Preconditioned State Research Articles

Preconditioning During Ischemia

In ischemic preconditioning a preliminary 5-min coronary occlusion is capable of reducing the extent of myocardial infarction after a subsequent prolonged coronary occlusion. This remarkable endogenous protection is triggered by release of adenosine during the brief ischemic period. Occupied adenosine receptors couple to pertussis toxin-senstive G proteins that in turn activate phospholipase C, leading to membrane phospholipid metabolism and release on diacylglycerol. The latter leads to stimulation of protein kinase C (PKC), presumably by cadsing translocation of inactive cytosolic enzyme to the cell membranes, where it resides in its activated state. As long as PKC is resident in the membranes approximately 1 hour in rabbits the cell is in the preconditioned state and PKC is poised to act to protect the cell. If there is recurrent ischemia within that hour renewed release of adenosine reinitiates intracellular signalling with the production of more diacylglycerol. Because PKC is already in the membranes restimulation can result in almost immediate phosphorviation of critical cell constituents one of which is the yet unidentified end effector of protection. In this scheme, adenosine release occurs twice and PKC is stimulated twice—to translocate during the first ischemia and to phospherylate during the second ischemia. Other receptors such as muscarinic M2 angiotensin ± AT2 α1-adrenergic, and bradykinin B2 are also known to couple to PKC. Exogenous administration of any of these in new of the brief ischemia can also precondition rabbit myocardium, Chronic receptor stimulation by exogenous administration of an adenosine analogue result in receptor downregulation and loss of the ability to protect the myocardium with additional adenosine analogue infusion. Therefore, clinical application must await development of an agent that can initiate the protection cascade without the development of tolerance. But this preconditioning phenomenon is the first described technique of clear and unequivocal salvage of ischemic myocardium and as such it holds great promise.

The effect of long-term low bacterial density on the growth kinetics of three marine heterotrophic nanoflagellates

Three marine helerotrophic nanoflagellates were each maintained under two cultural conditions for one month. They were either subcultured every 5 days in batch culture, to familiarise the cells to high bacterial density, or they were maintained in continuous culture in the presence of low bacterial density to familiarise the flagellate cells to this biotic state. The kinetic behaviour of each of the species and each preconditioned state were compared in batch culture grazing experiments. The maximum specific growth rate (/ gm max ) and half-saturation constant ( K s ) were the parameters used for comparison. After low prey preconditioning, Paraphysmnonas imperforuta was found to have a μ max value less than half that obtained if the culture was maintained with high prey density and this species did not increase its affinity for the bacterium under chemostat conditions. Bodo designis however, increased its affinity for the bacterium but the μ max value was severely reduced. Stephanoeca diplocostata increased its affinity for the bacterium and the μ amx value after low prey density conditions. It appeared that the maintenance of the cultures at < μ max in the chemoslats and the absence of bacterial aggregates may have reduced the performance of both P. imperforata, because it prefers being attached whilst feeding on bacteria in suspension, and B. designis, because it prefers to feed on attached bacteria. S. diplocostata on the other hand, appeared not to rely on aggregate formation for good performance and could therefore be a representative of pelagic flagellates known to occur as unattached cells in relatively oligotrophic waters.

Preconditioning for Stratospheric Sudden Warmings: Sensitivity Studies with a Numerical Model

Results are presented of numerical experiments performed to determine what constitutes preconditioning for stratospheric sudden warmings. A number of 20-d runs were performed with a 3D primitive equation model. Runs varied in that they had different (zonally symmetric) initial fields and different lower boundary planetary wave fields, both based on NMC data for particular days. Initial and boundary data were used from the two minor and the sole major sudden warmings of January-February 1979 and some minor and the sole major sudden warmings of January-February 1989. To isolate the properties of the initial conditions from preconditioned cases and those with no warming, hybrid initial fields were created by combining data from two different days, one preconditioned and the other not. Runs with the hybrid initial conditions indicate that it is the lower stratospheric winds that are important in characterizing a preconditioned flow. It is suggested that it would be possible for a preconditioned state to exist for at least a short time without a warming occurring.

Climatology of the Stratospheric Polar Vortex and Planetary Wave Breaking

We use the distribution of Ertel's potential vorticity (PV) on the 850 K isentropic surface to establish a climatology for the transient evolution of the planetary scale circulation in the Northern Hemisphere winter midstratosphere. We compute PV distribution from gridded NMC daily temperature and height maps for the 10 and 30 mb levels, and show that a very good approximation for 850 K PV can be derived from 10 mb heights and temperatures alone. We assume that reversals of the latitudinal gradient of PV, localized in longitude and latitude may be regarded as signatures of planetary wave breaking. Wave breaking identified by such signatures tends to occur mainly in the vicinity of the Aleutian anticyclone, with a secondary maximum over Europe. The area of the polar vortex, defined as the area enclosed by PV contours greater than a certain critical value, is strongly influenced by wave breaking. Erosion of the polar vortex due to transport and mixing of PV leads to a preconditioned state, when defined in terms of vortex area, that always occurs prior to major stratospheric warmings. During winters with little PV transport or mixing, the vortex area evolves rather uniformly in response to radiative forcing. During winters with major sudden warmings, the wave breaking signature as defined here first appears at low values of PV, then rapidly moves toward higher values as the vortex area is reduced and the “surf-zone” structure becomes well defined.

Stratospheric Sudden Coolings and the Role of Nonlinear Wave Interactions in Preconditioning the Circumpolar Flow

In a, series of idealized numerical experiments, Butchart et al (1982) have recently established that the configuration of the mean zonal wind occurring immediately before the wavenumber-2 major stratospheric warming of February 1979 was crucial in subsequently focusing upward propagating planetary wave-activity into the high latitude stratosphere. In this sense, it was concluded that the stratospheric circumpolar flow should evolve to some preconditioned state before a wavenumber-2 major warming could occur. In the present paper, the mechanisms responsible for the transition of the circumpolar flow from its normal midwinter state to this preconditioned state are investigated through a combination of observational numerical and theoretical studies. For the 1978–79 winter, this transition occurred during the substantial wavenumber-1 minor warming of January 1979, and the characteristic structure associated with the preconditioned mean zonal flow was established four days after the peak of this warming, during a period of intense high latitude acceleration. This latter phenomenon is referred to as a stratospheric sudden cooling. Observations of Eliassen-Palm flux cross-sections indicate that while wave, zonal mean-flow interaction theory could account for the qualitative evolution of the circumpolar flow during the warming, substantial nonlinear wave interactions were active during the cooling period, and these interactions significantly influenced the evolution of the circumpolar flow. In a series of numerical experiments using a truncated semi-spectral model, we show that this sudden cooling phenomenon can be realistically reproduced in an idealized integration in which wave-wave interactions are present. By contrast, we were unable to simulate this phenomenon with these interactions removed. Two different mechanisms are put forward to account for these nonlinearities. One mechanism is that of wave-breaking and associated potential vorticity mixing, as suggested by McIntyre (1982). The second mechanism is based on the notion of wave-activity, forced in the troposphere, propagating relative to isopleths of potential vorticity of some zonally asymmetric basic state. Results of the observational and numerical study suggest that the first mechanism was dominant, and that potential vorticity mixing in the outer regions of the polar vortex was central to the process of preconditioning. Nevertheless, we believe that the second mechanism plays an important role in the dynamics of the stratosphere.