The influence of hypoxia and metabolic inhibitors on the excitation process in atrioventricular node

Alterations in the ultrastructure of cardiac autonomic nervous system triggered by crotoxin from rattlesnake (Crotalus durissus cumanensis) venom

This study explored the toxic effects of crotoxin isolated from Crotalus durissus cumanensis venom on the ultrastructure of mice cardiac autonomic nervous system. Mice were intravenously injected with saline (control group) and crotoxin diluted in saline venom (study group) at a dose of 0.107 mg/kg mouse body weight. Samples from the inter-ventricular septum were prepared for electron microscopy after 6 h (G1), 12 h (G2), 24 h (G3) and 48 h (G4). The G1 group showed some cardiomyocyte with pleomorphic mitochondria. Capillary swollen walls, nerve cholinergic endings with depleted acetylcholine vesicles in their interior and other depletions were observed. A space completely lacking in contractile elements was noticed. The G2 group demonstrated a myelinic figure, a subsarcolemic region with few myofibrils and nervous cholinergic terminal with scarce vacuoles in their interior. The G3 group demonstrated a structure with a depleted axonic terminal, mitochondrias varying in size and enhanced electron density. In addition, muscular fibers with myofibrillar structure disorganization, a depleted nervous structure surrounded by a Schwann cell along with an abundance of natriuretic peptides, were seen. An amyelinic terminal with depleted Schwann cell and with scarce vesicles was also observed. Finally, axonic lysis with autophagic vacuoles in their interior and condensed mitochondria was observed in the G4 group. This work describes the first report of ultrastructural damage caused by crotoxin on mice cardiac autonomic nervous system.

Ionic mechanisms of the depression of automaticity and conduction in the rabbit atrioventricular node caused by hypoxia or metabolic inhibition and protective action of glucose and valine

The effects of hypoxia and a metabolic inhibitor, sodium cyanide (NaCN), on the spontaneous action potential and membrane current systems were studied in small preparations (0.2 x 0.2 x 0.1 mm) of rabbit atrioventricular node. When the PO2 of the superfusate was lowered from 500 to less than 20 mm Hg, all the preparations initially showed reductions in the spontaneous firing frequency (due to the decreased rate of diastolic depolarization) and maximal rate of depolarization, which were followed by a cessation of automatic activity with subsequent membrane hyperpolarization to -57 +/- 3 mV (n = 4). Voltage clamp experiments using double microelectrode techniques revealed that hypoxia reduced the slow inward current by 20 to 80% without affecting its inactivation kinetics. The delayed rectifying potassium current tail was also reduced or abolished by severe hypoxia, whereas the background outward current was greatly increased. The hyperpolarization-activated inward current was decreased by hypoxia. Similar changes in the spontaneous action potential and membrane currents were obtained on adding 0.1 to 0.5 mM NaCN to the superfusate under conditions of normal PO2. Hypoxia-induced suppression of automaticity was reversed completely by 5 mM glucose, and partially by 40 mM valine. These results suggest that (1) hypoxia impairs atrioventricular nodal conduction by reducing the slow inward current and the delayed rectifying potassium current; (2) hypoxia depresses automaticity by increasing the background outward current and, to some extent, by reducing the slow inward current; and (3) glucose and valine contribute to the generation of high-energy phosphates in cytoplasm and mitochondria, respectively, and protect the atrioventricular node from hypoxia.

Comparison of the direct effects of nifedipine and verapamil on the electrical activity of the sinoatrial and atrioventricular nodes of the rabbit heart

We compared the effects of nifedipine and verapamil on the rabbit sinus and atrioventricular nodes. Both drugs slowed the rate of impulse initiation by sinus node cells. Verapamil exerted a greater negative chronotropic effect at low concentrations, but at higher concentrations verapamil and nifedipine were equipotent. Nifedipine also reduced the amplitude of sinus node action potentials and the Vmax of phase O, effects which are identical to those previously described for verapamil. Both drugs slowed AV nodal conduction and prolonged refractory periods, but verapamil was more potent than nifedipine. Nifedipine reduced the amplitude of AV nodal action potentials and Vmax of phase O the same as verapamil. Nifedipine and verapamil, therefore, have nearly identical direct effects on the nodes. The failure of nifedipine to depress AV nodal conduction in situ and abolish reentrant AV nodal tachycardia is probably a result of the decreased sensitivity of the AV node to nifedipine compared to verapamil.

PR interval in middle-aged men with overt and latent coronary heart disease compared to PR in angionegative and normal men of similar age

PR was measured prospectively in 2014 apparently healthy men ranging in age from 40 to 59 years (P subjects), and retrospectively in 652 hospitalized men with a diagnosis of coronary heart disease (CHD) (R subjects). A cardiovascular survey examination suggested CHD in 115 of the 2014 P subjects, coronary angiography of 105 of these confirmed this suspicion in 69. The following observations concerning PR were made: (1) the shortest PR was found in angiopositive P subjects, intermediate PR in 1832 normal P subjects, and PR was longest in 36 angionegative subjects. (2) PR was shortest in angiopositive P subjects with triple-vessel disease, and longest in P subjects with single-vessel disease, (3) PR increased with age, and an inverse association was found between PR and resting heart rate in P-subjects. (4) In 98 of 1832 normal P subjects, all without signs or symptoms of heart disease, PR was greater than or equal to 0.22 s. (5) Prolonged PR was no more frequent among the 652 R subjects than among P subjects, despite significant CHD in 595, and a frequent use of drugs known to delay atrioventricular conduction. It is suggested that advanced stable clinical CHD only rarely gives rise to prolonged PR at rest, and that PR in subjects with latent CHD may have somewhat shorter PR than age counterparts without symptoms or signs of CHD. Conflicting data in the literature are probably related to differences in material and methods.