and on the functional impairment of the heart. The angiographically determined muscle mass is a good indicator of the susceptibility of the myocardium to ischemic injury and of its ability to quickly restore myocardial structure upon reperfusion. Tissue from extremely hypertrophied hearts exhibited numerous degenerative alterations.

Some electrical properties of right ventricles of neonatal rats and of aggregates from collagenase-dissociated cells from the same tissue are compared. The duration of the action potential does not change upon changing the stimulation frequency both in ventricles and aggregates; a decrease in temperature increases duration in both preparations to the same extent. The take-off potential and the maximal rate of rise of the action potential decrease in the same way in both preparations, while the time course of these changes is also comparable. It is concluded that the dissociation and aggregation procedure does not interfere with the membrane properties upon which the measured parameters are based; thus, aggregates are well suited for a realistic voltage-clamp analysis.

Phospholamban (molecular weight = 22,000), which serves as a regulator of Ca transport ATPase (molecular weight = 100,000) of cardiac sarcoplasmic reticulum (SR), becomes resistant to tryptic digestion upon phosphorylation by cAMP-dependent protein kinase (PK). The protective effect of phosphorylation is accompanied by persistence of the PK-induced stimulation of Ca transport. These findings indicate that structural alteration of phospholamban upon phosphorylation is closely associated with changes in the functional properties of cardiac SR. SR from fast-contracting skeletal muscle of rabbit does not contain a 22,000-dalton substrate for cAMP-dependent PK, nor is Ca transport stimulated by exogenous PK. SR preparation isolated from slow-contracting skeletal muscle of rabbit and dog contains phospholamban, and Ca transport was found to be increased by exogenous cAMP-dependent PK. In view of the distribution of phospholamban among different types of muscle, a hypothesis is presented to explain the relaxation-promoting effects of catecholamines in cardiac and slow-contracting skeletal muscle in which phospholamban is found. This may also account for the absence of a similar effect of catecholamines in fast-contracting skeletal muscle, which does not contain a similar substrate for PK.

Sarcolemma was isolated by fractionation of salt-extracted particles on two consecutive sucrose density gradients. Salt extraction of homogenates, rather than of washed particles, was found to preserve the activities of adenylate cyclase and ouabain-sensitive (Na+,-K+)-ATPase in the isolated sarcolemmal membranes. Purified sarcolemma contained substantial adenylate cyclase and guanylate cyclase activities that were stimulable by beta-adrenergic and muscarinic agonists, respectively. Significant ouabain-sensitive (Na+, K+)-ATPase activity as well as putative digitalis receptor activity was also present in sarcolemma. Cyclic nucleotide phosphodiesterases of sarcolemma, both cAMP- and cGMP-dependent, displayed positive cooperativity of substrate interactions; Ca2+ ions were found to increase the activity of the GMP-dependent enzyme.

Cardiac performance and metabolism of heart-lung preparation of rat were studied with acid, normal, and alkali perfusions. Cardiac output, glucose uptake, and myocardial content of lactate, malate, glycerophosphate, and CP were increased in alkali and decreased in acid perfusion of 20 min. On the other hand, when pH of the perfusate was abruptly changed, CP and ATP were decreased independent of the performance. FDP was high and PEP was low in acute acidifying experiments. From these findings it is concluded that cardiac performance and carbohydrate metabolism are accelerated in alkali and depressed in acid perfusion, and that myocardial metabolism could be affected by pH not only secondary to the change of performance but also by itself.

The P light chain of cardiac myosin is phosphorylated and dephosphorylated by highly specific enzymes. These reactions take place in the beating rabbit heart and there is evidence that dephosphorylation of the light chain occurs during the inotropic response produced by adrenaline. The extent of phosphorylation of cardiac troponin I is determined by the functional state of the beating heart. During perfusion of the rabbit heart the basal phosphate content of troponin I increased from the basal level of about 1.5 moles P per mole to about 2.7 moles P per mole at the height of the inotropic response to adrenaline. The three sites of phosphorylation on troponin I are probably located in the N terminal cyanogen bromide peptide of 48 residues.

Histochemistry and electron microscopic cytochemistry of cytochrome oxidase in the conduction system were studied with the 3,3'-diaminobenzidine (DAB) method in adult canine hearts. This enzyme was distinctly less active in the entire conduction system than in the working myocardium. By electron microscopy, enzymatic activity per cristal membrane was apparently similar in both specialized and working cardiocytes. However, the volume fraction of cell occupied by mitochondria and the density of cristal membranes in mitochondria were smaller in the specialized cells in the sinus node, atrioventricular (AV) node, His bundle, and Purkinje fibers. These observations define the nature of decreased histochemical activity of cytochrome oxidase in cells of the conduction system, which is caused entirely by the decrease in activity per unit of mitochondrial volume and unit of cell volume in the specialized cardiac tissues.

The heat-stable protein (protein kinase modulator), partially purified from fresh bovine heart, possessed the ability to inhibit and stimulate adenosine 3':5'-monophosphate (cAMP)-dependent protein kinase and guanosine 3':5'-monophosphate (cGMP)-dependent protein kinase activities, respectively. The inhibitory activity of protein kinase modulator on cAMP-dependent protein kinase was abolished almost completely by trypsin treatment, while the ability to stimulate cGMP-dependent protein kinase activity was resistant to trypsin. Fractionation by a linear potassium phosphate gradient on DEAE-cellulose column did not clearly separate both activities. Phosphorylation of cardiac microsomal component, "phospholamban" (molecular weight = 22,000), was inhibited almost completely by the saturating amounts of protein kinase modulator. This inhibition of phospholamban phosphorylation by protein kinase modulator was accompanied by a decreased Ca uptake rate that had been stimulated by cAMP-dependent protein kinase. These findings indicate that protein kinase modulator is functional in controlling the cAMP-dependent protein kinase-catalyzed phosphorylation of phospholamban and the rate of calcium transport, lending further support for the previously proposed mechanism, in which phospholamban is assumed to serve as a regulator of calcium transport in cardiac sarcoplasmic reticulum.