Lois Jane Heller

Myocardial and plasma renin-angiotensinogen dynamics during pressure-induced cardiac hypertrophy

Plasma and left ventricular (LV) renin and angiotensinogen concentrations were assessed in a rat model of pressure-overload cardiac hypertrophy to determine if myocardial levels remained proportional to plasma levels over time. Three days after subdiaphragmatic aortic constriction (AC), LV hypertrophy was evident and renin concentrations in both plasma and LV, although not significantly elevated, were positively correlated with relative cardiac mass. After 42 days AC, LV hypertrophy remained, plasma and LV renin and angiotensinogen levels were not different from shams, and there was no correlation between renin and relative cardiac mass. Furthermore, LV renin and angiotensinogen concentrations remained at ∼25 and 4%, respectively, of those in plasma throughout the experiment. Myocytes from 3-day AC and sham-treated rats contained little renin as did LV from 48-h anephric rats. Incubations using calculated concentrations of myocardial interstitial renin and angiotensinogen revealed significant angiotensin I generation. These data suggest that LV renin in this model varies directly with plasma renin, is confined to the interstitial space, and can generate significant intramyocardial angiotensin I.Plasma and left ventricular (LV) renin and angiotensinogen concentrations were assessed in a rat model of pressure-overload cardiac hypertrophy to determine if myocardial levels remained proportional to plasma levels over time. Three days after subdiaphragmatic aortic constriction (AC), LV hypertrophy was evident and renin concentrations in both plasma and LV, although not significantly elevated, were positively correlated with relative cardiac mass. After 42 days AC, LV hypertrophy remained, plasma and LV renin and angiotensinogen levels were not different from shams, and there was no correlation between renin and relative cardiac mass. Furthermore, LV renin and angiotensinogen concentrations remained at approximately 25 and 4%, respectively, of those in plasma throughout the experiment. Myocytes from 3-day AC and sham-treated rats contained little renin as did LV from 48-h anephric rats. Incubations using calculated concentrations of myocardial interstitial renin and angiotensinogen revealed significant angiotensin I generation. These data suggest that LV renin in this model varies directly with plasma renin, is confined to the interstitial space, and can generate significant intramyocardial angiotensin I.

Estimates of interstitial adenosine from surface exudates of isolated rat hearts

The adenosine concentration of exudate formed on the surface of isolated perfused rat hearts has been used to obtain estimates of interstitial values. At a constant perfusion of approximately 15 ml/min/g, exudate was collected from below ring seals that either fitted snugly (compressing seals) or that acted as wicks (wicking seals) to deflect venous effluent away from the apical surface. Steady state exudates flows obtained below each of these seals were 0.96 +/- 0.05 ml/min and 0.18 +/- 0.02 ml/min, respectively. Adenosine concentration of surface exudate and venous effluent from hearts with the compressing seal were 130 +/- 8 nM and 23 +/- 3 nM, respectively, and from those with the wicking seal were 770 +/- 93 nM and 36 +/- 9 nM, respectively. Interstitial adenosine concentration in a situation with no net filtration may be slightly higher than that achieved in the exudate from preparations with the wicking seal. Addition of exogenous adenosine to the perfusate (1.0 microM) decreased vascular resistance and automaticity of all preparations, increased the venous effluent adenosine concentration to 236 +/- 18 nM and 251 +/- 30 nM with the compressing and wicking seals, respectively, but did not significantly alter the exudate adenosine concentration with either of the seals. This finding suggests that increases in vascular adenosine may influence functional characteristics without altering interstitial levels. Perfusion with 10 microM adenosine increased adenosine concentration in both effluent and exudate in all preparations but the gradient was reversed so that effluent levels were significantly higher than exudate levels. We conclude that venous adenosine determinations significantly underestimate the interstitial adenosine concentration associated with endogenous adenosine production and significantly overestimate the interstitial levels achieved by infusion of exogenous adenosine.

Adenosine receptor blockade enhances isoproterenol-induced increases in cardiac interstitial adenosine

The failure of adenosine receptor antagonists to consistently attenuate metabolic coronary vasodilation suggests that adenosine is not a primary regulator of functional hyperemia. An alternative hypothesis, however, is that metabolic stimulation of the heart in the presence of an adenosine receptor antagonist results in enhanced interstitial levels of adenosine which then might overcome the blockade. To test this hypothesis, interstitial levels of adenosine and inosine were estimated by HPLC analysis of fluid which exudes from the epicardial surface of isolated rat hearts perfused with crystalloid solution at constant flow. Isoproterenol infusion (10 nM) produced increases in heart rate, left ventricular systolic pressure, rate of pressure development, myocardial oxygen consumption and adenosine and inosine concentrations of venous effluent and surface exudate and produced decreases in coronary vascular resistance. The presence of the adenosine receptor antagonist, 8-(4-sulfophenyl) theophylline (spT) (100 microM), in the perfusate had little or no effect upon most of the responses to isoproterenol except that it significantly enhanced the isoproterenol-induced increases in adenosine release and adenosine concentrations in the venous effluent and surface exudate. The isoproterenol-induced change in adenosine concentration per unit change in oxygen consumption was approximately 3-fold greater in the presence of spT than in its absence. This extra adenosine production may tend to overcome the competitive blocking effect of spT and help explain why agents such as spT are not always effective in blocking metabolic vasodilation.

Membrane Potentials and Contractile Events of Hypertrophied Rat Cardiac Muscle

Abstract Previous studies indicate that cardiac hypertrophy is associated with altered mechanical properties including prolonged contraction times and mechanical refractory periods and exaggerated aftercontractions in isolated rat papillary muscle preparations. The present study was designed to ascertain whether changes in transmembrane potentials accompany the altered mechanical properties. Responses of papillary muscles from hypertrophied hearts of deoxycorticosterone acetate-treated Wistar Kyoto (WKY) rats were compared to those of nonhypertrophied, sham-treated WKYs. Muscles connected to a force transducer in a bathing chamber containing a 27° oxygenated physiological salt solution were subjected to field stimulation and isometric tension measured. Intracellular microelectrodes were used to measure transmembrane potentials of single impaled cells. When compared to nonhypertrophied controls, the hypertrophied preparations had (1) longer action potentials and contraction times, (2) longer electrical and mechanical refractory periods, and (3) exaggerated transient depolarizations and aftercontractions following paired-pulse or high-frequency stimulation. Propranolol given to block the effect of endogenous catecholamine released by field stimulation decreased the electrical excitability and the tension output of all preparations but did not eliminate the differences between the hypertrophied and nonhypertrophied groups. Summary. Characteristics of simultaneously recorded membrane potentials and isometric tension responses of isolated papillary muscles from hypertrophied and nonhypertrophied rat hearts were determined. When compared with nonhypertrophied preparations, hypertrophied preparations were found to have longer action potentials, longer contraction times, and longer electrical and mechanical refractory periods. When subjected to paired-pulse or high-frequency, short-duration stimulation, hypertrophied preparations developed larger aftercontractions and larger transient depolarizations than did nonhypertrophied preparations. Propranolol treatment of the preparations did not eliminate these differences between hypertrophied and nonhypertrophied preparations, suggesting that endogenous catecholamines were not responsible for the differences.

Differential cardiotoxicity in response to chronic doxorubicin treatment in male spontaneous hypertension-heart failure (SHHF), spontaneously hypertensive (SHR), and Wistar Kyoto (WKY) rats.

Life threatening complications from chemotherapy occur frequently in cancer survivors, however little is known about genetic risk factors. We treated male normotensive rats (WKY) and strains with hypertension (SHR) and hypertension with cardiomyopathy (SHHF) with 8 weekly doses of doxorubicin (DOX) followed by 12weeks of observation to test the hypothesis that genetic cardiovascular disease would worsen delayed cardiotoxicity. Compared with WKY, SHR demonstrated weight loss, decreased systolic blood pressure, increased kidney weights, greater cardiac and renal histopathologic lesions and greater mortality. SHHF showed growth restriction, increased kidney weights and renal histopathology but no effect on systolic blood pressure or mortality. SHHF had less severe cardiac lesions than SHR. We evaluated cardiac soluble epoxide hydrolase (sEH) content and arachidonic acid metabolites after acute DOX exposure as potential mediators of genetic risk. Before DOX, SHHF and SHR had significantly greater cardiac sEH and decreased epoxyeicosatrienoic acid (EET) (4 of 4 isomers in SHHF and 2 of 4 isomers in SHR) than WKY. After DOX, sEH was unchanged in all strains, but SHHF and SHR rats increased EETs to a level similar to WKY. Leukotriene D4 increased after treatment in SHR. Genetic predisposition to heart failure superimposed on genetic hypertension failed to generate greater toxicity compared with hypertension alone. The relative resistance of DOX-treated SHHF males to the cardiotoxic effects of DOX in the delayed phase despite progression of genetic disease was unexpected and a key finding. Strain differences in arachidonic acid metabolism may contribute to variation in response to DOX toxicity.

Pseudomonas aeruginosa exotoxin A enhances automaticity and potentiates hypoxic depression of isolated rat hearts

Abstract The potent virulence factor exotoxin A, produced by Pseudomonas seruginosa, has been reported to suppress the synthesis of the α-subunit of cardiac G1 protein and may have general effects upon synthesis of other myocardial proteins. To determine whether such exotoxin A actions influence specific functional properties of the intact heart, characteristics of isolated perfused hearts obtained from rats receiving injections of exotoxin A 48 hr before sacrifice were compared with those of rats receiving no exotoxin A. Exotoxin A treatment increased the spontaneous beating rates and potentiated the suppressive effects of hypoxia upon heart rate, left ventricular systolic pressure, and rates of ventricular contraction and relaxation. On the other hand, exotoxin A treatment did not influence the magnitude or rate of pressure development under control conditions, the positive chronotropic and inotropic responses to isoproterenol, or the negative chronotropic responses to adenosine. Since a specific exotoxin Ainduced suppression of myocardial α-subunit of the G1 protein should confer hypersensitivity to isoproterenol and reduced sensitivity to adenosine, the absence of alterations in responses to these interventions suggests that exotoxin As effect was not confined to specific suppression of this protein. However, net effects of exotoxin A exposure included a pronounced increase in excitability of the hearts and enhanced vulnerability to hypoxic insults.

Cardiac Hypertrophy in Copper-Deficient Rats Is Not Attenuated by Angiotensin II Receptor Antagonist L-158,809

Abstract We tested the hypothesis that cardiac hypertrophy which accompanies copper (Cu) deficiency was mediated by angiotensin II (Ang II). Thirty 17-day-old male Holtzman rats were offered a semipurified low-Cu diet (0.45 mg Cu/kg) for 39 days. Fifteen controls (Cu adequate) were given supplemental Cu (20 μg/ml) in their drinking water, the other 15 (Cu deficient) were given deionized water. Five from each dietary treatment group were chronically infused by using osmotic pumps for 4 weeks with the Ang II receptor antagonist L-158,809, and five from each group were infused with propylene glycol vehicle. Five from each group were not implanted. Blood pressure (BP) changes to injection of Ang II, phenylephrine, and acetylcholine were monitored in cannulated rats. Cu-deficient rats had higher heart weight, left ventricular (LV)/body weight (BW), right ventricular (RV)/BW, lower mean BP, and coronary vascular resistance (CVR) than Cu-adequate rats. L-158–809 did not alter Cu levels or RV/BW in either dietary group, but did lower LV/BW, CVR, and mean BP in both dietary groups. Since Ang II blockage lowered HW/BW and LV/BW in both groups, the stimulus for cardiac hypertrophy in Cu-deficient rats remains unknown.

Cardiac Norepinephrine and Intrinsic Properties of Isolated Hypertrophied Hearts from DOCA Hypertensive Rats

Alterations in cardiac norepinephrine (NE) levels often accompany cardiac hypertrophy. The present study addresses the question of whether such differences in endogenous NE might be responsible for hypertrophy-dependent differences in acutely measured mechanical and metabolic properties of isolated rat heart preparations. Hypertrophied hearts (H) of deoxycorticosterone acetate-hypertensive rats and normal hearts (N) of sham-treated rats were perfused in Langendorff fashion with atrial tissue removed. Assessment of ventricular NE after 3 to 5 min indicated that the concentration was lower in H than N but total content of the left ventricle was not significantly different. Assessment of ventricular NE after three hours suggested that some washout or depletion of NE occurred in H but not N. Mechanical and metabolic characteristics of the isovolumic preparation were determined. When compared to N, H had greater spontaneous ventricular beating rates and, when paced with field stimuli, higher systolic pressures, longer relaxation times and mechanical refractory periods, and greater oxygen consumption. When the latter was normalized for ventricular mass and systolic pressure, H utilized less oxygen/g/100 mmHg pressure than N. Elimination of the acute effects of endogenous NE by either blockade (propranolol) or depletion (reserpine) eliminated the difference in ventricular automaticity between H and N, but did not influence the other differences (or similarities) between H and N. We conclude that aside from changes in ventricular automaticity, hypertrophy-dependent alterations in cardiac mechanical and metabolic properties are not likely to be a result of differing acute influences of endogenous norepinephrine.

Cardiac Anaphylaxis in Isolated Guinea Pig Hearts Perfused at Constant Flow or Constant Pressure

Abstract Acute responses to antigen-antibody interactions (anaphylactic reactions) in isolated guinea pig hearts are reported to include decreases in coronary flow, increases in heart rate, prolongation of impulse propagation, development of arrhythmias, and transient increases followed by substantial decreases in ventricular contractile force. It is not clear from these studies, however, whether all of the changes are direct effects of the mediators released by the antigen-antibody reaction or whether some of them are indirect results of the severe reduction in flow evoked by coronary vasoconstriction. Therefore, the present study was designed to assess cardiac anaphylactic events in isolated hearts of guinea pigs passively sensitized with IgG antibody to ovalbumin under conditions in which coronary perfusion pressure was maintained constant and to compare the responses to those of hearts in which coronary flow was maintained at a constant rate. Our data indicate that when coronary flow decreased during anaphylaxis (constant pressure perfusion), hearts responded to antigen challenge with greater (i) prolongation of the PR interval, (ii) duration of arrhythmias, (iii) suppression of left ventricular systolic pressure, and (iv) release of histamine and adenosine plus inosine into the venous effluent than when coronary flow was maintained during anaphylaxis (constant flow perfusion). The data suggest that maintenance of coronary flow during cardiac anaphylaxis may attenuate the severity of the functional derangement.

Relationship between alterations in atrial and ventricular histamine content and cardiac function during cardiac anaphylaxis of isolated guinea pig hearts.

Antigen-induced histamine release from sensitized cardiac tissue has been shown to contribute significantly to the increases in atrial rate, atrioventricular nodal conduction, and ventricular contractile force that occur during cardiac anaphylaxis. These findings suggest that there might be a strong negative correlation between changes in these variables and the residual histamine content in the tissue after the anaphylactic reaction. In the present study using isolated hearts of passively sensitized guinea pigs, antigen challenge evoked transient increases in atrial automaticity, P-R intervals, vascular resistance, and left ventricular pressure. The histamine content of atrial and ventricular tissue from antigen-challenge hearts (1,475 +/- 296 and 4,543 +/- 360 ng/g, respectively) was significantly less than that of nonchallenged hearts (2,652 +/- 335 and 6,298 +/- 251 ng/g, respectively). Significant correlations found were between the residual histamine content of the ventricular tissue or the total histamine released and the magnitude of the antigen-induced increase in left ventricular systolic pressure. These findings suggest that antigen-induced increases in left ventricular systolic pressure can be used as an index of the local ventricular histamine release. The lack of significant correlations between local residual histamine levels and changes in atrial rate, P-R intervals, or coronary vascular resistance suggests that complicated interactions between histamine and other mediators or factors may be involved in the antigen-induced chronotropic, dromotropic, and vasoconstrictive alterations in these preparations.