Static and dynamic stresses during valve closure of a bileaflet mechanical heart valve prosthesis.

The effect of contact geometry and component compliance on the magnitude, distribution, and state of various types of stresses on a bileaflet mechanical heart valve prosthesis during valve closure was analyzed using an Edwards-Duromedics mitral valve as example. Static and dynamic stresses developing on both the leaflet and pivot ball during valve closure were modeled using finite element analysis (FEA). Uniform contact between the leaflet and housing as well as between the pivot ball and pivot slot can significantly reduce both static and dynamic stresses around the contact area. The level of the dynamic flexural stresses can be an order of magnitude higher than that of the static stresses. When both the radial and axial compliance of the housing are taken into consideration, peak dynamic stress was more than 40% less than that generated through the impact between a moving leaflet and a non-compliant rigid housing.

Reactive capacity: a sensitive behavioral marker of movement initiation and nigrostriatal dopamine function.

Thirty-two Long Evans male rats with sham operations or unilateral 6-OHDA-induced damage to meso-telencephalic dopaminergic neurons were evaluated on a reactive capacity task that demanded high speed movement initiation. The task required lever manipulation to avoid signalled shock. The interval between the warning and the shock was incrementally reduced. A one-sleeved vest provided the opportunity to measure movement initiation of each limb independently. Extent of lesion was assessed by [3H]DA uptake, [3H]spiroperidol binding, or DA levels. Movement initiation latencies for each forelimb were found to be linearly related to interhemispheric striatal DA asymmetry induced by microinjections of 6-OHDA. Even those lesions resulting in small to moderate decreases in DA function, including deficits causing no chronic posture or sensory asymmetries, resulted in reactive capacity deficits and greatly slowed reaction time in the paw contralateral to the lesion. Following severe lesions, small yet substantial deficits were also seen in ipsilateral paw performance, which may be related to DA depletions found in the non-lesioned striatum. Thus, a reactive capacity task which requires the animal to react with maximal speed appears to be a potentially good index of nigrostriatal dopamine integrity even when the depletion is not severe.

The effects of exercise training on [3H]-spiperone binding in rat striatum.

Thirty male Sprague-Dawley rats 100 days of age were divided into three groups: interval trained, endurance trained, and pair-weighted controls. Both trained groups ran up to one hour per day, 6 days per week for 12 weeks. The interval trained group ran up to 20 repeat intervals at 54 meters per minute for 30 seconds, while the endurance trained group ran at 27 meters per minute for 60 minutes. The animals were sacrificed, and the effects of aerobic training were documented by measuring cytochrome oxidase activity in the mixed quadriceps muscles. The cytochrome oxidase activity of the interval and endurance trained groups increased 49%, and 31% respectively, above the control group. [3H]-spiperone was used to label dopamine receptors in the striatum. The endurance group was not significantly different from the interval group in [3H]-spiperone receptor binding, so the two exercise groups were combined to form one group of runners. The runners had significantly higher [3H]-spiperone receptor binding than the controls, F(1,26) = 4.87, p less than 0.05. The mean and standard error for receptor binding was 89 +/- 13 fmoles/mg protein for the runners and 60 +/- 5 fmoles/mg protein for the controls.

Speed of movement initiation performance predicts differences in [3H]spiroperidol receptor binding in normal rats.

Speed of movement initiation is altered in normal aging and in neurological disorders such as Parkinson's disease. This study was undertaken to extend our previous results, which suggested a relationship between nigrostriatal dopamine function and an animal model of movement initiation speed (reactive capacity). Fisher 344 rats exhibiting exceptionally fast or slow reactive capacity but otherwise normal were examined for differences in the striatal binding of the dopaminergic ligand, [3H]spiroperidol. Rats with fast reactive capacity (Fast rats) exhibited significantly higher binding than did rats with slow reactive capacity (Slow rats). Also, Fast rats responded nearly maximally on the reactive capacity task regardless of the duration of time provided in which to respond, whereas Slow rats reacted more slowly when more response time was provided. The differences in [3H]spiroperidol binding and the differential influence of time provided to respond on the response latency of these two normal groups of rats was similar to that observed in old or model Parkinson's disease rats having nigrostriatal dopamine deficits. These results strengthen the relationship between an animal model of reactive capacity and nigrostriatal dopamine function.