Effects of sodium nitroprusside and nitroglycerin on tension prolongation of cat papillary muscle during recovery from hypoxia

Relation of cyclic nucleotide ratios to ischemic and reperfusion injury in nitric oxide-donor treated rat hearts

Nitric oxide (NO) donors given during ischemia possibly protect the myocardium by increasing tissue cyclic guanosine monophosphate (cGMP) and decreasing cytosolic Ca2+ levels. However, NO donors also elevate ischemic cyclic adenosine monophosphate (cAMP) levels, which exacerbates ischemic-reperfusion injury. The authors propose that suppression of this NO donor-induced increase in cAMP would improve the cardioprotective properties of these compounds. Langendorff perfused rat hearts were treated with sodium nitroprusside (SNP, 0.1 mM ) or glyceryl trinitrate (GTN, 1.0 microM ) and/or adenylyl cyclase (SQ, 50 microM ) or guanylyl cyclase (ODQ, 30-300 microM ) inhibitors during 40-min low-flow (0.2 ml/min) ischemia. Control reperfusion rate-pressure product (RPP) recoveries were 47 +/- 3% (n = 9) and improved to 59 +/- 1% (n = 11) (p < 0.05) with SNP treatment. Ischemic ODQ treatment decreased RPP recovery to 33 +/- 3% (n = 10) (p < 0.05). ODQ eliminated the cardioprotective effects of SNP (RPP recovery: 40 +/- 5% [n = 7] vs. 59 +/- 1% [p < 0.05]). Adenylyl cyclase inhibition improved RPP recovery from 59 +/- 1% (SNP) to 72 +/- 4% (SNP + SQ) (n = 11) (p < 0.05). The authors conclude that (a) suppression of the NO donor-induced elevations in ischemic cGMP levels (ODQ) worsened reperfusion RPP, (b) suppression of the NO donor-induced elevation in ischemic cAMP levels (SQ) further improved reperfusion RPP in NO donor-treated hearts, and (c) the severity of ischemic-reperfusion injury in the NO donor-treated heart was inversely related to ischemic-tissue cGMP levels and often directly related to the ischemic-tissue cAMP-to-cGMP ratio.

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Effects of isosorbide-5-mononitrate and isosorbide-2-mononitrate on the contractile and electrical activity and on the content of cyclic nucleotides in isolated heart muscles of the guinea-pig and dog

Isosorbide-5-mononitrate and isosorbide-2-mononitrate, the metabolites of isosorbide dinitrate, were studied for their effects on the contractile and electrical activity and on the content of cyclic nucleotides in isolated heart muscles of guinea-pig and dog. Isosorbide-5-mononitrate at all concentrations tested and isosorbide-2-mononitrate at low concentrations did not produce significant changes in the mechanical activity of guinea-pig heart preparations. Isosorbide-2-mononitrate in concentrations above 10(-4) M inhibited the contractility and shortened the action potential of guinea-pig and dog heart muscle. Neither mononitrate had an effect on cAMP concentration but both induced a significant elevation of cGMP content in guinea-pig heart tissue, the effect of isosorbide-2-mononitrate being greater.

Differential effects of reoxygenation on intracellular calcium and isometric tension

We used the bioluminescent Ca2+ indicator, aequorin to record intracellular calcium transients during reoxygenation of hypoxic ferret ventricular muscle in order to determine whether alterations in the amplitude and time course of isometric contraction are mediated by changes in [Ca2+]i. Papillary muscles less than or equal to 1 mm in diameter were removed from the hearts of male ferrets and perfused with a bicarbonate-buffered physiologic salt solution at 30 degrees C. Muscles were stimulated to contract isometrically at 0.33 Hz and were loaded with aequorin by a chemical procedure. Hypoxia was induced by changing the gas mixture bubbling the perfusate to 95% N2, 5% CO2; reoxygenation was accomplished by switching the gas mixture to 95% O2, 5% CO2. Hypoxia produced a decrease in peak Ca2+ and tension that was reversed by reoxygenation. However, the effects on tension of changes in oxygenation were greater than expected from the degree of change in [Ca2+]i. The time courses of the Ca2+ transient and isometric twitch moved in opposite directions and were respectively prolonged/abbreviated by hypoxia and abbreviated/prolonged by reoxygenation. These results indicate that changes in the amplitude and time course of the isometric twitch induced by hypoxia and reoxygenation cannot be attributed alone to changes in intracellular Ca2+ availability and are caused in part by a significant decrease in the calcium sensitivity of the contractile apparatus.

A mechanism for the nitroglycerin-induced downward shift of the left ventricular diastolic pressure-diameter relation

Nitroglycerin has been shown to cause a downward shift in the left ventricular (LV) pressure-volume relation in patients. To test the hypothesis that this shift is mediated by an alteration in pericardial pressure, 13 patients undergoing diagnostic cardiac catheterization were studied. LV and right ventricular (RV) pressure (micromanometers) and LV diameter (2-dimensional echocardiography) were measured simultaneously before and after sublingual administration of 0.3 to 0.6 mg of nitroglycerin. In the 11 patients with hemodynamic effects from nitroglycerin, mean LV end-diastolic pressure decreased from 12.7 +/- 5 mm Hg (mean +/- standard deviation) to 7.3 +/- 3 mm Hg (p less than 0.002) and mean RV end-diastolic pressure declined from 7.7 +/- 3 mm Hg to 5.0 +/- 1 mm Hg (p less than 0.001). However, nitroglycerin caused only a slight (6%) reduction in LV minor axis diameter, from 52 +/- 8 mm to 49 +/- 9 mm (p less than 0.05). Diastolic pressure-diameter plots constructed from early and late diastolic measurements demonstrated a downward shift in the relation. However, when RV end-diastolic pressure was used as an estimate of pericardial pressure (a procedure validated by studies in our laboratory), the transmural pressure-diameter points before and after administration of nitroglycerin defined a single curve. These observations are in keeping with the conclusions that nitroglycerin did not alter the elastic properties of the myocardium and that the decrease in LV end-diastolic pressure induced by nitroglycerin was primarily attributable to a reduction in external constraint.

Mechanisms of hypoxia-induced decrease of load dependence of relaxation in cat papillary muscle

Relaxation of mammalian cardiac muscle is very sensitive to the prevailing load, but becomes largely load-independent during hypoxia. This effect was previously ascribed to a delayed removal of activating myoplasmic calcium. To further elucidate the underlying mechanisms of this effect of hypoxia, relaxation was now studied in 26 cat papillary muscles, in which hypoxia-induced decrease of load dependence of relaxation was compared with the effects of low [Ca2+]o (1.0, 0.5, 0.375 mM), verapamil (1 microM) and nifedipine (0.1 microM). Load dependence of relaxation was quantified by comparing force and time coordinates at the onset of the isometric relaxation phase in several after-loaded isotonic twitch contractions with the relaxation of the isometric control contraction. Hypoxia, low [Ca2+]o, verapamil and nifedipine decreased load dependence of relaxation. Although low [Ca2+]o, verapamil and nifedipine had a more marked negative inotropic effect on the contraction phase than hypoxia, the decrease of load dependence with hypoxia was significantly more pronounced and it included two phases: an early and fast drop, followed by a slower and longer-lasting decrease. The early fast phase was neutralized in low [Ca2+]o and also diminished after administration of verapamil or nifedipine. An impaired calcium reuptake by the sarcoplasmic reticulum would underly only the second phase of decreased load dependence. The first phase on the other hand originated mainly from changes in the isometric relaxation of the papillary muscles.

Variable effects of intracoronary nitroglycerin on myocardial blood flow and segmental shortening according to dose and degree of coronary occlusion

The effects of intracoronary nitroglycerin were determined in 21 open-chest dogs. Six dogs with severe stenosis (25 mm Hg diastolic perfusion pressure) and nine dogs with moderate stenosis (40 mm Hg) received a 12 micrograms bolus followed by 44 micrograms/min intracoronary nitroglycerin. In addition, six dogs with moderate stenosis (40 mm Hg) received 5 micrograms followed by 5 micrograms/min nitroglycerin. Myocardial blood flow was measured with radioactive microspheres and segment shortening with ultrasonic crystals. At 40 mm Hg, high-dose but not low-dose nitroglycerin raised epicardial blood flow, while at 25 mm Hg nitroglycerin had no effect. Subendocardial blood flow was not affected in any group. Partial occlusion resulted in a decrease in segment shortening in the 25 mm Hg group but not at 40 mm Hg. High-dose nitroglycerin had no effect on shortening at either level of occlusion. Thus, in the presence of coronary vascular reserve, high-dose nitroglycerin may overcome coronary autoregulation. A dose equivalent to one that simulates the amount of nitroglycerin delivered to the coronary circulation by a systemic infusion did not affect myocardial blood flow. In addition, even a large dose of nitroglycerin did not affect segment shortening.

Effect of isosorbide dinitrate and its metabolites on rabbit atrial myocardial force-frequency relationships in vitro

Isosorbide dinitrate (ISDN) and its two mononitrate metabolites were studied for their effects on the force-frequency relationship of isolated rabbit left atrial preparations. ISDN (4.9 and 15 microgram/ml) (50-150 times maximum therapeutic plasma concentrations) produced no significant effect on the frequency-dependent changes in contractile force. A concentration of 90 micrograms/ml (approximately 900 times therapeutic concentrations) produced a depression of contractility normally observed at frequencies of 80-400 beats/min. The two mononitrate metabolites of ISDN (endo- and exo-nitratoisosorbide) (90 micrograms/ml and 450 micrograms/ml) failed to alter the normal-force-frequency relationship over the range of 10-400 beats/min. These concentrations are several thousand times greater than plasma levels achieved in man with therapeutic dosing of ISDN. The basic lack of effect of ISDN and its metabolites on this fundamental property of myocardial contractility under the test conditions suggests that their therapeutic action is not mediated through direct effects on myocardial excitation-contraction coupling mechanisms.

Comparative effects of nitroglycerin and nifedipine on myocardial blood flow and contraction during flow-limiting coronary stenosis in the dog

Both nifedipine and nitroglycerin are used to treat angina pectoris. The comparative effects of these agents on myocardial blood flow and contraction in the setting of flow-limiting coronary stenosis are poorly understood. Thus 24 open chest dogs underwent carotid to left anterior descending coronary arterial perfusion with coronary flow probe and perfusion pressure monitoring. Segment length was measured with ultrasonic crystals in the subendocardial ischemic and nonischemic zones. Myocardial blood flow was measured with radioactive microspheres. Partial coronary occlusion was performed to attain a diastolic perfusion pressure of 40 mm Hg. Twelve dogs received intravenous nifedipine, 3 micrograms/kg per min, and 12 received intravenous nitroglycerin to reduce aortic pressure by 20 mm Hg. Partial occlusion resulted in a slight but significant decrease in segment shortening in the ischemic zone. Neither nitroglycerin nor nifedipine affected shortening in the ischemic zone. After occlusion, blood flow decreased in the subendocardial ischemic zone but was unchanged in the subepicardium. Nifedipine increased subendocardial blood flow in the nonischemic zone and decreased it in the ischemic zone but caused no change in subepicardial flow in the ischemic zone. In contrast, nitroglycerin decreased subendocardial and subepicardial blood flow in both the ischemic and nonischemic zones. In the setting of coronary stenosis, different classes of vasodilators may have varying effects on myocardial blood flow, suggesting different sites and mechanisms of action. In addition, segment function may not always reflect changes in myocardial blood flow.

Cardiovascular management in acute hypoxemic respiratory failure

This paper reviews recent data concerning the interactions among pulmonary edema, intrapulmonary shunt and cardiac output in acute hypoxemic respiratory failure. In canine oleic acid edema, a 5 mm Hg reduction in pulmonary wedge pressure significantly reduces edema, but a corresponding increase in colloid osmotic pressure does not. When pulmonary wedge pressure is lowered, cardiac output can be maintained with infusions of nitroprusside, dopamine or dobutamine. Each vasoactive agent improves ventricular pumping function, and the increase in cardiac output is due in part to peripheral circulatory actions of the drugs. Although pulmonary shunt increases with these vasoactive agents, increased shunt is due to their pulmonary vasoactivity but to the associated increase in pulmonary blood flow. Positive end-expiratory pressure reduces venous return by raising right atrial pressure, and it does not depress ventricular pumping function. Rather, positive end-expiratory pressure increases ventricular filling pressure t a given end-diastolic volume; it does not reduce and probably increases edema, yet it reduces shunt by redistributing the edema. These interpretations suggest several goals for cardiovascular management in acute hypoxemic respiratory failure: (1) the lowest pulmonary wedge pressure consistent with adequate cardiac output; and (2) the least positive end-expiratory pressure consistent with saturation of adequate circulating hemoglobin on nontoxic inspired oxygen.

Treatment of acute low pressure pulmonary edema in dogs: relative effects of hydrostatic and oncotic pressure, nitroprusside, and positive end-expiratory pressure

Severe pulmonary edema sometimes develops despite normal pulmonary capillary wedge pressure (Ppw). The equation describing net transvascular flux of lung liquid predicts decreased edema when hydrostatic pressure is reduced or when colloid osmotic pressure is increased in the pulmonary vessels. We tested these predictions in a model of pulmonary capillary leak produced in 35 dogs by intravenous oleic acid. 1 h later, the dogs were divided into five equal groups and treated for 4 h in different ways: (a) not treated, to serve as the control group (Ppw = 11.1 mm Hg); (b) given albumin to increase colloid osmotic pressure by 5 mm Hg (Ppw = 10.6 mm Hg); (c) ventilated with 10 cm H(2)O positive end-expiratory pressure (Peep) (transmural Ppw = 10.4 mm Hg); (d) phlebotomized to reduce Ppw to 6 mm Hg; (e) infused with nitroprusside, which also reduced Ppw to 6 mm Hg. Phlebotomy and nitroprusside reduced the edema in excised lungs by 50% (P< 0.001), but Peep and albumin did not affect the edema. Pulmonary shunt decreased on Peep and increased on nitroprusside, and lung compliance was not different among the treatment groups, demonstrating that these variables are poor indicators of changes in edema. Cardiac output decreased during the treatment period in all but the nitroprusside group, where Ppw decreased and cardiac output did not. We conclude that canine oleic acid pulmonary edema is reduced by small reductions in hydrostatic pressure, but not by increased colloid osmotic pressure, because the vascular permeability to liquid and protein is increased. These results suggest that low pressure pulmonary edema may be reduced by seeking the lowest Ppw consistent with adequate cardiac output enhanced by vasoactive agents like nitroprusside. Further, colloid infusions and Peep are not helpful in reducing edema, so they may be used in the lowest amount that provides adequate circulating volume and arterial O(2) saturation on nontoxic inspired O(2). Until these therapeutic principles receive adequate clinical trial, they provide a rationale for carefully monitored cardiovascular manipulation in treating patients with pulmonary capillary leak.

Haemodynamic effects of combined nitroglycerin- and dobutamine-infusions after coronary by-pass surgery. With one nitroglycerin-related complication

The haemodynamic effects of separate and simultaneous nitroglycerin (NTG)--and dobutamine (DOB) infusions were determined in order to evaluate the nature of the effects of combined treatment. Ten patients were studied, 6 h after coronary by-pass surgery. NTG alone reduced significantly systemic and pulmonary arterial, as well as ventricular filling pressures, and brought about a concomitant 10% decrease in the cardiac index (P < 0.05). Dobutamine (6 microgram/kg/min) did not affect systemic or pulmonary arterial pressures, nor ventricular filling pressures. As a result of the 20 beats/min rise in heart rate produced by DOB, the cardiac index was increased by 24%, whereas the stroke volume and work indices remained unchanged during DOB-infusion. The haemodynamic changes brought about by either NTG or DOB separately were unaffected by the simultaneous infusion of the other vasoactive drug. The haemodynamic effects of the combined treatment were simply the sum of their separate effects, thereby yielding reduced arterial and ventricular filling pressures, increased heart rate, and slightly augmented cardiac index with significantly reduced systemic vascular resistance; myocardial oxygen consumption was unaffected by the combination therapy. One patient, whose case is discussed separately, had a hypotension-bradycardia complication during NTG-infusion.

Contractile state of the left ventricle in man as evaluated from end-systolic pressure-volume relations

End-systolic pressure (PES), volume (VES), wall tension (TES) and circumference (CES) of the human left ventricle were studied at cardiac catheterization in 24 subjects with varying degrees of left ventricular dysfunction. Acute alterations in systolic load consistently resulted in changes in VES and CES, with a smaller volume and circumference characterizing the lower systolic load in each subject. End systolic pressure-volume lines were constructed by plotting PES against VES at the higher and lower systolic load in each subject. The slope of the resultant lines was considerably steeper for normal than for poorly contractile left ventricles. Vo, the volume axis intercept of the line (i.e., the theoretical VES at PES = O) was significantly smaller for normal than for poorly contractile ventricles. Similar findings were noted for Co, the theoretic end-systolic circumference at zero end-systolic ventricular wall tension. Post-extrasystolic potentiation resulted in decreased VES and CES with no change in PES and only a slight fall in TES. In conclusion, end-systolic pressure-volume and tension-circumference relations reflect the contractile state of left ventricular myocardium. Quantitation of these relationships may provide a useful new approach to the assessment of myocardial function in man.

Effects of sodium nitroprusside on left ventricular diastolic pressure-volume relations

The effect of sodium nitroprusside on the relationship between left ventricular pressure and volume during diastole was studied in 11 patients with congestive heart failure. Nitroprusside was infused to lower mean arterial pressure approximately 20-30 mm Hg. High fidelity left ventricular pressures were recorded in all patients simultaneously with left ventricular cineangiography (biplane in eight and single plane in three patients), allowing precise measurement of pressure and volume throughout the cardiac cycle. Left ventricular diastolic pressure-volume curves were constructed in each patient from data obtained before and during nitroprusside infusion. In 9 of 11 patients there was a substantial downward displacement of the diastolic pressure-volume curve during nitroprusside infusion, with left ventricular pressure being lower for any given volume with nitroprusside. Serial left ventricular cineangiograms performed 15 min apart in six additional subjects who did not receive sodium nitroprusside showed no shift in the diastolic pressure-volume relation, indicating that the shift seen with nitroprusside was not due to the angiographic procedure itself. A possible explanation for the altered diastolic pressure-volume relationships with nitroprusside might be a direct relaxant effect of nitroprusside on ventricular muscle, similar to its known relaxant effect on vascular smooth muscle. Alternatively, nitroprusside may affect the diastolic pressure-volume curve by affecting viscous properties or by altering one or more of the extrinsic constraints acting upon the left ventricle.