Effect of hypoxia on cellular interaction in the myocardium

Ultrastructural-functional basis for spontaneous termination of ventricular fibrillation in mammals

Ventricular fibrillation in humans is generally sustained (SVF), but it can be also transient (TVF), reverting spontaneously to sinus rhythm. In previous studies we have shown that: a) TVF appears in all young mammals and varies according to age and species; b) it requires synchronization of myocardial cell activity; c) infusion of certain drugs may change the type of ventricular fibrillation from sustained into transient. We hypothesize that the synchronization required for TVF depends on the electrical conductivity of intercellular structures. These intercellular couplings differ among species and decrease with age. Comparison between the inter- and intra-specific variations of intercellular connective structure described in the literature with the type of ventricular fibrillation found in our previous studies on various animals of different ages showed a clear relationship between these histological variations and the changes in the type of ventricular fibrillation. In this study we examined intercellular connective structures ultrastructurally in 3 groups of cats: a. control, untreated cats exhibiting sustained ventricular fibrillation; b. untreated cats exhibiting transient ventricular fibrillation; c. treated cats exhibiting sustained ventricular fibrillation before infusion of a defibrillating drug and transient ventricular fibrillation thereafter. It was found that the intercellular connective structure in cats exhibiting sustained ventricular fibrillation differs significantly from that in cats exhibiting transient fibrillation. In hearts exhibiting sustained ventricular fibrillation, many intercellular connective structures are widened and the degree of widening is pronounced, forming a continuous line, while in hearts exhibiting transient ventricular fibrillation the widened junctions are rare and isolated and the widening is relatively small. These preliminary results strongly support our above-mentioned hypothesis, providing an explanation for the origin of transient ventricular fibrillation and a tool for the development of new defibrillating drugs.

Effects of pCai and pHi on cell-to-cell coupling

Internal longitudinal resistance (ri), a determinant of cardiac conduction, is affected by changes in intracellular calcium and protons. However, the role and mechanism by which H+ and Ca2+ may modulate ri is uncertain. Cable analysis was performed in cardiac Purkinje fibers to measure ri during various interventions. In some experiments, intracellular pH (pHi) was recorded simultaneously to study the pHi-ri relation. Both intracellular Ca2+ and H+ independently modified ri. However, internal resistance of cardiac fibers was insensitive to pHi changes compared to other tissues. A latent period preceded the pHi-related changes in ri and the amount of change depended upon methodology. The results suggest that direct action of protons or ri may be subordinate to other regulatory processes. Ionic regulation of internal longitudinal resistance may occur by more than one mechanism: i) direct cationic binding to sites on junctional membrane proteins; and ii) H+- or Ca2+-dependent phosphorylation of junctional proteins.

Electrophysiology of the normal-to-hypoxic transition zone

The "normal-to-hypoxic" transition zone was modeled after the chamber technique for perpendicular and parallel orientation with respect to the direction of fiber axis. The evidence obtained from the recording of the transmembrane action potentials suggests the presence of a preservation phenomenon in the hypoxic area, based on the utilization of energy stores of the normoxic area. Better intercellular coupling in longitudinal than in transverse fiber direction results in the anisotropic properties of the preservation phenomenon. The preservation phenomenon provides a basis for the existence of critical size of the viable hypoxic area compared to the size of the transitional zone. The crucial role of electrotonic coupling was demonstrated, as well as the possible contribution to the preservation phenomenon mechanism of the cell-to-cell diffusion of metabolites.

Effects of hypoxia on passive electrical properties of canine ventricular muscle

The effect of hypoxia, either in the presence or in the absence of glucose, on the passive electrical properties of canine ventricular muscle fibers was examined, employing a single sucrose gap method. The significant changes after 30 min of hypoxia (PO2 = 35-45 mm Hg) were an increase in the specific internal longitudinal resistance (Ri) and a decrease in the space constant (lambda). The values during the control (PO2 greater than 450 mm Hg) were 198 +/- 77 omega cm for Ri and 0.81 +/- 0.15 mm for lambda, and they changed to 245 +/- 90 omega cm and 0.70 +/- 0.10 mm, respectively, after 30 min of hypoxia. Hypoxia decreased the specific membrane resistance (Rm), but the changes were not statistically significant. The membrane time constant (tau m) and capacity (Cm) were not affected significantly. The absence of glucose during hypoxia was found to cause more profound changes than hypoxia alone in the passive electrical properties, especially Ri and lambda, suggesting that glucose might counteract the effects of hypoxia on these parameters of ventricular muscles.

Decreased intercellular coupling after prolonged rapid stimulation in rabbit atrial muscle

Driving rabbit atrial trabeculae at a rapid rate for 15 minutes resulted in a decrease in the space constant for electrotonic decay from an average of 670 to 440 micrometers. Input resistance, Rin, as measured by use of a double-barrelled microelectrode, increased from a mean value of 380 kOhms to one of 600 kOhms. The time to return to control values after the end of rapid driving was 20-60 minutes. Similar effects of rapid driving were observed in the presence of atropine, propranolol, and atropine plus propranolol and phentolamine. According to the theory of current spread in a three-dimensional syncytium, a rise of input resistance should be interpreted mainly as an increase of cell-to-cell resistance. We advance the hypothesis that, when driven at their maximal possible rate (or when fibrillating), cardiac cells gain Na+ ad Ca2+, and that this results in partial but reversible uncoupling.