Catecholamine modulation of magnesium plasma levels in the rat.

Epinephrine (0.3 microgram/min) or isoproterenol (0.2 microgram/min) were infused for 40 min in rats to determine the role of catecholamines in the short-term control of plasma magnesium levels. Phentolamine (0.2 and 1 mg/kg) or propranolol (5 mg/kg), injected i.p. 10 min following the beginning of the infusion with catecholamines, were used to block alpha- or beta-adrenoceptors. Epinephrine alone elevated the systolic while decreasing the diastolic blood pressure and initially increased heart rate by 8% to 455 +/- 25 beats/min. Plasma potassium declined by 24% but magnesium levels remained constant. Following 0.2 mg/kg of phentolamine, systolic and diastolic blood pressure decreased by 40% and 55% to 95 +/- 11 and 39 +/- 3 mmHg, respectively. Potassium levels were further reduced by 8%. Administration of this dose of phentolamine increased magnesium levels by 8%. Magnesium levels were elevated by 19% to 2.38 +/- 0.20 mEq/l after 1 mg/kg of phentolamine, possibly due to excessive hemodynamic derangements. Propranolol reversed the hypokalemic and hemodynamic effects of the epinephrine infusion without altering plasma magnesium. Similarly, propranolol abolished the tachycardia and hypokalemia elicited by isoproterenol without affecting magnesium levels. These results suggest that catecholamines play no major role in the short-term control of plasma magnesium in the rat.

Myocardial mechanical, biochemical, and structural alterations induced by chronic ethanol ingestion in rats.

To determine the effects of moderate ethanol consumption on the mechanical, biochemical, and structural characteristics of the heart, myocardial mechanical performance, contractile protein enzyme activity, and the number and size of myocytes were measured in male Fischer 344 rats after the ingestion of 30% oral ethanol. Papillary muscles removed from the left ventricle were greater in length, weight, and cross-sectional area than the corresponding muscles from the right side. However, no differences were found between control and ethanol-treated myocardium when either the left or right side was compared separately. Chronic ethanol ingestion resulted in an increase in resting tension in left ventricular muscles, with no alteration in peak developed tension. Moreover, time to peak tension was significantly prolonged, whereas a depression was observed in the peak rate of isometric tension development. Isotonically, left muscles from ethanol-treated rats revealed a prolongation of time to peak shortening and a marked depression in the velocity of shortening at physiological loads. No changes were noted in muscles from the right ventricle. Contractile protein enzyme activity revealed no differences in myofibrillar Mg(2+)-ATPase activity in right and left ventricular myocardium between control and ethanol-treated rats in the presence of EGTA. However, at physiological activating levels of calcium, an upward shift of the myofibrillar Mg(2+)-ATPase activity-calcium curve occurred in left myocardium, whereas a depression in this relation was seen in the right ventricle. As a result of chronic ethanol intake, a decrease was noted in the volume percent of myocardium occupied by myocytes, and that myocyte cell volume per nucleus was found to remain essentially constant throughout the various layers of the ventricular wall. Importantly, a 14% significant decrease in the total number of myocyte nuclei was demonstrated in the left ventricular myocardium of rats on chronic ethanol consumption. Thus, chronic but moderate alcohol ingestion resulted in depressed contractile performance, alterations in myofibrillar Mg(2+)-ATPase activity, and myocyte loss. These events may serve to function as preliminary indicators of the onset of heart failure of alcoholic origin in this animal model.

Left ventricular dysfunction induced by chronic alcohol ingestion in rats.

To determine whether moderate ingestion of alcohol for protracted periods of time affects normal cardiac performance and produces myocyte damage, male Fischer 344 rats at 4 mo of age were given 30% ethanol in their drinking water every day for a period of 8 mo. Experimental animals and age-matched controls were examined hemodynamically and morphometrically at 12 mo of age. Body and cardiac growth were depressed in alcoholic animals by 15 and 12%, respectively. Although left ventricular (LV) weight was reduced by 14% in alcoholic rats, no difference in right ventricular (RV) weight was noted, and consequently the ratio of RV weight to body weight increased by 12%. Systemic arterial pressures as well as LV peak systolic pressure decreased in alcoholic rats despite an unchanged heart rate. Myocardial contractility in alcoholic rats was further depressed as revealed by a significant decrease in the peak rate of ventricular pressure decay. Importantly, end-diastolic pressure was elevated 5.2-fold in the left ventricle and 2.9-fold in the right ventricle after 8 mo of ethanol consumption. LV diastolic chamber volume increased through myocardial remodeling as the longitudinal axis and transverse diameters from the base to the apex increased in experimental animals while the thickness of the LV diminished. Structural and hemodynamic alterations resulted in a 571% increase in the volume of diastolic circumferential wall stress on the left ventricle. Damage to the myocardium was increased in alcoholic animals with the volume percent of myocardial lesions increasing 342% in the wall of the left ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ethinyl estradiol on isoproterenol-induced death in ventricular fibrillation in DOCA-salt pretreated male and female rats.

To assess the effects of ethinyl estradiol on the incidence of death in ventricular fibrillation induced by isoproterenol in DOCA-salt pretreated rats we implanted male and female rats simultaneously with a 20 mg DOCA pellet and pellets containing either ethinyl estradiol or vehicle (wax). Rats drank saline after implantation. After 6 days rats were challenged with a single, sc dose of 150 micrograms of isoproterenol. The average daily dose of estradiol per rat was estimated on the basis of the quantity of pellet lost during 6 days. In male rats the average daily dose of 61.2 +/- 20.2 micrograms/rat of ethinyl estradiol decreased the incidence of mortality by 80%, from 73.3% (11/15) in vehicle treated to 13.3% (2/15) in estradiol treated rats. Death occurred within 19.2 +/- 8.0 minutes from the injection of isoproterenol and was due to ventricular fibrillation. Serum levels of magnesium and potassium were comparable in the two groups both before and after isoproterenol. Isoproterenol induced death in 9 of 11 DOCA-salt pretreated, ovariectomized rats within 22.3 +/- 9.8 minutes. Only 3 of 11 DOCA-salt ovariectomized rats receiving the average daily dose of 28.4 +/- 12.1 micrograms/rat of ethinyl estradiol died. None of 10 ovariectomized untreated rats died from isoproterenol challenge. Serum levels of magnesium and potassium were comparable in the estradiol and vehicle treated groups. The average daily dose of 2.8 +/- 0.42 micrograms/rat of ethinyl estradiol elicited uterine growth but did not influence the incidence of mortality, since 9 out of 16 and 10 out of 16 rats died following isoproterenol in vehicle and estradiol treated DOCA-salt ovariectomized rats. We conclude that only pharmacological doses of estradiol exert protective effects against DOCA-salt induced myocardial sensitization to isoproterenol and that this protection is not associated with relevant changes in serum potassium or magnesium.

Effects of alcohol on isoproterenol-induced ventricular fibrillation in adult rats.

To determine whether the pattern of alcohol consumption, intake, and withdrawal modulates the incidence of malignant ventricular arrhythmias and sudden cardiac death due to beta-adrenergic stimulation in a species susceptible to administration of isoproterenol alone, a regimen was formulated in which both continuous and interrupted alcohol ingestion was obtained. After alcohol treatment of fully mature, adult rats for 7 weeks, a single subcutaneous injection of 150 micrograms/kg of isoproterenol was given to control (non-alcohol-treated), alcoholic (continuous consumption), and alcohol withdrawal (interrupted) rats. The incidences of malignant ventricular arrhythmias and related deaths were compared. The results revealed that arrhythmias and arrhythmic deaths were highest in the "alcohol withdrawal group," 92% and 54%, respectively. As judged from baseline studies, withdrawal of alcohol produced a 16% decrease in serum K+ concentration compared with controls, whereas continuous alcohol ingestion resulted in a 20% elevation in magnesium concentration. These electrolyte changes were further affected by isoproterenol and may have contributed to the differential response to beta-adrenergic stimulation as a result of the pattern of alcohol intake.

Spasm of small coronary arteries and ischemic myocardial injury induced by hypothalamic stimulation in the rat.

Electrical stimulation of the lateral hypothalamus resulted in electrocardiographic evidence of acute myocardial ischemia in 35% of normal adult rats under anesthesia. Mean arterial blood pressure was also elevated. Study of vascular corrosion casts disclosed that spasm of smaller branches of the coronary circulation, rather than the major epicardial arteries, was the main cause of the ischemic response. The histologic changes of the same experimental treatment in a separate group of animals revealed multiple focal areas of tissue damage throughout the myocardium, which were quantitatively assessed. The results may be relevant for the clinical problem of various forms of ischemic heart disease in which little evidence is found for organic (atherosclerosis) or dynamic (spasm) stenosis involving the major coronary arteries.

Coronary artery spasm: involvement of small intramyocardial branches.

A study was conducted to determine if the small (resistance) vessels of the coronary circulation could undergo spasm comparable to that of the major conductance (epicardial) arteries which in the rat measure 275-300 micron in diameter. This information may be relevant to the growing evidence of ischemic myocardial disease without significant coronary atherosclerosis or even spasm of the larger vessels. Vascular corrosion casts of the coronary circulation were prepared in the rat 20 min after intravenous injection of arginine vasopressin, a powerful coronary constrictor substance, under continuous electrocardiographic monitoring. Electrocardiographic changes observed consisted of S-T segment elevation and conduction disturbances, implying ischemic effects on the myocardium. Corrosion casts revealed spasm of smaller arteries only (50-150 micron diameter). Controls (vehicle-injected or untreated) showed no abnormalities of the coronary vasculature. These results suggest that myocardial vessels of this size are comparable in their potential for spasm to the large conductance arteries. Similar findings in patients involving smaller vessels could explain ischemic myocardial events in the absence of significant spasm, or organic stenosing pathology of major coronary arteries. As a corollary, it is suggested that the term "coronary artery spasm" could be enlarged in its definition to include other levels of the coronary circulation rather than that of the large conductance arteries alone.

Increased incidence of isoproterenol-induced ventricular fibrillation in aging rats.

Prolonged beta-adrenergic stimulation obtained by subcutaneous injection of isoproterenol in unanesthetized, unrestrained rats elicited ventricular fibrillation in approximately 80% of animals at 10-12 months of age. Ventricular fibrillation failed to occur in 1-month-old rats and involved only 12% of rats at 2 months. Senescence appeared not to increase the frequency of ventricular fibrillation since a similar incidence was seen in rats at 10-12 and 19-21 months. In all instances, ventricular fibrillation was preceded by ECG changes consistent with acute subendocardial ischemia. To evaluate whether acute beta-adrenergic stimulation elicits comparable cardiovascular effects in animals of different age, a dose-response curve to intravenous injection of isoproterenol was performed in anesthetized rats. Changes in heart rate, systemic arterial pressure, left ventricular pressure, and dP/dt were not different among animal groups. It was concluded that the arrhythmogenic potential of isoproterenol may not be related to differences in cardiac beta-receptor sensitivity with age as suggested by the comparable changes in the inotropic and chronotropic actions of isoproterenol in the animal groups studied.

Myocyte cell loss and myocyte hypertrophy in the aging rat heart.

To determine the effects of age on the myocardium, the functional and structural characteristics of the heart were studied in rats at 3, 10 to 12 and 19 to 21 months of age. Systemic arterial pressure, left ventricular pressure and its first derivative (dP/dt) and heart rate were comparable in the three animal groups. In the interval between 3 and 10 to 12 months, mean myocyte cell volume per nucleus increased 53 and 26% in the left and the right ventricle, respectively. The total number of myocyte nuclei remained constant in either ventricle. In the following period, between 10 to 12 and 19 to 21 months, a 39% further cellular hypertrophy on the left side of the heart was found in association with an 18% loss of cells in the ventricle. Cell loss was accompanied by discrete areas of interstitial and replacement fibrosis in the subendocardium. In contrast, no myocardial damage was observed in the right ventricle, and the measured 35% additional enlargement of myocytes occurred without a change in cell number. Thus, the aging left ventricle is composed of a smaller number of hypertrophied cells. Cellular hypertrophy may explain the unaltered cardiac function of the aged myocardium.

Response of the border zone to myocardial infarction in rats.

The response of the surviving myocardium 30 days after coronary artery occlusion was measured morphometrically in the regions bordering and remote from infarcts of different sizes. Mean cell volume per nucleus increased with infarct size in both zones, but the rate of change was greater in the border than in the remote portion of the unaffected myocardium. Capillary numerical density within the uninjured tissue progressively decreased with infarct size leading to an increased diffusion distance for oxygen. Although the magnitude of changes in capillary density was similar in the two regions of the ventricle, the analysis of the individual values in each heart showed that in infarcts comprising more than 11% of the ventricular wall capillary concentration and the path length for oxygen supply to the myocytes were affected more in the border zone than in the myocardium remote from the scar. In conclusion, the border zone participates in the hypertrophic recovery process after infarction, but the inadequate growth of the capillary microvasculature suggests that this region is more susceptible to additional ischemic episodes.

Single-dose pharmacokinetics of oral ciprofloxacin in patients with cystic fibrosis.

The single-dose pharmacokinetics of oral ciprofloxacin were studied in ten patients with cystic fibrosis aged 18 to 34 years. Each patient received three different drug doses (500 mg, 750 mg, and 1,000 mg) at successive one-week intervals. Dosing and drug assays were double blinded. Blood and urine were assayed over the 48 hours following each dose. Ciprofloxacin was absorbed from the gastrointestinal tract. Peak serum concentrations averaged 2.8, 4.5, and 4.6 micrograms/mL respectively at the three doses, well above the mean inhibitory concentrations of most isolates of Pseudomonas aeruginosa. Time to peak concentration was approximately two hours. The range of sputum levels in three patients was 1.1-2.1 micrograms/mL at four hours after the three doses. The serum elimination half-life was 3.7 hours and was independent of dose. Urinary recovery was 26%; greater than 90% of urinary excretion occurred within the first 12 hours. The results of this study indicate that ciprofloxacin has potential for use in the treatment of P aeruginosa infections in patients with cystic fibrosis.

DOCA-salt induced myocardial sensitization to ventricular fibrillation by isoprenaline in rats. Role of the autonomic nervous system.

The possible role of the autonomic nervous system in the development of myocardial sensitization to isoprenaline produced by DOCA-salt pretreatment was evaluated in bilaterally unanaesthetized rats subject to chronic ganglionic blockade with chlorisondamine or hexamethonium and to treatment with 6-hydroxydopamine. Isoprenaline produced ventricular fibrillation and death in rats pretreated for 6 days with DOCA (20 mg pellet, s.c.) and 0.9% saline. The incidence of mortality was dose dependent and was 76.6% with 150 microgram/kg of isoprenaline. Adrenalectomy did not alter the mortality rate. The combination of adrenalectomy with concurrent chronic ganglionic blockade significantly decreased the incidence of mortality and delayed the time of death. In contrast, 6-hydroxydopamine treatment and adrenalectomy did not alter the incidence of mortality nor time of death. No differences in myocardial noradrenaline turnover were detected in rats susceptible and nonsusceptible to death in ventricular fibrillation. We conclude that myocardial sensitization to isoprenaline induced by DOCA-salt requires a neurogenic component, which is not, however, manifested by changes in myocardial noradrenaline turnover.

Ventricular fibrillation and the Brattleboro rat.

Despite considerable evidence from both clinical and animal experiments, the role of beta-adrenergic stimulation in the development of life-threatening cardiac arrhythmias is not well understood. Recent studies indicate that the posterior pituitary hormone, arginine vasopressin, may have a modulatory effect on cardiovascular sympathetic tone, especially with respect to blood pressure and heart rate. Employing the homozygous Brattleboro rat, an animal genetically deficient in the synthesis of arginine vasopressin, the following report offers evidence that the neuropeptide may also act on central pathways to suppress arrhythmogenic activity elicited by beta-adrenoceptor stimulation.

Left ventricular failure induced by myocardial infarction. I. Myocyte hypertrophy.

To determine whether left ventricular failure after acute myocardial infarction is associated with a growth response of the myocytes that tends to compensate for the loss of muscle mass and function, the left coronary artery in rats was ligated near its origin, and the animals were killed 3 days later. Elevated left ventricular end-diastolic pressure and decreased first derivative of left ventricular pressure and systolic arterial pressure indicated significant impairment of ventricular function. Absolute infarct size, determined morphometrically by measurement of the fraction of myocyte nuclei lost, averaged 57%. Hypertrophy of surviving left ventricular myocytes was 28%, involving a 14% increase in cell length and a 6% increase in diameter. Right ventricular myocyte volume per nucleus increased 21% by a 10% enlargement of cellular diameter with no change in length. These results show on a cellular basis that myocardial hypertrophy in the left ventricle is accomplished by cellular shape changes characteristic of a combination of pressure and volume overload hypertrophy, whereas cellular growth in the right ventricle is consistent with pressure overload hypertrophy.

Left ventricular failure induced by myocardial infarction. II. Tissue morphometry.

Three days after myocardial infarction involving 57% of the left ventricle in rats, the viable tissue of the left ventricle expanded 29%, whereas myocardial hypertrophy in the right ventricle was 19%. To determine whether tissue oxygenation in the hypertrophied ventricles was supported by a proportional growth of the capillary network, morphometric analysis was used to measure capillary luminal volume and surface densities and the diffusion distance for O2. The volume fraction of capillary lumen and the luminal surface of capillaries, related to O2 availability and diffusion, were altered by -21 and -19%, respectively, in the left ventricle and by -23 and -20%, respectively, in the right ventricle. The path length for O2 transport was found to be increased by 12 and 15% in the left and right ventricle, respectively. In contrast, myocyte mass expanded in proportion to tissue growth in the left ventricle and exceeded tissue growth by 5% in the right ventricle. Myocyte mitochondria and myofibrils both grew in proportion to the cells, so that their volume ratio was not changed in either ventricle. The relatively inadequate adaptation of the capillary vasculature suggests that hypertrophy after severe myocardial infarction may initially leave the heart more vulnerable to additional ischemic episodes.

Enhanced incidence of isoproterenol-induced ventricular fibrillation in the magnesium-deficient rat.

The electrocardiogram was recorded and serum and bulk myocardial electrolytes were determined in male Sprague Dawley rats, subjected to dietary magnesium deficiency for various periods, to assess the time course of development and cessation of the enhanced arrhythmogenic action of isoproterenol (150 micrograms/kg, subcutaneously) and to establish possible relationships between electrolyte changes and severe ventricular dysrhythmias. Ventricular fibrillation occurred within 60 min following isoproterenol injection in 25, 25, 62.5, 50, and 62.5% of rats on magnesium deficient diet for 4, 7, 11, 15, and 19 days (N = 8), respectively, and resulted in death in most animals (83%). Reintroduction of normal chow following a 30-day period on magnesium-deficient diet normalized serum magnesium (from 1.42 +/- 0.23 to 1.90 +/- 0.08 mEq/liter, mean +/- SD) but did not significantly reduce the incidence of ventricular fibrillation. Magnesium deficiency did not produce statistically significant alterations in bulk myocardial content of sodium, potassium, magnesium, and calcium. However, sodium was elevated and potassium diminished in hearts from rats that died in ventricular fibrillation, but not in those that had recovered. Magnesium-deficient rats sacrificed 30 min after isoproterenol injection, that is before the occurrence of ventricular fibrillation, exhibited hypomagnesemia and hypokalemia as well as elevated sodium and diminished potassium and magnesium in the myocardium. In contrast, rats on Purina Chow exhibited hypermagnesemia, but also showed hypokalemia and diminished cardiac potassium. The results indicate that magnesium deficiency enhances the arrhythmogenic propensity of isoproterenol and that the development of ventricular fibrillation is preceded by serum and myocardial electrolyte alterations.

Cardiovascular changes produced by the injection of aconitine at the area of the locus coeruleus in unanesthetized rats.

Blood pressure, heart rate and electrocardiogram were monitored in unanesthetized, unrestrained rats while the area of the locus coeruleus was stimulated by the injection of 0.03 to 10 micrograms of aconitine to pharmacologically evaluate its cardiovascular action. Aconitine elicited a dose-dependent elevation of mean blood pressure to 200 mmHg; at doses of 1-10 micrograms, it increased heart rate by more than 100 beats/min and produced arrhythmias. Phentolamine and the serotonergic antagonist, 2-bromo-lysergic acid diethylamide, prevented aconitine-induced cardiovascular changes. Diminution of aconitine-induced tachycardia by propranolol was attributed to the antagonist's direct peripheral effects. Atropine (5 mg/kg, subcutaneously) did not alter aconitine's cardiovascular actions. It is suggested that stimulation of the area of the locus coeruleus with aconitine elicits specific cardiovascular effects, which may involve adrenergic as well as serotonergic pathways.

Death in ventricular fibrillation induced by isoproterenol in DOCA-salt pretreated rats preceded by changes in myocardial electrolytes.

Serum and tissue content of sodium, potassium, magnesium and calcium was determined in controls and desoxycorticosterone acetate (DOCA)-salt treated rats to determine whether electrolyte changes preceded the development of isoproterenol-induced death in ventricular fibrillation. Control Sprague Dawley, male rats, were injected subcutaneously (s.c.) with either saline (Group A) or actinomycin D (0.1 mg/kg; Group B) once daily for 4 days. Other rats received 20 mg of DOCA by implantation, drank normal saline and were injected with either saline (Group C) or actinomycin D (Group D) once daily for 4 days. In the first part of the experiment, it was determined that none of 15 rats from Group C died when challenged with isoproterenol (150 micrograms/kg, s.c.) six days later: however, 13 out of 15 rats from Group D died within 29.1 +/- 15.0 minutes (mean +/- S.D.) from isoproterenol injection. Myocardial sodium was elevated (48.8 +/- 3.8 versus 36.3 +/- 1.9) and potassium decreased (60.4 +/- 3.4 versus 70.6 +/- 3.3, meq/kg wet weight, mean +/- S.D.) in rats that had succumbed to isoproterenol. In the second part of the experiment serum and tissues were removed from control and DOCA-saline pretreated rats before they died in ventricular fibrillation, 20 minutes after isoproterenol. DOCA-saline pretreated rats were hypernatremic and hypokalemic and exhibited higher sodium and lower potassium in skeletal muscle than control rats. Isoproterenol elicited hypokalemia in all rats, but it only elevated sodium and decreased potassium content in the myocardium of rats of Group D, that were more prone to die in ventricular fibrillation. It is concluded that myocardial electrolyte changes precede the onset of ventricular fibrillation and may be associated with the development of this dysrhythmia.