Contractility in mammalian heart muscle; calcium and osmolality

Contraction-initiated NO-dependent lymphatic relaxation: a self-regulatory mechanism in rat thoracic duct

The objectives of this study were to evaluate the physiological importance of the flow and shear generated by phasic contractions of lymphatic vessels and the mechanisms responsible for the influences of such shear on lymphatic pumping. Lymphatic segments of the rat thoracic duct were isolated, cannulated and pressurized. The diastolic diameters were measured in phasically non-active segments. The diastolic and systolic diameters, half-relaxation time (HRT), contraction frequency, ejection fraction and fractional pump flow were determined in phasically active segments. Since imposed flow was excluded, flow and shear occurred only as a result of the intrinsic contractions in phasically active segments whereas in phasically non-active segments contraction-generated flow and shear were absent. The influences of incrementally increased transmural pressure (from 1 to 5 cmH(2)O) were examined in control conditions and after NO synthase blockade (l-NAME 10(-4) m) or cyclooxygenase blockade (indomethacin 10(-5) m). The spontaneous phasic contractions produced a flow-dependent diastolic relaxation. This reduction of the lymphatic tone is a regulatory mechanism that maintains pumping in thoracic duct in an energy-saving/efficient mode: it improves diastolic filling (enhanced lusitropy - lowering HRT), makes lymphatic contractions stronger (enhanced inotropy - higher contraction amplitude) and propels more fluid forward during each contraction (elevated ejection fraction) while decreasing contraction frequency (reduced chronotropy). The findings also demonstrated that the NO pathway, not the cyclooxygenase pathway is responsible for this reduction of lymphatic tone and is the prevailing pathway responsible for the self-regulatory adjustment of thoracic duct pumping to changes in lymph flow pattern.

Hypertonic saline

A key feature in the successful resuscitation of dehydrated or endotoxemic ruminants is the total amount of sodium administered. Administration of small volumes of HS and HSD offer major advantages over large volumes of isotonic saline because HS and HSD do not require intravenous catheterization or periodic monitoring, and are therefore suitable for use in the field. Hypertonic saline and HSD exert their beneficial effect by rapidly increasing preload and transiently decreasing afterload. Contrary to early reports, HS and HSD decrease cardiac contractility and do not activate a pulmonary reflex. The osmolality of HS and HSD should be 2400 mOsm/L (7.2% NaCl solution, 8 times normal plasma osmolality). Use of HS and HSD solutions of different osmolality to 2400 mOsm/L should be avoided at all costs, as too low a tonicity removes the main advantages of HS (low cost, decreased infusion time), whereas too high a tonicity may cause rapid vasodilation and decreased cardiac contractility, resulting in death. Rapid administration (> 1 mL/kg-1/min-1) of HS (2400 mOsm/L) should be avoided, as the induced hypotension may be fatal when coupled with a transient decrease in cardiac contractility. For treating dehydrated adult ruminants, HS (2400 mOsm/L, 4-5 mL/kg i.v. over 4-5 minutes) should be administered through the jugular vein and the cow allowed to drink water. This means that 2 L of HS should be administered to adult cattle. HSD should be administered in conjunction with isotonic oral electrolyte solutions to all calves 8% or more dehydrated (eyes recessed > or = 4 mm into the orbit, cervical skin tent duration > 6 seconds) or calves with reduced cardiac output (fetlock temperature < 29 degrees C when housed at 10-24 degrees C). For treating dehydrated calves, HSD (2400 mOsm/L NaCl in 6% dextran-70, 4-5 mL/kg i.v. over 4-5 minutes) should be administered through the jugular vein and the calf allowed to suckle an isotonic oral electrolyte solution. This means that 120-200 mL of HSD of HSD should be administered to a calf. HSD should be routinely administered to severely depressed or comatose calves, as HSD provides the fastest method of resuscitation while rapidly reversing the effects of hyperkalemia.

Sodium-hydrogen exchange in guinea-pig ventricular muscle during exposure to hyperosmolar solutions

1. The effect on intracellular pH (pHi) and intracellular Na+ activity (aNai) of exposure to hyperosmolar solutions was investigated in guinea-pig ventricular muscle using ion-sensitive microelectrodes. 2. Exposure of tissue to solution made hyperosmolar by the addition of 100 mM-sucrose produced an intracellular alkalinization of 0.10 pH units and hyperpolarization of the membrane potential. 3. When extracellular Na+ was reduced to 15 mM by substitution of NaCl with choline chloride, exposure to hyperosmolar solutions caused a decrease in pHi. Identical experiments using LiCl as the sodium substitute resulted in an increase in pHi of a magnitude similar to that seen at physiological Na+ levels. 4. In the presence of 50 microM-5-(N,N-dimethyl)amiloride (DMA), an inhibitor of Na(+)-H+ exchange, pHi decreased upon exposure to hyperosmolar solution. 5. The recovery of pHi from an intracellular acidosis (induced by brief exposure to NH4Cl) was enhanced in hyperosmolar solution when compared to recovery in isosmolar solution. This enhancement was observed even when aNai was markedly elevated (greater than 25 mM) by inhibition of the Na(+)-K+ pump. 6. There was an increase in aNai during exposure to hyperosmolar solutions. When the Na(+)-K+ pump was inhibited with dihydro-ouabain a component of this increase in aNai was sensitive to DMA. 7. We conclude that exposure of cardiac tissue to hyperosmolar solutions results in an intracellular alkalosis due to activation of the sarcolemmal Na(+)-H+ exchanger. Such changes should be considered when exposure to hyperosmolar solutions is used in the study of excitation-contraction coupling and cardiac muscle mechanics.

Small-volume hypertonic saline dextran resuscitation from canine endotoxin shock

This study evaluated resuscitation of endotoxin shock with 7.5% hypertonic saline dextran (HSD 2400 mOsm) by measuring hemodynamic and regional blood flow responses. Endotoxin challenge (1 mg/kg) in adult dogs caused a significant decrease in mean arterial blood pressure (MABP), cardiac output (CO), left ventricular +/- dP/dt max, and regional blood flow (radioactive microspheres). Cardiocirculatory dysfunction and acid-base derangements persisted throughout the experimental period in untreated endotoxin shock (group 1, n = 10). In contrast both regimens of fluid resuscitation (group 2, n = 11: bolus of 4 mL/kg HSD followed by a constant infusion of lactated Ringer's [LR] to maintain MABP and CO at baseline values; group 3, n = 10; LR alone given as described for group 2) improved regional perfusion and corrected acid-base disturbances similarly in all dogs. Hypertonic saline dextran enhanced all indices of cardiac contraction and relaxation more than LR alone. The total volume of LR required to maintain MABP and CO at baseline values was less in the HSD group (59.2 +/- 6.8 mL/kg) than in the LR alone group (158 +/- 16 mL/kg, p = 0.01). The net fluid gain (infused volume minus urine output and normalized for kilogram body weight) was five times greater in the LR (24.8 +/- 6.2 mL/kg) than in the HSD group (4.6 +/- 1.2 mL/kg, p = 0.01). Lung water was similar in all dogs, regardless of the regimen of fluid resuscitation. Hypertonic saline dextran effectively resuscitates endotoxin shock in this canine model.

Diltiazem and left ventricular hypertrophy in renovascular hypertensive rats

The effects of diltiazem treatment (40-50 mg/kg/day orally for 8 weeks) of left ventricular hypertrophy on systemic and coronary hemodynamics and mechanical cardiac performance were investigated in renovascular hypertensive rats (Goldblatt, two-kidney, one clip). Systemic and coronary hemodynamics were determined by using radioactive microspheres in conscious, unrestrained rats. Mechanical performance was measured on isolated papillary muscle from the same animal. Nine treated hypertensive rats were compared with control groups: 12 untreated hypertensive and nine sham-operated rats. Diltiazem treatment led to an effective but incomplete control of blood pressure (from 208 +/- 5 mm Hg in the untreated hypertensive group to 155 +/- 3 mm Hg in the treated hypertensive group; p less than 0.01) associated with a significant but incomplete decrease of the left ventricular mass (from 3.10 +/- 0.19 mg/g in untreated hypertensive rats to 2.35 +/- 0.04 mg/g in treated hypertensive rats; p less than 0.01). A close correlation was found between left ventricular mass and systolic blood pressure in untreated, treated, and pooled groups (r = 0.84, p less than 0.001, n = 30). The left ventricular weight to systolic blood pressure ratio was equivalent in all three groups, so that the reduction of left ventricular mass in diltiazem-treated rats was commensurate with the reduction of blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the mechanical behavior of isolated papillary muscle preparations of the ferret

The characteristics of isometric and isotonic contraction were studied in isolated papillary muscle preparations from the right ventricle of the ferret and compared with those previously reported for comparable preparations from cat and rat. The effects of stimulation rate, temperature, extracellular calcium concentration, caffeine, verapamil, and ouabain were measured. Afterloaded contractions and load clamps were used to study the mechanics of relaxation. The contractile characteristics of ferret papillary muscle preparation appear to be intermediate in many respects between those reported for cat and rat preparations.

Effects of a new nonionic and a conventional ionic contrast agent on coronary sinus ionized calcium and left ventricular hemodynamics in dogs

Transient myocardial depression associated with intracoronary injections of contrast medium has been attributed to hypertonicity and to calcium binding. To further assess the importance of calcium binding, a new technique for continuous monitoring of coronary sinus ionized calcium with an intravascular calcium-selective electrode was used. With this calcium-selective electrode the effects of intracoronary injection in dogs of a conventional ionic contrast agent, sodium meglumine diatrizoate (Renografin-76), and a new nonionic agent, iohexol, were assessed and compared. Left ventricular pressure was measured with a micromanometer catheter. After bolus injection of 0.2 ml/kg body weight of Renografin-76 (n = 10), coronary sinus pCa increased by 0.27 from 2.98 +/- 0.02 to 3.25 +/- 0.03, indicating a decrease in ionized calcium from 2.0 to 1.1 mEq/liter. With iohexol (n = 9), pCa increased by only 0.05 +/- 0.01 (p less than 0.001), indicating a decrease in ionized calcium from 2.0 to 1.8 mEq/liter. Peak changes occurred approximately 6 seconds after injection. Renografin-76 caused a marked decrease in left ventricular systolic pressure (140 +/- 7 to 106 +/- 8 mm Hg) and in heart rate (122 +/- 7 to 101 +/- 5 beats/min) with an increase in end-diastolic pressure (5 +/- 1 to 12 +/- 1 mm Hg), whereas iohexol did not significantly alter these variables. Using Renografin-76 with calcium added to achieve an ionized calcium level of 2 (n = 4), 4 (n = 4) or 6 (n = 4) mEq/liter, the changes in coronary sinus pCa were abolished and the hemodynamic changes attenuated. These findings indicate that Renografin-76 results in greater myocardial depression than the new nonionic agent iohexol.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-dependent effects of cadmium on energy metabolism and function of perfused rat heart

Postequilibrated isolated rat hearts were perfused for 60 min with a standard supporting electrolyte buffer containing one of the following calcium concentrations: 0.9, 1.8, 3.5, or 5.0 mM, either with or without added cadmium. Doses of cadmium which proved to be minimally (0.03 microM Cd)--and maximally (3.0 microM Cd)--effective at 0.9 mM Ca were studied at all other calcium concentrations. A dose-dependent positive inotropy that persisted throughout the 60-min perfusion period was induced by the graded increases in the perfusate calcium concentration throughout the range from 0.9 to 5.0 mM. Atrioventricular node conductivity was prolonged significantly in hearts perfused with 0.9 mM Ca as compared to hearts perfused with higher calcium concentrations. Increasing the perfusate calcium concentration caused a dose-dependent increase in heart glycerol 3-phosphorylcholine (GPC) content. The other measured phosphatic metabolites of the heart were not altered significantly by varying the perfusate calcium level. In contrast, cadmium (3.0 microM Cd) induced extensive functional and metabolic aberrations which varied in magnitude as an inverse function of the perfusate calcium concentration. Contractile tension, rate of tension development (dT/dt), heart rate, coronary flow rate, and atrioventricular node conductivity were decreased significantly in response to cadmium perfusion. Moreover, these hearts characteristically had significantly elevated low energy phosphate (inosine monophosphate and inorganic phosphate) and decreased high energy phosphate (ATP, PCr) levels relative to their respective calcium controls. Furthermore, various phosphorylated intermediates of glycolysis (glucose 6-phosphate, fructose 6-phosphate, glucose 1-phosphate), as well as glycerol 3-phosphate, and uridine diphosphoglucose accumulated significantly in hearts perfused with cadmium at certain calcium concentrations below 5.0 mM. The calcium-activated increase in heart GPC was inhibited completely by 3 microM cadmium. At the minimally effective dose of cadmium (0.03 microM), demonstrable changes were apparent only at the lowest perfusate calcium concentration examined (0.9 mM). These findings are consistent with the hypothesis that cadmium interferes with calcium-activated and calcium-mediated physiologic and biochemical processes of the mammalian heart. The primary mechanistic basis for the action of cadmium appears to be linked to a competition with calcium for membrane and possibly intracellular binding and activation sites.

Direct pharmacologic and osmolal effects of contrast media on the mechanics of heart muscle isolated from cats

We studied 56 right ventricular papillary muscles isolated from cats to determine whether the direct mechanical effects of contrast media are due to their pharmacologic action or to the hyperosmolality caused by them. A 10% solution of Renografin-76 abruptly decreased force 22% below control, then slowly increased it to 22% above control. To elucidate these changes we also studied: (1) Renografin-60 (lower concentration of diatrizoate); (2) meglumine and diatrizoate; (3) meglumine alone; (4) diatrizoate sodium; (5) ethylene diamine tetra-acetic acid (EDTA) (same concentration as in Renografin-76); (6) sucrose to increase osmolality; and (7) Amipaque (metrizamide), a contrast medium without meglumine or diatrizoate. All solutions containing meglumine, and EDTA alone decreased force. Diatrizoate sodium also decreased force initially, presumably due to the high sodium concentration. Meglumine alone decreased force abruptly and substantially. In contrast, metrizamide and 100 mmol sucrose only increased force. Thus, the abrupt decrease in force was caused by the pharmacologic action of meglumine, EDTA, or high sodium in the contrast media, whereas the slow increase in force was caused by hyperosmolality.

Relation between ionized calcium concentration and ventricular pump performance in the dog under hemodynamically controlled conditions

The effect of plasma ionized calcium concentration on left ventricular function was studied in the canine heart on right heart bypass. Stroke volume, mean arterial pressure and heart rate were controlled. Plasma ionized calcium was lowered to 0.58 +/- 0.01 mM by citrate infusion and raised to 1.70 +/- 0.01 mM by calcium chloride infusion in random order in each dog. Left ventricular function at each of these ionized calcium levels was compared with that in an immediately preceding normocalcemic period. At a constant stroke work (16.9 +/- 0.2 g-m), sustained hypercalcemia was associated with a small decrease in left ventricular end-diastolic pressure (1.7 +/- 0.7 cm H2O, p less than 0.05) despite a marked increase in peak left ventricular dP/dt (first derivative of ventricular pressure) averaging 34 percent (p less than 0.001). Coronary blood flow, tension-time index and myocardial oxygen consumption were not significantly altered. Stroke work determined at a left ventricular end-diastolic pressure of 14 cm H2O, by interpolation in left ventricular function curves, was 11 +/- 4.4 percent above that at control normocalcemia (p less than 0.05). At a constant stroke work (16.9 +/- 0.2 g-m), sustained hypocalcemia was associated with a marked depression of left ventricular function as demonstrated by a substantial increase (from 4.9 +/- 0.3 to 12.7 +/- 1.1 cm H2O, p less than 0.0001) in left ventricular end-diastolic pressure (p less than 0.0001), decreased mean systolic ejection rate (p less than 0.01) and decreased peak left ventricular dP/dt (p less than 0.0001). Coronary blood flow increased (p less than 0.05) whereas myocardial oxygen consumption did not change significantly. A marked displacement of left ventricular function curves to the right (compared with curves obtained during normocalcemia) was observed, and stroke work determined at a left ventricular end-diastolic pressure of 14 cm H2O was 52 +/- 5.4 percent below control level (p less than 0.001). It appears that hypercalcemia, when initiated from a normal control level, provides only a small enhancement of ventricular pump performance (as indexed by the stroke work-left ventricular end-diastolic pressure relation) despite a marked increase in peak left ventricular dP/dt, whereas marked improvement of left ventricular performance may be expected when calcium infusion is initiated from an ionized calcium level that is below normal.

The effect of mannitol following permanent coronary occlusion

Quantitative comparisons of infarcts 24 hours after ligation of the left anterior descending coronary artery (LAD) via thoracotomy were made in 13 control and 13 dogs treated with i.v. 25% mannitol, 2 ml/min for 4 hours following occlusion. Mannitol increased serum osmolarity by 44+/-4 mOsm/L (mean +/- 1 SE) with hemodynamic effects limited to a small increase in left ventricular dP/dt. Nonperfusible tissue measured by planimetry at 24 hours was similar in both groups (46+/-4% of area defined with dye injected into the distal LAD for control versus 48+/-5% for mannitol treated dogs, P = NS). Creatine phosphokinase activity in infarcted tissues was also similar in both groups. Myocardial blood flows measured with radioactive microspheres were also similar in both groups. Collateral conductance calcultaed from retrograde flow and aortic pressure increased with the 24 hour period by 146+/-23% in the control dogs; in the mannitol treated dogs, collateral increased only 38+/-14% (P less than 0.001). Thus mannitol had no effect on ultimate infarct size. Moreover, mannitol appeared to hinder the development of collateral vessels.

Effect of sodium bicarbonate on canine left ventricular function

Left ventricular performance was evaluated in seven canine right heart bypass preparations during and after the intraauricular administration of sodium bicarbonate (100 ml, 1100 Osm, 60 mEq). Under conditions of constant cardiac input and left ventricular volume, sodium bicarbonate produced mild and brief left ventricular depression, as manifested by a decline of left ventricular dp/dt with simultaneous rise of left ventricular end-diastrolic and left atrial pressures. These findings occurred as aortic impedance fell. Identical results accrued in paced and unpaced preparations. The data lends credence to the proposition that a sudden rise in blood pCO2 produces increased transport of CO2 across the cardiac membranes with intracellular acidosis. However, the data is also compatible with hyperosmolality induced contractile depression. Possible clinical implications are discussed.