Myocardial energy production and consumption remain balanced during positive inotropic stimulation when coronary flow is restricted to basal rates in rabbit heart.

The effect on myocardial energy balance of increasing oxygen demand without altering basal myocardial perfusion rate was assessed in isolated, isovolumic, retrograde blood perfused rabbit hearts. Myocardial energy requirements were increased with paired stimulation. The capacity of rapid paired stimulation to increase mechanical energy consumption was demonstrated in the presence of increased perfusion with the rate X pressure product and oxygen consumption increasing 86 and 148%, respectively, compared with control values. In contrast, rapid paired stimulation under constant, basal flow conditions did not alter the rate X pressure product, while oxygen extraction and consumption increased only 40% relative to control. Myocardial ATP, creatine-phosphate, and lactate content were identical under control and constant flow-paired stimulation conditions. The results of this study indicate that no detectable energy imbalance was produced by rapid paired stimulation with flow held constant at basal rates. These results suggest that the myocardium does not increase mechanical energy expenditure in response to inotropic or rate stimulation in the presence of restricted flow reserve and are inconsistent with the concept of "demand-induced" or "relative" myocardial ischemia.

Effect of temperature on sodium-calcium exchange in sarcolemma from mammalian and amphibian hearts.

We have investigated temperature dependence of Ca2+ uptake by the cardiac sarcolemmal Na(+)-Ca2+ exchanger from dog, rabbit and bullfrog. In native rabbit sarcolemmal vesicles, Ca2+ affinity of the Na(+)-Ca2+ exchanger is unchanged from 7 to 37 degrees C; however, the initial velocity of Ca2+ uptake declines much more steeply below 22 degrees C than above 22 degrees C. In native dog sarcolemma, the temperature dependence of Na(+)-Ca2+ exchange velocity is similar to that of native rabbit. However, in frog heart the velocity of Na(+)-Ca2+ exchange declines much more slowly with decreasing temperature at both temperature ranges. Reconstitution of the Na(+)-Ca2+ exchanger into artificial lipid vesicles consisting of either asolectin or phosphatidylserine, phosphatidylcholine, and cholesterol has little effect on temperature dependence of Na(+)-Ca2+ exchange velocity in any of the three species. We conclude that the lesser temperature sensitivity of the cardiac sarcolemmal Na(+)-Ca2+ exchanger of a poikilothermic species is at least partly an intrinsic property of the transport protein.

Molecular cloning and functional expression of the guinea pig cardiac Na(+)-Ca2+ exchanger.

The cDNA of the guinea pig cardiac Na(+)-Ca2+ exchanger was cloned from a lambda ZAP cDNA library. The deduced sequence of the protein corresponds to 970 amino acids and is 98% identical to the canine cardiac exchanger. The leader peptide region shows substantial variation among species. The cloned cDNA can induce Na(+)-Ca2+ exchange activity when in vitro transcribed cRNA is injected into Xenopus laevis oocytes.

Influence of cardiac motion on Doppler measurements using in vitro and in vivo models.

OBJECTIVES: Using both in vitro and in vivo techniques, we investigated the extent to which cardiac motion alters Doppler-measured blood flow velocity and thus potentially can alter the calculation of valve areas or pressure gradients. BACKGROUND: Blood flow velocity measured by Doppler ultrasound represents the net motion of the blood relative to the transducer. It is widely assumed that the measured velocity represents the actual flow. It has been demonstrated that cardiac motion generates regularly occurring low velocity Doppler signals that are commonly treated as artifact. METHODS: We used an in vitro model that allowed us to measure and independently control the flow of a liquid through a chamber and the motion of the chamber relative to the Doppler beam. A cornstarch-water slurry was driven by a pulsatile pump through tubing to simulate the blood flow within the heart, and the tubing was cyclically moved by a piston to simulate the heart motion. We also measured cardiac motion using M-mode and two-dimensional echocardiography and compared the results with the Doppler signal derived from cardiac motion in subjects without cardiac disease. RESULTS: In the in vitro model, alteration in the motion of the tubing resulted in apparent changes in the measured maximal velocity of the fluid. The Doppler spectrum of the combined motion of the tubing and the fluid was the algebraic sum of their Doppler signals. In human subjects, the maximal slope of the M-mode tracing of the aortic annular motion and the peak Doppler signal due to cardiac motion were compared and were highly correlated. CONCLUSIONS: Cardiac motion alters the Doppler signal derived from blood flow. This effect can be demonstrated in vitro and in vivo.

Measurement of sarcolemmal vesicle orientation by beta-adrenergic receptor binding.

To assess the orientation (inside-out vs. outside-out) of purified cardiac sarcolemmal vesicles, we developed a new method utilizing the known outward-facing binding site of the beta-adrenergic receptor. We compared the binding of the lipid-insoluble ligand 3H-CGP-12177, which binds to beta-adrenergic receptors on outside-out sarcolemmal vesicles only, to the binding of the lipid soluble ligand 125I-iodocyanopindolol, which binds to beta-adrenergic receptors in sarcolemmal vesicles of either orientation. The ratio of CGP to ICYP binding is equal to the fraction of outside-out sarcolemmal vesicles. Sidedness measurements by beta-adrenergic receptor-binding showed similar mean values but less scatter than sidedness assessments by measurement of 3H-ouabain-binding or Na+,K(+)-ATPase activity in the presence or absence of membrane permeabilizing agents.

Sodium pump inhibition in sarcolemma from ischemic hearts.

Ischemic myocardial cells lose K+ and accumulate Na+. The role of the Na+/K(+)-pump in these changes was investigated by measuring both Na+/K(+)-ATPase activity and Na+ pumping in highly purified sarcolemmal vesicles from rabbit hearts made globally ischaemic for 1 h compared to non-ischemic controls. Purification of the sarcolemma was similar for control, 31 +/- 8-fold, and ischemia, 38 +/- 10-fold. The fraction of intact inside-out vesicles, in which Na+ pumping could be measured, was also the same for control, 60 +/- 16%, and ischemic, 56 +/- 8% as measured by 3H-ouabain binding in the presence and absence of detergent. Scatchard analysis of ouabain binding revealed a 26% increase in binding sites in ischemia compared to control. The Na+/K(+)-ATPase in the inside-out vesicles, measured as monensin-stimulated activity, was not affected by ischemia: 22 +/- 9 v 21 +/- 9 mumol Pi mg -1 h -1 for control and ischemic respectively. However, the initial velocity of ATP-dependent Na+ pumping into inside-out vesicles, assayed by subsequent exchange of Na+i for 45Ca2+ by the Na(+)-Ca2+ exchanger present in the vesicles was inhibited in ischemia. At 18 mM Na+o the velocity for control vesicles was 2.4 +/- 0.2 nmol mg -1 s -1 compared to 1.1 +/- 0.1 for ischemia vesicles. Passive sarcolemmal Na+ permeability was unchanged after 1 h of ischemia. The large reduction in Na+ pumping with unchanged Na+/K(+)-ATPase suggests uncoupling of the Na+/K(+)-pump in ischemia and a decreased ability to extrude Na+ despite the increase in number of pump sites in the sarcolemma.

Verapamil protection of ischemic isolated rabbit heart: dependence on pretreatment.

Verapamil may protect ischemic myocardium by several mechanisms: prevention of Ca overload as a direct effect of blocking Ca influx through slow channels, coronary vasodilatation, decreased contractility, or cardioplegia produced by high doses. We manipulated the experimental situation to ask whether the first mechanism alone could be protective. We studied isovolumically contracting rabbit hearts perfused at 37 degrees C, paced at 150/min, and maximally vasodilated by dipyridamole. Hearts were subjected to 60 min of low flow ischemia followed by 60 min reperfusion. Two groups were exposed to verapamil 0.5 microM beginning either 2 to 4 min before ischemia or 10 min after the onset of ischemia (when pressure development had ceased) and continuing until reperfusion. Developed pressure recovered during reperfusion to 70 +/- 4% of its initial value in hearts treated with verapamil before ischemia compared to 40 +/- 5% for control hearts and 35 +/- 11% for hearts treated with verapamil 10 min after the onset of ischemia. There was significant preservation of phosphocreatine at 10 min of ischemia and of ATP at 60 min in the early verapamil group compared to the other two. When verapamil was present before ischemia, pressure development during early ischemia was reduced to about 50% of control. Consequently there was substantial sparing of high energy phosphates and enhanced recovery of mechanical function. If verapamil was added 10 min after the onset of ischemia, when it no longer could affect cardiac work, there was no protection. Therefore, in the isolated rabbit heart, verapamil had an important protective effect only by reducing contractility of ischemic myocardium.

Effects of physical training on end-diastolic volume and myocardial performance of isolated rat hearts.

We studied the performance of ventricular muscle and cardiac function of hearts from rats conditioned by swimming (CH) and from sedentary rats (SH) in an isolated working heart apparatus modified to measure end-diastolic volume by dye dilution. Instantaneous aortic flow, left ventricular (LV) pressure and oxygen consumption were measured. Heart rate and mean aortic pressure were kept constant, and atrial filling pressure was varied from 5 to 20 cm H2O. Heart weights of SH and CH were equal and end-diastolic pressures and volumes were similar at all atrial pressures. However, ejection fraction, calculated circumferential fiber velocity, peak systolic pressure, peak aortic flow, cardiac output, and stroke work were all greater in CH than in SH, and the differences increased as atrial pressure was increased. Maximal negative dP/dt was greater in CH than SH at all preloads (P less than 0.001). Oxygen consumption of CH was increased in proportion to the increase in work. These results indicate that the improved pumping performance of CH is due to a change in ventricular muscle function. Faster relaxation is a prominent effect of physical training on the rat heart and may foster more complete filling at high heart rates.

Lysophosphatidylcholine and sodium-calcium exchange in cardiac sarcolemma: comparison with ischemia.

Lysophosphoglyceride accumulation in ischemic myocardium has been hypothesized to be a mechanism for altered sarcolemmal properties that underlie electrophysiological changes and Ca2+ accumulation in ischemia. We find that in vitro application of lysophosphatidylcholine to normal canine sarcolemmal vesicles at a concentration of 0.3 mumol/mg sarcolemmal protein inhibits Na(+)-Ca2+ exchange. Both maximum velocity (Vmax) for Ca2+ transport and Ca2+ affinity are reduced by lysophosphatidylcholine, whereas in ischemia only Vmax is reduced [M. M. Bersohn, K. D. Philipson, and J. Y. Fukushima. Am. J. Physiol. 242 (Cell Physiol. 11): C288-C295, 1982]. This amount of lysophosphatidylcholine does not affect sarcolemmal passive permeability to either Ca2+ or Na+. Treatment of sarcolemma with phospholipase A2 sufficient to inhibit Na(+)-Ca2+ exchange velocity by 50% causes large increases in sarcolemmal lysophosphatidylcholine and lysophosphatidylethanolamine. On the other hand, 1 h of ischemia in rabbit hearts does not affect sarcolemmal phospholipid composition. Thus, although in vitro treatment with lysophosphatidylcholine or phospholipase A2 has profound effects on sarcolemmal properties, sarcolemmal accumulation of lysophosphatidylcholine cannot account for the effects of ischemia as measured in highly purified sarcolemmal vesicles from ischemic hearts.

Sarcolemmal calcium transporters in myocardial ischemia.

To determine the potential pathophysiologic role of alterations in sarcolemmal Ca2+ transport mechanisms, we investigated the effects of up to 120 min of global ischemia in the rabbit heart on the three major Ca2+ transport proteins in the sarcolemma: the Na+-Ca2+ exchanger, the ATP-dependent Ca2+ pump, and the L-type Ca2+ channel. We purified sarcolemmal vesicles from control rabbit hearts and rabbit hearts made ischaemic for 20, 30, 60, 90, and 120 min. Purification of K+-p-nitrophenylphosphatase activity was about 30-fold compared to the initial homogenate, and was the same for control and ischemic hearts. We measured the initial velocity of Na+-Ca2+ exchange and found no inhibition after 20 min of ischemia, a 22% reduction in Vmax after 30 min of ischemia, and approximately a 50% reduction in Vmax after 60, 90, and 120 min of ischemia. At no time was there any change in the Ca2+ affinity of the Na+-Ca2+ exchanger. Solubilization and reconstitution of the Na+-Ca2+ exchanger into asolectin vesicles restored the velocity to the same level as control reconstituted vesicles after 60 min of ischemia, but not after 90 or 120 min of ischemia. In contrast to Na+-Ca2+ exchange, the initial velocity of the sarcolemmal ATP-dependent Ca2+ pump was unaffected by up to 2 h of ischemia. The number of L-type Ca2+ channels, measured by nitrendipine binding, was reduced by 21% after 60 min of ischemia. Decreased Ca2+ efflux due to direct inhibition of the Na+-Ca2+ exchanger, as well as inhibition by low pH and increased intracellular Na2+ in ischemia, probably contribute to Ca2+ overload and irreversible myocyte injury. Conversely, decreased Ca2+ influx due to decreased availability of L-type Ca2+ channels, as well as decreased capacity for reversed Na+-Ca2+ exchange, could contribute to contractile dysfunction during ischemia and myocardial stunning following reperfusion.

Sodium-calcium exchange and sarcolemmal enzymes in ischemic rabbit hearts.

We have investigated alterations in sarcolemmal function that occur during myocardial ischemia. Rabbit ventricles were incubated at 37 degrees C for time periods ranging from 5 min to 2 h. The ischemic tissue was homogenized, and activities of the sarcolemmal enzymes Na+-K+-ATPase, K+-p-nitrophenylphosphatase (K+-pNPPase), and adenylate cyclase were measured in the crude homogenate. Na+-K+-ATPase and K+-pNPPase were substantially inhibited after only 10 min of ischemia, and activities for all three enzymes declined progressively up to 1 h of ischemia, when activities were 37-59% of control. Highly purified sarcolemmal membranes prepared from control tissue and myocardium that had been made ischemic for 1 h showed similar purification of sarcolemmal enzymes, passive Ca2+ binding, and passive permeability to Ca2+. However, the velocity of Na+-Ca2+ exchange in ischemic sarcolemmal vesicles was reduced approximately 50% due to a reduction in Vmax. Although the parallel decline in activities of several sarcolemmal functions might suggest a change in membrane structure, phospholipid and cholesterol contents in ischemic sarcolemma were the same as control.

Effect of increased magnesium on recovery from ischemia in rat and rabbit hearts.

Perfusates containing high magnesium concentrations have been suggested to have a protective effect for ischemic myocardium, but the mechanism for such an effect is unclear. We investigated the recovery of isolated perfused rabbit and rat hearts from ischemia under varied conditions of increased Mg. Hearts were made ischemic in the presence of normal 1.2 mM Mg or elevated 15 mM Mg. Rabbit hearts, which show minimal alteration in contractility in the presence of 15 mM Mg, were not protected from ischemia by high Mg perfusate. Rat hearts, which have a large negative inotropic response to 15 mM Mg, exhibited significantly better recovery of mechanical function following ischemia in the presence of high Mg than following ischemia with normal Mg. This protective effect was abolished by increasing both Ca and Mg in the perfusate to prevent the decline in contractility that normally occurred with Mg. Reperfusion with 15 mM Mg after ischemia also had no protective effect if the rat heart had been made ischemic in the presence of normal Mg. We conclude that elevated Mg protects ischemic myocardium only under circumstances in which it has a negative inotropic effect before the onset of ischemia, i.e., in the rat heart perfused with normal Ca. These results suggest that the mechanism of protection by high Mg involves sparing of ATP. However, the different responses to Mg of the species studied in these experiments should be a caution against extrapolating such results from rat hearts to other species.

Effects of pH on Na+-Ca2+ exchange in canine cardiac sarcolemmal vesicles.

Using highly purified sarcolemmal vesicles isolated from dog ventricles, we examined the effects of pH on Na+-Ca2+ exchange. The initial rate of Nai+-dependent Ca2+ uptake is a sigmoid function of pH. The Ca2+ uptake is inhibited at pH 6 and stimulated at pH 9 (as compared with uptake at pH 7.4). This dependence on pH suggests that the ionization state of a histidine residue may be important in Na+-Ca2+ exchange. The effects of H+ on Nai+-dependent Ca2+ uptake are partially competitive with Ca2+, although this relationship is complex. Nao+-dependent Ca2+ efflux is also sensitive to H+ and increases monotonically with pH. These effects of pH appear to be due to intrinsic interactions with the Na+-Ca2+ exchange system and are not due to an alteration of Na+-H+ exchange or membrane permeability. The effects of pH on vesicular Na+-Ca2+ exchange are apparent only at low Ca2+ and Na+ concentrations. Thus modulation of vesicular Na+-Ca2+ exchange by pH is manifest only under ionic conditions which exist intracellularly in intact myocardium. Since the negative inotropy caused by acidosis is thought to reflect a fall in internal pH, these results suggest that alteration of sarcolemmal Ca2+ transport (medicated by Na+-Ca2+ exchange) by internal pH may contribute to the regulation of myocardial contractility by pH.

Placement of a defibrillation lead in the left subclavian vein from the right cephalic vein.

This case report highlights the feasibility and stability of transvenous placement of a second defibrillation lead in the left subclavian vein from a right cephalic vein approach. This was undertaken in a right-sided implant of an active can cardioverter defibrillator to lower defibrillation thresholds that would have otherwise precluded implant.

Synthesis and use of radio cobaltic EDTA as an extracellular marker in rabbit heart.

A new gamma-labeled marker for extracellular space is the cobaltic form of 58Co-ethylenediaminetetraacetic acid (58Co-EDTA). The cobaltic ion has a much higher affinity for EDTA than the cobaltous ion; it is prepared as a potassium salt, K+(58Co3+-EDTA4-), and is apparently biologically inert. Testing by equilibration in intact rabbits and comparing the myocardial content with that of [14C]sucrose give values of the volume of distribution in the myocardium of 0.294 +/- 0.052 ml/g for 58Co-EDTA and 0.303 +/- 0.051 ml/g for [14C]sucrose (SD, n = 130, for two hearts), with the ratios of 58Co-EDTA/sucrose averaging 0.973 +/- 0.043 (n = 130). The average value of the extracellular fluid measured in isolated rabbit interventricular septum using Co-EDTA was 0.51 +/- 0.05 ml/g (SD, n = 16) and 0.46 +/- 0.04 ml/g using [14C]sucrose as an extracellular fluid space (ECF) marker. Flushing with a high concentration of nontracer Co-EDTA does not reveal any release from binding sites. The gamma-energy (811 KeV), long half-life (71.4 days), stability, and lack of binding to tissue components make 58Co-EDTA a useful marker for ECF.

Mechanically mediated pacemaker pseudomalfunction.

A case is described of pacemaker pseudomalfunction caused by intermittent displacement of the ventricular lead. The displacement was shown to be caused by critically timed atrial contractions, dislodging the looped ventricular lead, and resulting in intermittent failure to stimulate the heart.

Amiodarone decreases Na,K-ATPase alpha 2 and beta 2 expression specifically in cardiac ventricle.

Studies have suggested that chronic amiodarone treatment (a class III antiarrhythmic) may have effects on the myocardium that resemble those of hypothyroidism. Since hypothyroidism is associated with decreased expression of Na,K-ATPase alpha 1, alpha 2 and beta 1 subunits in heart and alpha 2 subunit in skeletal muscle, we aimed to test the hypothesis that chronic in vivo administration of amiodarone would result in changes in the expression of Na,K-ATPase isoforms that resembled those seen in hypothyroid rat. Rats were treated with 40 micrograms/g body weight amiodarone for 3 and 6 weeks. Na,K-ATPase alpha 1, alpha 2, beta 1 mRNA and protein levels and beta 2 mRNA were measured in cardiac ventricle and skeletal muscle. After 3 weeks of amiodarone cardiac alpha 1, alpha 2 and beta 1 protein levels were decreased to 0.65, 0.42, and 0.54 of control, respectively. After 6 weeks of treatment, cardiac alpha 1 and beta 1 levels returned to control and alpha 2 remained depressed. At the mRNA level alpha 2 and beta 2 decreased to 0.6-fold of control after 6 weeks treatment. There was no effect of amiodarone on skeletal muscle Na,K-ATPase expression at either time point. We conclude that the effects of amiodarone on myocardial Na,K-ATPase expression are similar to those of hypothyroidism after 3 weeks of amiodarone treatment, but by 6 weeks the similarities are limited to the decrease in alpha 2 and beta 2 expression.(ABSTRACT TRUNCATED AT 250 WORDS)