In the present studies, the addition of HOCl to the bathing medium for isometrically contracting papillary rat papillary muscles induced the development of contracture. This was characterized by a decline in developed tension (DT) in combination with a rise in resting tension (RT), a prolongation in relaxation kinetics, and a sensitivity of the muscles to stimulation voltage. This response to HOCl was potentiated by preincubation with low extracellular Ca$\sp{2+}$ or the Ca$\sp{2+}$ channel antagonist nifedipine. Conversely, the response was attenuated by preincubation with high extracellular Ca$\sp{2+}$ concentrations or with the Ca$\sp{2+}$ channel agonist Bay K 8644. The development of contracture was prevented by preincubation with 1 nM ryanodine, an alkaloid compound known to alter Ca$\sp{2+}$ handling by the sarcoplasmic reticulum. In isolated whole-cell clamped cardiac myocytes, using the Ca$\sp{2+}$ fluorescence indicator Fura-2 to monitor (Ca$\sp{2+}$) $\sb{\rm i}$, HOCl induced a steady rise in diastolic (Ca$\sp{2+}$) $\sb{\rm i}$ even in the absence of extracellular Ca$\sp{2+}$. This rise in (Ca$\sp{2+}$) $\sb{\rm i}$ was prevented by pre-exposure of the cell to 5 mM caffeine, a compound known to deplete the SR of stored Ca$\sp{2+}$. In oxalate loaded SR vesicles isolated from rat hearts perfused with HOCl, both the activity of the SR Ca$\sp{2+}$-ATPase (determined by inorganic phosphate production) and the uptake of $\sp{45}$Ca$\sp{2+}$ were significantly depressed as compared to Control. I conclude that the exposure of the rat myocardium to HOCl inactivates the SR Ca$\sp{2+}$-ATPase through the oxidation of protein thiols, leading to a cytosolic Ca$\sp{2+}$ overload and the development of contracture. Cursery evidence also suggests that the Ca$\sp{2+}$ pump of the sarcolemma and the Na$\sp+$/Ca$\sp{2+}$ exchanger may also be inactivated. Subsequent exposure of the HOCl-treated myocardium to the disulfide reducing agent dithiothreitol (DTT) resulted in a restoration of contractile function, characterized by a decline in RT concomitant with a recovery of DT in the isolated papillary model, or a recovery of left ventricular end diastolic pressure and developed pressure in the perfused rat heart. We conclude that DTT was able to cross cellular membranes where it restored cellular protein thiol levels by reducing HOCl-induced disulfide formation. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/7601 |
| Date | January 1991 |
| Creators | Eley, Douglas W. |
| Contributors | Korecky, B., |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 314 p. |
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