Tachycardia preconditions infarct size in dogs: role of adenosine and protein kinase C.

BACKGROUND: Myocardial ischemic preconditioning is a well-known phenomenon, however there is scant information in regard to nonischemic preconditioning. METHODS AND RESULTS: We studied in anesthetized dogs the preconditioning effect of tachycardia and the mediation of adenosine and protein kinase C in this process. In a control group the anterior descending coronary artery was occluded for 60 minutes and reperfused for 270 minutes. Heart rate was kept constant at 120 +/- 5 cycles/min and aortic pressure changes were damped. The infarct size (necrotic volume/risk region volume x 100) was 15.8 +/- 1.5%. In another group of dogs a similar protocol was followed, but five periods of tachycardia (213 +/- 12 cycles/min), 5 minutes in duration each, with 5 minutes of intervening periods at control heart rate, were induced previous to the coronary occlusion. The infarct size was reduced by 46% (P<.001) with respect to the nonpreconditioned group. This effect was not due to changes in collateral flow nor risk region size. During tachycardia, myocardial interstitial adenosine increased about twofold (P<.05); no metabolic, hemodynamic, or ECG evidences of ischemia were observed and the transmural vasodilatory reserve was preserved. The blockade of adenosine receptors with 8 phenyltheophylline, before or after the preconditioning tachycardia, reverted its protecting effect but it did not modify infarct size in nonpreconditioned dogs. No changes in cytosolic or particulate protein kinase C activity or translocation of alpha-, beta-, epsilon-, and zeta- protein kinase C isozyme by effect of tachycardia or ischemia were observed between preconditioned and nonpreconditioned dogs. CONCLUSIONS: Tachycardia, in the absence of ischemia, mimics the preconditioning effect of ischemia in the dog. This effect is mediated by adenosine but not by changes in protein kinase C activity or its translocation.

[Endothelium and coronary circulation]. / El endotelio y la circulacion coronaria.

In the last few years, so many different substances produced by the endothelium have been discovered that this structure is considered today a paracrine organ. Among these substances, there are at least three with marked vascular effects: prostacyclin (PGI-2) and the endothelium-derived relaxing factor (EDRF) are vasodilators, platelet stabilizers and anti-atherogenic. On the other hand, endothelin-1 (ET-1) is a potent vasoconstrictor and probably pro-atherogenic. There are many agents that stimulate the liberation of these substances by the endothelium and most of them stimulate simultaneously the production of the three substances. Even though it is not possible yet to define the exact participation of the endothelium in the normal regulation of coronary blood flow it is highly probably that a disfunction of this structure secondary to hypercholesterolemia, hypertension, atheromatosis, diabetes and smoking may decrease the coronary reserve, induce coronary spasm and facilitates the development of atheroma.

Effect of left intraventricular pressure on magnitude of vascular waterfall in the epicardial coronary veins.

STUDY OBJECTIVE: The aim was to test the hypothesis of a vascular waterfall in the epicardial veins due to compression by the left ventricular (LV) pressure. If this were so, the epicardial venous pressure should be a direct function of the LV pressure. DESIGN: Canine arrested hearts were used, without autoregulation and perfused with the Langendorff technique. Coronary flow and outflow pressure were measured in the great cardiac vein, which was the only outflow of the system. The pressure in an epicardial vein was also measured. The measurements were done with LV pressures varying from zero to 40 mm Hg. The outflow pressure was progressively increased until a steady decrease in flow occurred. This pressure was considered the critical outflow pressure. EXPERIMENTAL MATERIAL: 19 mongrel dogs, 18-25 kg, were used. The animals were anaesthetised and the hearts perfused in the Langendorff manner with homologous blood. MEASUREMENTS AND MAIN RESULTS: The epicardial venous pressure before outflow pressure was increased was higher than LV pressure (even at zero LV pressure), and increased linearly with the increase in LV pressure (epicardial venous pressure = 0.74 LV pressure +6.18; r = 0.93). The increase in outflow pressure did not modify flow until it reached a value (critical outflow pressure) similar to epicardial venous pressure. Critical outflow pressure was linearly related to LV pressure (critical outflow pressure = 0.65 LV pressure +5.13, r = 0.87). CONCLUSIONS: The results support the hypothesis of a waterfall circulation in the epicardial veins during diastole, the magnitude of which is a function of the LV pressure.

Neuropeptide Y (NPY): a coronary vasoconstrictor and potentiator of catecholamine-induced coronary constriction.

The vasoactive effect of neuropeptide Y (NPY) a peptide commonly found in perivascular nerves, including those of the heart, was assessed in the coronary circulation of the isolated perfused dog heart and in superfused segments of isolated canine coronary arteries. The intracoronary administration of 0.7-23.5 nmol NPY to hearts during beta adrenergic blockade produced a dose-dependent increase in coronary vascular resistance ranging from 0.10 to 0.49 mmHg.min-1.ml-1.100 g-1 without changes in myocardial oxygen consumption. The potency of NPY as a coronary vasoconstrictor was about 250 times that of noradrenaline. Pretreating the coronary system of these hearts with NPY caused a marked potentiation of the vasocontractile effect of noradrenaline, displacing its dose-response curve to the left in a non-parallel fashion. The addition of 0.2-3.7 nmol NPY did not induce contraction in superfused helical segments of large coronary arteries but it potentiated the tension developed in response to 0.18 microM adrenaline in a concentration-dependent manner. Pretreatment of these arteries with 3.7 nmol NPY caused a significant leftward displacement of the adrenaline contractile effect. These results show that NPY is a potent coronary vasoconstrictor and a potentiator of the contractile effect of catecholamines and support the hypothesis that NPY may participate in the regulation of coronary vascular resistance.

Effects of alpha-1 adrenergic blockade on regional flow in the ischemic heart.

Although ischemia induces strong coronary vasodilation, some vasoconstrictive tone persists in the ischemic myocardium. To assess whether this tone is mediated through alpha adrenergic receptors, coronary blood flow was measured with radioactive microspheres in the normal and in the ischemic left ventricular wall of the dog before and during alpha blockade with Trimazosin. Ischemia was accomplished by decreasing the coronary perfusion pressure to 22 +/- 1.4 mmHg. Heart rate and aortic pressure were kept constant in each experiment. Trimazosin significantly increased flow in the normal left ventricular wall, but to a greater extent in the subepicardium than in the subendocardium with a decrease of the inner/outer flow ratio from 1.38 +/- 0.12 to 1.20 +/- 0.11 (p less than 0.05). In the ischemic region, Trimazosin did not change total transmural flow, but flow decreased in the subendocardium and increased in the subepicardium with a decrease in the inner/outer flow ratio from 0.63 +/- 0.09 to 0.38 +/- 0.06 (p less than 0.01). These results show that a vasoconstrictive tone mediated through alpha-1 adrenergic receptors persists in the ischemic myocardium, the blockade of which is detrimental for the subendocardium.

Effects of alpha-1 adrenergic blockade on regional flow in the ischemic heart.

Although ischemia induces strong coronary vasodilation, some vasoconstrictive tone persists in the ischemic myocardium. To assess whether this tone is mediated through alpha adrenergic receptors, coronary blood flow was measured with radioactive microspheres in the normal and in the ischemic left ventricular wall of the dog before and during alpha blockade with Trimazosin. Ischemia was accomplished by decreasing the coronary perfusion pressure to 22 +/- 1.4 mmHg. Heart rate and aortic pressure were kept constant in each experiment. Trimazosin significantly increased flow in the normal left ventricular wall, but to a greater extent in the subepicardium than in the subendocardium with a decrease of the inner/outer flow ratio from 1.38 +/- 0.12 to 1.20 +/- 0.11 (p less than 0.05). In the ischemic region, Trimazosin did not change total transmural flow, but flow decreased in the subendocardium and increased in the subepicardium with a decrease in the inner/outer flow ratio from 0.63 +/- 0.09 to 0.38 +/- 0.06 (p less than 0.01). These results show that a vasoconstrictive tone mediated through alpha-1 adrenergic receptors persists in the ischemic myocardium, the blockade of which is detrimental for the subendocardium.

Changes in total and transmural coronary blood flow induced by ethanol.

We studied the effects of ethanol on total coronary resistance and on the resistance across the left ventricular wall in the isolated empty beating heart of the dog. The coronaries were perfused with homologous fresh blood, thermoregulated at 37 degrees C and equilibrated with a gas mixture of O2 (95%) and CO2 (5%). Coronary flow distribution was measured with radioactive microspheres. In 22 experiments in which coronary flow was kept constant, ethanol (calculated concentration in the perfusing blood, 2.9 +/- 0.2 g . litre-1) produced a significant decrease in perfusion pressure (from 14.2 +/- 0.5 to 11.9 +/- 0.5 kPa, P less than 0.005). This decrease in perfusion pressure was not caused by metabolic autoregulation since ethanol produced a decrease in the oxygen consumption of the heart (2.19 +/- 0.43 to 1.62 +/- 0.31 cm3 . min-1 . 100 g-1, P less than 0.05). It was not caused, either, by a decrease in extravascular compression since ethanol did not produce any further decrease in perfusion pressure after maximal dilatation of the coronaries with dipyridamole. In experiments performed either at constant coronary flow or at constant perfusion pressure, flow across the left ventricular wall was redistributed towards the subendocardium during the vasodilatory effect of ethanol. Since in our experimental conditions, changes in the neurohumoral, metabolic and cardiac mechanical factors that influence coronary flow were discarded, this study demonstrates a direct vasodilatory effect of ethanol on the coronary vessels with a redistribution of flow towards the subendocardium.

Intramyocardial pH as an index of myocardial metabolism during cardiac surgery.

At present, a practical method for continuous monitoring of the state of tissue metabolism in the individual patient's heart during cardiac operations is not available. We have explored the use of miniature electrode measurements of myocardial interstitial pH to provide this monitoring capability, making comparisons with intracellular pH in left ventricular biopsy specimens and with tissue PCO2 measured by mass spectrometry. The electrode system consisted of a hydrogen ion-sensitive glass miniature electrode, housed in the beveled end of a 21 gauge (0.8 mm diameter) hypodermic needle, and a 2 mm diameter reference electrode, with an internal silver-silver chloride electrode coupled to tissue through a saline bridge (150 mM/L sodium chloride) saturated with silver chloride. Accuracy in blood at 37 degrees C was compared with conventional instrumentation (Radiometer BMS-3 MK-2 Blood Micro System) over a pH range of 7.4 to 6.4 with linear regression analysis (n = 26) revealing a high correlation (r = 0.997) and a mean difference in paired observations of only 0.01 +/- 0.004 (mean +/- SEM) pH units. In two groups of dogs on cardiopulmonary bypass, the pH needle and reference electrodes were inserted into the anterior wall of the left ventricle. Ischemic arrest of the heart at 37 degrees C was used to vary myocardial pH. In Group 1 (n = 8), intracellular pH was estimated from left ventricular biopsy specimens (400 mg each) taken over a microelectrode pH range of 7.37 to 6.37, snap frozen, and homogenized. In Group II (n = 6), tissue PCO2 in the anterior wall of the left ventricle was determined by mass spectrometry (sampling catheter 1.3 mm diameter). Miniaturized electrode (interstitial) pH exceeded biopsy (intracellular) pH under control conditions by 0.28 +/- 0.025 pH units (p less than 0.001), but below an electrode pH of 6.8 the results of the two techniques did not differ significantly. The tissue PCO2 rose from 69 +/- 2 mm Hg to a final plateau of 419 +/- 25 mm Hg, which was similar to the predicted value of 427 +/- 28 mm Hg calculated from the pH change (7.37 +/- 0.01 to 6.01 +/- 0.07), providing a further independent check on the pH electrode technique. These data indicate that our intramyocardial pH measurements do reflect intracellular metabolism during elective arrest of the heart and may have potential for clinical use.

Regional diastolic coronary blood flow during diastolic ventricular hypertension.

The effect of diastolic ventricular hypertension on regional diastolic coronary flow was measured with radioactive microspheres in the canine heart paced at a constant rate and perfused only during diastole with a constant coronary perfusion pressure. Diastolic ventricular hypertension produced an homogenous increase of diastolic flow across the left ventricular wall when the metabolic coronary autoregulation was intact, but produced a decrease in subendocardial diastolic flow when the autoregulation was abolished. These results suggest that diastolic ventricular hypertension produces a higher subendocardial than subepicardial mechanical resistance to diastolic coronary flow.

Coronary vascular resistance during halothane anesthesia.

To study the effect of halothane on the coronary circulation, the circumflex diastolic coronary vascular resistance was measured in the working heart and total mean coronary resistance (TCR) in the isolated nonworking heart of the dog during administration of 100 per cent oxygen and during administration of 2--3 per cent halothane in oxygen. In the working heart, when the diastolic aortic pressure was kept at a nearly control level, halothane induced decreases of 12 per cent in circumflex diastolic coronary vascular resistance and 18 per cent in left ventricular arteriovenous oxygen content difference and no significant change in diastolic coronary blood flow. This effect occurred in spite of the absence of any significant change of myocardial oxygen consumption. In the nonworking beating, arrested or fibrillating heart, halothane induced a decrease of 24 per cent in total mean coronary resistance. Since the decrease in circumflex diastolic coronary vascular resistance in the working heart connot be attributed to myocardial hypoxia and since the results in the isolated nonworking heart eliminate the influences of mechanical and neurohumoral factors on coronary resistance, it is concluded that the observed decrease in resistance is probably due to vasodilation produced by a direct action of halothane on the coronary vessels. This effect was not modified by beta-adrenergic blockade.

Effect of heart rate on regional coronary blood flow.

The effect of tachycardia on the distribution of coronary blood flow was studied in the dog using the radioactive microsphere technique. Increasing heart rate from 1.7 to 3.0 Hz produced an increment of flow to the inner and outer layer of both ventricular walls. However, the flow ratio, inner layer to outer layer, increased in the right ventricular wall and decreased in the left wall. Measurements performed in ventricular regions with previous vasodilatation and during right ventricular hypertension revealed that the comparatively larger vasodilatation reserve of the right ventricular subendocardium is due to the lower pressure this ventricle develops normally.

Mechanical effects of heart contraction on coronary flow.

The mechanical effects of heart contraction on coronary flow were studied in the dog heart by implanting vessels in the subendocardial and subepicardial layers of the left ventricular wall and perfusing them independetly of the aortic pressure. At a perfusion pressure of 4 kPa (30 mm Hg), with spontaneous aortic pressure and heart rate, subendocardial flow was 40% less than subepicardial flow. Increasing the aortic pressure or the heart rate produced a comparatively larger decrease of the subendocardial flow. The results suggest that these changes are due to variations of the period of systolic time during which the vessels remain close.