Effect of low temperatures on rat myocardial Mg-ATPase and NaK-ATPase

Novel method to avoid serious injurious effects on the atrioventricular nodal (AVN) conduction during catheter ablation of the AVN slow pathway utilizing cryofreezing energy

BACKGROUND

Slow pathway elimination of the atrioventricular node (AVN) is essential to treat AVN reentrant tachycardia (AVNRT). However, injury to the AVN conduction (IAVN) is one of the serious complications. Cryofreezing energy is expected to reduce the incidence of IAVN. This study aimed to investigate the usefulness of a novel method to avoid IAVN during cryoablation of AVNRT.

METHODS

A total of 157 patients (average age, 65.8 years; male, 71) suffering from AVNRT were included. Once the AVNRT terminated during cryo-ablation, then rapid atrial constant pacing (RACP) was performed during freezing at a rate lower 10 bpm than that inducing Wenchebach AV block in 74 (47.1 %) patients (Group A). The RACP rate was decreasingly reduced by 10 bpm in case of the occurrence of IAVN. When the RACP reached 100 bpm, the cryoablation was prematurely terminated. Group B patients (83 = 52.9 %) underwent cryoablation during sinus rhythm. All patients were allocated in a randomized fashion. We compared the severity of the IAVN between Groups A and B.

RESULTS

There were no significant differences at 12 months regarding the freedom from the AVNRT between Groups A and B. However, the duration of the IAVN was significantly longer in Group B than A (p = 0.02). There were no significant differences regarding the distance between the His recording sites and successful ablation sites between Groups A and B. No permanent IAVN requiring pacemaker implantation was provoked in either group.

CONCLUSION

RACP was useful to avoid sustained and serious IAVN during cryoablation of AVNRT.

Effect of low temperature on the cytosolic free Ca2+ in rat ventricular myocytes

The effect of low temperature on the cytosolic free Ca2+ [( Ca2+]i) has been investigated in isolated ventricular myocytes from adult rats using the fluorescent probe Indo-1. The distribution of Indo-1 between the mitochondrial and cytoplasmic compartments was first determined in the isolated myocytes using the digitonin and Triton X-100 treatments. By subtracting the mitochondrial [Ca2+]i from the total [Ca2+]i measured with Indo-1, the average cytosolic [Ca2+]i was found to increase significantly (P less than 0.05) from 139 nM to 255 and 297 nM when the temperature was decreased from 37 degrees C to 15 degrees and 5 degrees C, respectively. A marked increase in cytosolic [Ca2+]i to a new steady state level was observed when the membrane of myocytes was depolarized by 60 mM KCI; the average magnitude of increase being 110, 243 and 186 nM, at 37 degrees, 15 degrees and 5 degrees C respectively. Our results support the hypothesis that the cardiac arrhythmia typically observed in the hypothermic rat is due to an increased cytosolic [Ca2+]i with decreasing body temperature.

The effect of temperature on the basal metabolism of cardiac muscle

Using independent methods, measurements were made of the rate of oxygen consumption of quiescent rat ventricular tissue slices and of K+-arrested rabbit hearts at different temperatures. Experiments were designed such that the effect of temperature could be separated from the effect of time. The rate of oxygen consumption of both cardiac muscle preparations declined with time following cardiectomy. Likewise the rate of resting oxygen consumption of both preparations was relatively insensitive to temperature: Q10 = 1.3. By contrast, the rate of oxygen consumption of rat liver slices showed a Q10 of 2.6. The low Q10 values of the cardiac preparations do not appear to be due to an inadequate supply of oxygen. The results are in close agreement with those of myothermic studies.

Functional characteristics and responses to adrenergic stimulation of isolated heart preparations from hypothermic and hibernating subjects

Contemporary studies of isolated hearts from hibernators and nonhibernators are presented. Original experiments with isolated perfused hamster hearts are reported. Such hearts can maintain left ventricular function at temperatures as low as 7 degrees C. Generated left ventricular pressure was 40 +/- 9 mm Hg and heart rate was 7 +/- 1 beats/min. During cooling heart rate dropped dramatically, coronary flow increased, and ventricular pressure decreased initially, plateaued, and then fell as 7 degrees C was approached. Norepinephrine can cause increased heart rate and left ventricular pressure at 22 and 7 degrees C. This positive inotropic and chronotropic response was associated with increased cAMP at 30 sec after stimulation at 22 degrees C but not at 7 degrees C. Furthermore, cAMP was also not changed at peak response at 7 degrees C. Isoproterenol increased cAMP content in 37 degrees C ventricular slices but not at hypothermic temperatures. Possible mechanisms of nonadenylate cyclase mediation of inotropic and chronotropic responses at 7 degrees C are discussed.

Erythrocyte sodium-potassium-stimulated adenosine triphosphatase activity is not related to obesity

Altered erythrocyte sodium potassium (Na,K)-stimulated adenosine triphosphatase (ATPase) activity has been cited as having pathophysiologic significance in morbidly obese man. Previous studies have failed to consider obese patients after weight loss and, therefore, did not clarify the role of ATPase deficiency as a cause or effect of the obese state. To define more completely the possible alteration of cellular thermogenesis in obesity, a study was made of three groups of people: (1) normal weight controls; (2) morbidly obese; and (3) formerly morbidly obese patients who had lost over 100 pounds after gastric bypass surgery. Erythrocyte ATPase activity was determined by use of an assay that coupled ATPase activity with NADH oxidation in the presence of excess pyruvate kinase, lactic dehydrogenase, and phosphoenolpyruvate. This coupled assay produced a continuous slope so that activity could be calculated from the initial, maximal, linear portion of the decay trace. Results did not demonstrate any statistically significant differences in Na,K-ATPase activity between groups by analysis of variance. A nonsignificant correlation of 0.086 was seen between obesity index and Na,K-ATPase activity. It is concluded that (1) erythrocyte Na,K-ATPase activity is similar in both normal and obese individuals, (2) erythrocyte Na,K-ATPase does not change with weight loss, and (3) therefore, disordered erythrocyte thermogenesis does not have a role in the development or maintenance of obesity.