Myocardial Cell Membrane Research Articles

Effect of Astragalus Membranaceus on Ca2+ influx and CVB3 RNA replication in cultured neonatal rat heart cells infected with CVB3

Here was investigated the effect of Radix Astragalus Membranaceus (AM) on Ca2+ influx across the myocardial plasma membrane and coxsackie virus B3(CVB3)-RNA replication in cultured neonatal rat heart cells infected with CVB3. It was found that the Ca2+ influx could be inhibited significantly by AM both in heart cells infected with CVB3 for 48 hours and in normal control heart cells. In addition, the Ca2+ influx and the amounts of CVB3-RNA in myocytes simultaneously infected with CVB3 and treated with AM for 48 hours were statistically decreased compared with that in CVB3-infected control cells. These phenomena suggested that AM could exert the effects of decreasing the secondary Ca2+ damages, improving the abnormal myocardial electric activity and inhibiting replication of CVB3-RNA in myocardium. Thus, it is a rational choice to treat patients with AM in viral myocarditis.

Expression of the subtype 1A dopamine receptor in the rat heart.

The subtype 1A dopamine receptor (D1A) has recently been detected in the rat kidney. In the present study using light microscopic immunohistochemistry, electron microscopic immunocytochemistry, and in situ amplification of mRNA, we demonstrate the D1A receptor in Sprague-Dawley and Wistar Kyoto rat hearts. For immunohistochemistry and immunocytochemistry, anti-peptide polyclonal antibodies were directed toward amino acid sequences of the third extracellular and intracellular domains of the native receptor. Selectivity was validated by recognition of the D1A receptor expressed in stably transfected LTK- cells. D1A receptor mRNA was detected with a novel transcription-based isothermal in situ amplification system as well as with reverse transcription-polymerase chain reaction. D1A receptor protein was distributed throughout the atrium and ventricular myocardium. Preimmune and preabsorption controls were negative. Electron microscopic immunocytochemistry using the protein A gold method demonstrated the D1A receptor along the cellular membranes of coronary smooth muscle cells and ventricular myocytes and in the myosin thick filaments and M-lines. D1A receptor mRNA was present in coronary vessels and myocardium in amplified but not in unamplified sections. Western blot analysis showed specific D1A bands in transfected LTK- cells and the atrium but not in nontransfected LTK- cells and the ventricle. The selective D1-like receptor agonist SKF38393 stimulated adenylyl cyclase in ventricular myocardial plasma membranes in a dose-related fashion, and the response was abolished by the selective D1-like receptor antagonist SCH23390. These results demonstrate that the D1A receptor gene and protein are expressed in normal rat heart. The physiological and pathophysiological roles and predominant cell signaling mechanism or mechanisms of this receptor remain to be determined.

Myocardial rubidium-82 tissue kinetics assessed by dynamic positron emission tomography as a marker of myocardial cell membrane integrity and viability.

Recent reports have demonstrated the clinical use of rubidium-82 chloride (Rb-82) in combination with positron emission tomography (PET) not only as a tracer of myocardial blood flow but also as a marker of cell membrane integrity using static imaging early and late after tracer injection. The purpose of this study was to compare myocardial Rb-82 kinetics assessed by dynamic PET imaging as a marker for tissue viability with regional fluorine-18 fluorodeoxyglucose (FDG) uptake in patients with coronary artery disease. Twenty-seven patients with angiographically proven coronary artery disease and 5 subjects with a low likelihood for coronary artery disease underwent dynamic PET imaging under resting conditions using Rb-82 and FDG. Both image sequences served as input data for a semiautomated regional analysis program. This program generated polar maps representing Rb-82 tissue half-life and FDG utilization assessed by Patlak's approach. Myocardial tissue viability was visually determined from static Rb-82 and FDG images. Regions were categorized as normal, ischemically compromised, and scar tissue. Their coordinates were subsequently copied to the functional polar maps for further analyses. In normal subjects, Rb-82 tissue half-life was homogeneous throughout the left ventricle (90 +/- 11 seconds). In coronary patients, differences between Rb-82 tissue half-lives in normal and scar tissue were highly significant (95 +/- 10 and 57 +/- 15 seconds, respectively; P < .0001). FDG uptake in these two tissue groups was 78 +/- 12% and 40 +/- 13%, respectively (P < .0001). Ischemically compromised tissue with reduced perfusion but maintained FDG uptake displayed an Rb-82 half-life of 75 +/- 9 seconds, indicating active cellular tracer retention, which was significantly different from scar tissue. Overall agreement of tissue categorization as either viable or scar was 86% between Rb-82 kinetics and FDG utilization. In a subgroup of 11 patients with all three tissue types within one image set, Rb-82 tissue half-life discriminated between normal, ischemic, and scar tissue (97 +/- 9, 75 +/- 9, and 60 +/- 15 seconds, respectively; P < .01). This study demonstrated a significant relationship between cell membrane integrity as assessed by dynamic Rb-82 PET imaging and myocardial glucose utilization as a marker for tissue viability. In regions with reduced perfusion, Rb-82 kinetics was different in compromised but metabolically active and irreversibly injured myocardium. The predictive value of this approach must be evaluated in follow-up studies.

Experimental Study on Microthrombi and Myocardial Injuries

The experiment was conducted with SD male rats. After they had been each given an intravenous injection of high molecular weight dextran (0.8 ml/100 g body wt) once a day for 4 days, they were brought under the observation of ECG and mesentery microcirculation. Microthrombi were found in the venules and capillaries of each rat of the experimental group, while in the microcirculation of the control group rats, no microthrombi were found. No changes were found in the ECGs of the rats (n= 6) in the control group after the injections, while the rats in the subject group all suffered a rise in the S-T segment of ECG, an indication of myocardial injuries. The rise was significantly in positive correlation to the increase in microthrombi in number (r= 0.944,P< 0.01). The erythrocytes of the rats in the subject group clustered to become rouleau-like, and platelets aggregated by tens and hundreds to form microthrombi. Their blood also showed a significant decrease in number of platelets. The degree of platelet aggregation and the scores of the rise on ECG were significantly in positive correlation as shown by the results:y= 20 + 94x,r= 0.94,P< 0.01. The plasma TXB2of the subject group increased obviously but the change of 6-K-PGF1αin the blood was not significant. The content of plasma TXB2and the scores that indicated the rise in the S-T segment of the ECG showed significantly a positive correlation by the analysis of linear regression equation:y= 109.997 ± 116.25x,r= 0.889,P< 0.05. The activity of Na+-K+-ATPase on the myocardial cell membranes of the rats with microthrombi was significantly reduced as compared with that of the rats in the control group (P< 0.01). The activity of the Na+-K+-ATPase was significantly in negative correlation to the rise in the S-T segment of ECG (P< 0.05). This study demonstrates that the above changes are the causes of myocardial injuries in rats with circulatory thrombi.

Alterations in G-protein-regulated transmembrane signalling induced in murine myocardium by coxsackievirus B3 infection

Cardiomyopathy is usually associated with marked alterations in myocardial transmembrane signalling. Although acute viral myocarditis may result in chronic cardiomyopathy in some cases, the possible consequences of viral infection on function of the myocardial signal-transducing complex have not been explored. Therefore, the present study was designed to investigate the G-protein-regulated adenylyl cyclase signalling system in murine myocardium during myocarditis induced by coxsackievirus B3 (CVB3) infection. We examined the functional characteristics of adenylyl cyclase as well as the function and distribution of beta-adrenoceptors, m-cholinoceptors and G-proteins in myocardial plasma membranes isolated from the hearts of mice with acute (7 days pi) or late phase (21 days pi) myocarditis and the obtained results were compared with the corresponding data determined in age-matched controls. While the basal adenylyl cyclase activity was not significantly altered, the ability of forskolin, sodium fluoride and GTP gamma S to activate adenylyl cyclase was lowered by about 20% in samples from virus-infected animals. The level of Gs alpha in myocardial plasma membranes as well as the functional activity of Gs alpha was not affected by viral infection, but the Gi alpha content was increased by about 20%. The total number of beta-adrenoceptors in myocardial plasma membranes increased by about 12-15% due to higher content of the beta 2-adrenoceptor subtype. Although the agonist-binding parameters of beta-adrenoceptors were not significantly altered, the ability of these receptors to mediate stimulation of adenylate cyclase was markedly diminished (by 56-80%). The total number of m-cholinoceptors in samples derived from virus-infected mice increased considerably (by 29-59%) and a significant proportion of the receptors shifted to a higher affinity status, but their ability to transduce agonist signals was impaired. These data are the first to demonstrate that several different sites of the myocardial G-protein-regulated adenylyl cyclase signalling complex are significantly altered in acute as well as in late phase of CVB3-induced myocarditis.

Isolation of myocardial membrane long-chain fatty acid-binding protein: Homology with a rat membrane protein implicated in the binding or transport of long-chain fatty acids

Abnormal myocardial long-chain fatty acid uptake is suspected of being involved in certain types of heart disease, but the mechanism by which the heart takes up long-chain fatty acids remains unclear. The sulfo-N-succinimidyl derivatives of long-chain fatty acids have been reported to undergo covalent binding to a membrane protein and to irreversibly inhibit the transport of long-chain fatty acids by rat adipocytes (Harmon et al., 1991). It has been suggested that the membrane protein bound by these derivatives is a candidate transporter for long-chain fatty acids in adipocytes. However, myocardial membrane long-chain fatty acid-binding proteins have not yet been fully investigated. Rat hearts were isolated and perfused with a sulfo-N-succinimidyl derivative of tritium-labeled palmitate ([3H]SSP). Then the [3H]SSP-binding protein was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) autoradiography and histological autoradiography. Myocardial palmitic acid uptake was examined after pretreatment of isolated perfused rat hearts with SSP. The SSP-binding protein was isolated from bovine hearts by successive chromatography, and the amino acid sequences of lysylendopeptidase-digested peptide fragments were determined. SDS-PAGE autoradiography revealed that [3H]SSP bound to an 85-90 kDa protein derived from the myocardial microsomal fraction, and histological autoradiography demonstrated that [3H]SSP radioactivity was localized to the myocardial cell membrane. Pre-incubation with SSP inhibited palmitic acid uptake by isolated perfused rat hearts. A [3H]SSP-binding protein was also found in canine and bovine hearts, and was isolated from the bovine cardiac membrane fraction. Amino acid sequencing revealed that four peptide fragments showed strong sequence homology with rat adipocyte membrane protein, which is implicated in the binding or transport of long-chain fatty acids (Abumrad et al., 1993). We conclude that the SSP-binding protein is localized to the myocardial cell membrane and might be involved in the uptake or transport of long-chain fatty acids.

Effect of hydralazine on myocardial plasma membrane fatty acid binding protein (PM-FABP) during diabetes mellitus.

Chronic treatment with the antihypertensive drug hydralazine did not affect the hyperglycemic state of streptozotocin (STZ)-diabetic rats but did prevent the serum hyperlipidemia that is synonymous with these diabetic animals. After 6 weeks, untreated STZ-diabetic rats exhibited a 659% increase in serum triglycerides and 292% increase in serum cholesterol versus age-matched non-diabetic rats. Hydralazine-treated STZ-diabetic rats had serum triglyceride and cholesterol levels that did not differ from controls. Myocytes from control rats showed a preference for binding of the unsaturated fatty acid analog cis-parinaric acid vs the saturated fatty acid analog trans-parinaric acid. This preference was not altered in STZ-diabetic rat myocytes; hydralazine-treatment of STZ-diabetic rats also showed no change in fatty acid preference. STZ-diabetes caused a decrease in the affinity (Kd) for the trans, but not the cis-parinaric acid. However, total binding of both analogs was increased in STZ-diabetes. Hydralazine treatment of STZ-diabetic rats resulted in even greater total binding of both analogs. Affinity for the trans analog was further decreased in these hydralazine-treated rats, but the affinity for the cis analog was increased beyond control animals. These results suggest that the diabetic state influences the binding characteristics of the myocardial PM-FABP and that hydralazine, while reducing serum lipids in diabetes, has complex effects on the fatty acid binding by this protein.

Multiple Laminin Binding Proteins in Human Fetal Heart: (fetal heart/laminin binding proteins/cation dependent protein/cation independen protein/developmental changes).

The possibility of occurrence of laminin binding proteins in cardiac tissue under different stages of growth was examined by affinity chromatography of the soluble fraction of human fetal myocardial plasma membrane over Ln-Sepharose. A 67 kDa protein was isolated by elution with glycine/HCl buffer containing 1 M NaCl and visualized as a coomassie stainable band on SDS gel electrophoresis under reducing conditions. Dot blot assays of the radioiodinated protein revealed the binding of 67 kDa protein with high affinity to laminin in a cation independent manner. This protein appears to be present in relatively higher amounts in tissues from early stage fetus. The occurrence of cation dependent laminin binding proteins was also examined by affinity chromatography. Electrophoresis of the EDTA eluate under reducing conditions followed by silver staining showed two prominent bands with average molecular size 130 and 174 kDa which under non-reducing conditions appeared as two bands with average molecular weight of 115 and 135 kDa. Using radioiodinated protein in dot blot assays, its binding to Ln was found to be maximum in the presence of Mn++ ions. Immunoblotting using anti-β1 integrin antibodies showed that 115 kDa protein is a β1 integrin suggesting the possibility of this protein belonging to the integrin group of receptors. The occurrence of multiple laminin binding proteins and the relative abundance of one of these proteins viz. the 67 kDa protein during early stages than in late stage tussue suggest a possible role for these proteins in cellular interactions with laminin during myocardial tissue development.

Effects of fatty acids on myocardial calcium control during hypothermic perfusion

Although hypothermia is regarded as providing protection of the myocardium during cardiac operations, rapid cooling of the myocardium in the nonarrested state may have detrimental effects on the function of the myocardial cell membrane as a permeability barrier. We therefore measured total cellular calcium in isolated working rat hearts, receiving either glucose (11.1 mmol/L) or glucose plus palmitate (1.2 mmol/L), before, during, and after a 40-minute hypothermic arrest (10° C, Langendorff perfusion). In both groups a rise in total cellular calcium, measured by45Ca2+ technique, was observed during hypothermia, followed by a decline on rewarming. However, the rise in total cellular calcium during hypothermia was significantly (p < 0.05) higher in hearts perfused with palmitate (from 1.0 ± 0.2 to 3.5 ± 0.2 nmol/mg dry weight) compared with that in glucose-perfused hearts (from 1.1 ± 0.13 to 2.6 ± 0.2 nmol/mg dry weight). Palmitate-perfused, but not glucose-perfused, hearts showed arrhythmias and delayed pressure development 1 to 2 minutes after rewarming. In addition cardiac output of these hearts was significantly lower (p < 0.025) than that of glucose-perfused hearts 5 to 10 minutes after rewarming. These data show that hypothermia per se causes a net calcium uptake in isolated rat hearts and that this effect is aggravated by high concentrations of fatty acids. Thus the impaired recovery of myocardial function in palmitate-perfused hearts can possibly be related to a distorted calcium handling. (J THORAC CARDIOVASC SURG 1994;107:233-41)

Ethmozine and ethacizine--new antiarrhythmic drugs with defibrillating properties.

Ventricular fibrillation (VF) is a life-threatening arrhythmia that leads to death unless electrical defibrillation is applied in time. Recent publications indicate that VF can be either sustained (SVF), requiring electrical defibrillation, or transient (TVF), reverting spontaneously into sinus rhythm. Since VF cannot be totally prevented by drugs, a new antiarrhythmic therapeutic approach has been proposed: drug-induced enhancement of the ability of the heart to defibrillate by itself. In this study we examined the defibrillating potency of two antiarrhythmic phenothiazines, ethmozine (ETM) and ethacizine (ETA), as well as their effects on catecholamine uptake and on the electrophysiological properties of the myocardial cell membrane. The antiarrhythmic-defibrillatory activity was examined in cats; the inhibitory effect on [3H]-norepinephrine (NE) uptake was examined in rat brain synaptosomes, and the electrophysiological membrane effects were examined by microelectrode recordings in perfused strips of heart ventricle from guinea-pigs. The results indicate that: 1. ETA exhibits similar but stronger antiarrhythmic-defibrillating and NE reuptake inhibitory effects than ETM; 2. ETA at 10-6 M decreases ventricular conduction time and increases Vmax while ETM at this concentration does not change them; 3. The defibrillating ability of the drugs can be related to their inhibitory potency on NE reuptake. We suggest that the risk of sympathomimetic arrhythmogenicity is prevented by the previously described, membrane stabilizing Class 1 antiarrhythmic properties of these drugs.

Smoke-induced lung tissue changes

The aim of this study was to investigate the changes of myocardial Gsα mRNA expression and ATPase and the effects of Panax notoginseng saponins (PNS) on that after burns in rats. Wistar rats were exposed to a 95 °C water bath for 10 s to produce 30% TBSA skin-full-thickness scalds. Myocardial Gsα mRNA level, cAMP content and adenyl cyclase (AC) activities were determined with dot blotting hybridization, radioimmunoassay and indirect method, respectively. The ATPase activities in plasma membrane of cardiomyocytes and erythrocytes were measured colorimetrically. At 3, 6, 9 hours after scalds, the myocardial Gsα mRNA expression decreased significantly (P<0.01). PNS (100, 200 mg kg−1, i.p.) markedly increased these levels (P<0.01). The elevation was correlated significantly with PNS dose (r=0.95, P<0.05). At 3, 6, 9, 12, 24, 48 hour after the scalds, the myocardial cAMP content and AC basal activity was reduced significantly. PNS (100, 200 mg kg−1, i.p.) increased cAMP content and enhanced AC activity markedly in comparison with the 3rd hour postburn group. The activities of (Ca2+–Mg2+)-ATPase and (Na+–K+)-ATPase in plasma membrane of myocardial cells and red blood cells in scald group were significantly lower than those in the normal control group (P<0.01). PNS (100 mg kg−1, i.p.) improved these indexes significantly after scalds (P<0.01 or 0.05). These data suggested that the effects of PNS on myocardium in burned rats involved its action to increase myocardial Gsα mRNA expression and AC activity, cAMP content as well as ATPase activities.

Functions of myocardial plasma membrane of model animal with Keshan disease

Functions of myocardial plasma membrane of model animal with Keshan disease

Histochemical investigation of horse radish peroxidase (HRP) penetrated myocardial plasma membrane

Histochemical investigation of horse radish peroxidase (HRP) penetrated myocardial plasma membrane

Cardiac contractile effects of ethanolism and hemorrhagic shock.

Moderate ethanol consumption, associated with cardiac depression, occurs in greater than 50% of trauma. Hemorrhagic shock, a significant component of trauma in the clinical setting, causes intrinsic cardiac contractile dysfunction. In this study, we used an isolated heart model to determine whether acute ethanolism increases the cardiovascular risk associated with hemorrhagic shock. We hypothesized that hemorrhagic shock in the acutely intoxicated subject would cause significantly greater cardiac dysfunction compared with that observed in a nonintoxicated subject. A total of 116 guinea pigs was divided into four groups: control (no ethanol, no shock), ethanol intoxication alone (1 mg/kg iv), hemorrhagic shock alone (mean arterial blood pressure, 30 mmHg for 2 h), and a combination of hemorrhagic shock plus ethanol. Half of the hearts in each group were used for isolated heart studies, and half were used to assess myocardial cell membrane integrity. Ethanol alone reduced peak isovolumic pressure by 36%, maximal rate of left ventricular pressure (LVP) rise by 27%, and maximal rate of LVP fall by 35%; however, contractile depression was significantly greater in the intoxicated, hemorrhaged, group compared with the nonintoxicated, hemorrhaged, group (P less than 0.05). Both ethanol and hemorrhage independently altered myocardial cell volume regulation; however, abnormalities in myocardial cell volume regulation induced by hemorrhage were similar in the intoxicated and nonintoxicated groups. Our data show that hemorrhagic shock causes significantly greater cardiac contractile dysfunction in the intoxicated subject.

Β-Adrenoceptor density on mononuclear leukocytes and right atrial myocardium in infants and children with congenital heart disease

Sympathetic regulation of myocardial performance has been shown to be altered in congestive heart failure. Right atrial tissue of children with severe acyanotic and cyanotic congenital heart disease (CHD) showed a significantly lower beta-receptor density than that of children with less severe defects. Since mononuclear leukocytes (MNL) contain a homogeneous population of beta 2-adrenoceptors which have similar properties to those of cardiac beta 2-adrenoceptors, they are frequently used for studying the beta-adrenergic system. In a group of 37 children with CHD of different types and severity who underwent cardiac surgery, we compared the MNL beta-adrenoceptor density to the type and severity of CHD and looked for a possible relationship to plasma catecholamine levels and to the right atrial beta-adrenoceptor density. Membranes of MNL and myocardial cells were radiolabeled with (-)3-[125I]Iodocyanopindolol [( 125I]ICYP). A significantly higher beta-adrenoceptor density on MNL was found in patients with moderate acyanotic CHD (group I) than in those with severe acyanotic (group II) and cyanotic CHD (group III). Patients of group I showed approximately 50% higher myocardial beta-receptor density than those of groups II and III. ICI 118.551-[125I]ICYP competition studies revealed that in groups II and III significantly lower proportions and densities of beta 1-receptors were found compared to group I. Noradrenaline (NA) plasma levels in group II and group III were significantly higher than those in group I. The adrenaline plasma levels were found to be very high in all children with CHD.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical markers of activation in cardiac transplant rejection

Immunocytochemical analysis of endomyocardial biopsies from cardiac transplant patients has defined changes in expression of antigens, expressed on both the myocardium and endothelium which are characteristic of rejection. Biopsies taken from normal donor heart (prior to transplantation) have been compared with biopsies showing histological signs of rejection. There is induction of MHC class I antigen on the normally negative myocardial plasma membrane and induction of the adhesion molecule ICAM-1 on the intercalating discs. Capillary endothelial cells, which constitutively express Class II DR antigen and ICAM-1 in normal heart show increased of endothelial antigens Pal-E and FVIII-RA during rejection. These results demonstrate perturbation of the endothelial system during rejection and possibly indicate damage to the capillary endothelial cells.

The pathophysiology of myocardial stunning: reversibility, accumulation and continuity of the ischemic myocardial damage after reperfusion.

In order to understand the pathophysiology of myocardial stunning, reversibility, accumulation and continuity of ischemic myocardial damage after reperfusion should be studied. Then, to analyze these three factors, myocardial function, metabolism and morphology under ischemia and reperfusion were studied in anesthetized, open-chest dogs. When myocardial ischemia was induced by occlusion of the left anterior descending coronary artery, percentage regional systolic shortening (%SS) of ischemic myocardium sharply decreased and became stable 10 min after occlusion. After reperfusion, ischemic myocardium showed active shortening after within 30-min occlusion, but did not after more than 60-min occlusion. During 90-min of ischemia, extracellular K+ concentration (Ke) steeply increased for first 10 min and was almost stable for next 10 min. Then, Ke straightly increased till 90 min. Metabolic rates, calculated from myocardial tissue CO2 and pH, steeply increased for first 20 min and sharply decreased for next 10 min. After 30 min, these two variables were almost stable, near zero. By electron-microscopy with cytochemistry, distribution of Na/K ATPase to myocardial cell membrane was observed to be almost after 15-min occlusion but distinctly sparse with destruction of cell membrane after 30-min occlusion. Therefore, irreversible myocardial damage appears after about 20-min ischemia and is almost complete after 60 min. Reversibility of damage to ischemic myocardium after reperfusion may mainly occur within 60-min ischemia. Although stunned myocardium in a narrow sense is may appear after reperfusion within less than 20-min of ischemia, stunned myocardium in a broad sense may appear within less than 60-min ischemia. When reversible myocardial ischemia (4- or 15-min occlusion) was repeated after short time intervals (20-min reperfusion), %SS of ischemic myocardium was gradually decreased with each ischemic episode. Active shortening of ischemic myocardium disappeared after more than two episodes of 15-min occlusion. Fluctuation of PCO2, pH and Ke of ischemic myocardium was gradually depressed with each occlusion. Metabolic viability of ischemic myocardium was cumulatively depressed by repeated brief occlusion. Naturally, myocardial damage was more severe after repeated 15-min occlusion than after 4-min occlusion. Accumulation of ischemic myocardial damage may arise as brief ischemia, which only induces reversible damage, is repeated. At last, continuity of ischemic myocardial damage was studied. The effect of 5-min occlusion to %SS of ischemic myocardium was apparently reversed after 90-min reperfusion. Early contractile failure was advanced even after very short duration of ischemia. Thus, myocardial function will be latently damaged.(ABSTRACT TRUNCATED AT 400 WORDS)

A model study of stability and oscillations in the myocardial cell membrane

As a step towards an improved understanding of cardiac arrhythmias caused by abnormal automaticity, we perform a stability analysis of a Hodgkin-Huxley model of the myocardial cell membrane (modified Beeler-Reuter, MBR). The bifurcation structure of the model is obtained as a function of three parameters: the intensity of an applied constant current; the potassium equilibrium potential representing the accumulation of K+ ions in the external medium; and the maximum conductance of the slow inward current mimicking the local application of catecholamines on the membrane. For a range of parameter values, the model exhibits either stable automaticity or bistability between two quiescent states or between a quiescent state and an oscillatory state. These transformations of the bifurcation structure are shown to depend on the interrelationship between three elements: the activation of the slow inward current, the region of high slope conductance of the time-independent potassium current functions, and the slow variables controlling the activation of the potassium current and the inactivation of the slow inward current. Reduced two- and three-dimensional models are shown to reproduce the main stability properties of the full MBR model and to facilitate the understanding of its dynamic behavior. The onset of instability and the oscillatory features of the MBR model are in good agreement with relevant experimental results, and possible sources of disagreement on certain points are discussed.

Properties of calcium channels in cardiac muscle and vascular smooth muscle

The voltage-dependent slow channels in the myocardial cell membrane are the major pathway by which Ca2+ ions enter the cell during excitation for initiation and regulation of the force of contraction of cardiac muscle. The slow channels have some special properties, including functional dependence on metabolic energy, selective blockade by acidosis, and regulation by the intracellular cyclic nucleotide levels. Because of these special properties of the slow channels, Ca2+ influx into the myocardial cell can be controlled by extrinsic factors (such as autonomic nerve stimulation or circulating hormones) and by intrinsic factors (such as cellular pH or ATP level). The slow Ca2+ channels of the heart are regulated by cAMP in a stimulatory fashion. Elevation of cAMP produces a very rapid increase in number of slow channels available for voltage activation during excitation. The probability of a slow channel opening and the mean open time of the channel are increased. Therefore, any agent that increases the cAMP level of the myocardial cell will tend to potentiate ISi, Ca2+ influx, and contraction. The myocardial slow Ca2+ channels are also regulated by cGMP, in a manner that is opposite to that of cAMP. The effect of cGMP is presumably mediated by means of phosphorylation of a protein, as for example, a regulatory protein (inhibitory-type) associated with the slow channel. Preliminary data suggest that calmodulin also may play a role in regulation of the myocardial slow Ca2+ channels, possibly mediated by the Ca2(+)-calmodulin-protein kinase and phosphorylation of some regulatory-type of protein. Thus, it appears that the slow Ca2+ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of extrinsic and intrinsic factors. VSM cells contain two types of Ca2+ channels: slow (L-type) Ca2+ channels and fast (T-type) Ca2+ channels. Although regulation of voltage-dependent Ca2+ slow channels of VSM cells have not been fully clarified yet, we have made some progress towards answering this question. Slow (L-type, high-threshold) Ca2+ channels may be modified by phosphorylation of the channel protein or an associated regulatory protein. In contrast to cardiac muscle where cAMP and cGMP have antagonistic effects on Ca2+ slow channel activity, in VSM, cAMP and cGMP have similar effects, namely inhibition of the Ca2+ slow channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics and pharmacodynamics of phosphodiesterase-III inhibitors

T HE NEW phosphodiesterase fraction III (PDE-III) inhibitors are unique nonglycosidic, nonsympathomimetic drugs that have both positive inotropic effects and the ability to produce vasodilation independent of the PI-adrenergic receptor.‘** Catecholamines stimulate the /3,-adrenergic receptors at myocardial cell membranes to activate intracellular adenylate cyclase. Adenylate cyclase catalyzes cyclic adenosine monophosphate (CAMP) formation, the second messenger that controls the level of calcium available for the contractile mechanism of cardiac muscle. Patients with chronic congestive heart failure (CHF) have down-regulation of the firadrenergic receptor with a decrease in receptor density and altered responses to catecholamine administration.3 Inhibition of PDE activity also increases CAMP levels by preventing its breakdown. The bipyridine derivatives (amrinone and milrinone), and the imidazolone derivatives (enoximone and piroximone), although structurally different, selectively inhibit PDE-III, a CAMPspecific PDE enzyme, to augment myocardial cellular CAMP levels and increase contractility.2 In addition to positive inotropic effects on the heart, PDE inhibition in vascular smooth muscle causes vasodilation in both arterial and capacitance beds.4.5 Amrinone and enoximone increase cardiac output (CO), decrease pulmonary capillary wedge pressure (PCWP), and decrease systemic and pulmonary vascular resistance (SVR, PVR) in patients with biventricular failure.lm6 Thus, the inodilating properties of PDE-III inhibitors present a novel approach to the management of biventricular dysfunction in cardiac surgical patients. This discussion will review the pharmacokinetics and pharmacodynamics of the two major intravenous (IV) PDE-III inhibitors, amrinone and enoximone.