Physiological Resistance

Larkin injected several peroxidases at 0.1-10 ppm into one-half of a tobacco leaf and found some protection. He suggested that peroxidase, which is often associated with plant stress conditions, may be important in physiologic resistance. It is doubtful that any one mechanism of action exists. It is important that we understand the mechanism of ozone injury and resistance in plants, so that we can determine better what chemicals may play a role in protecting plants against oxidants. 

Physiological resistance has been shown to involve three factors, i.e., reduced penetration, enhanced detoxification, and target site insensitivity. Generally, these resistance factors do not occur alone and are known to interact with each other, especially penetration and metabolism, to enhance the level of resistance. 

Fig. 12. Although advances in cancer treatment have improved quality of life for patients, the actual incidence and death rates in the United States from cancer have continued to increase in the past 20 years. (Source American Cancer Society.) Our hypothesis is that the effectiveness of current treatment methods is limited by physiological resistance and by intrinsic or acquired cellular resistance. The former impedes the delivery of blood-borne agents to solid tumors, and the latter reduces their effectiveness once these agents have reached the target cells. A better understanding of transport in tumors may help develop strategies to overcome or to exploit the physiological resistance for improved cancer treatment (Jain, 1994).

It must be noted that physiological resistance of biofilms to oxidising biocides is much less pronounced than for non-oxidising biocides. Combinations of both oxidising and non-oxidising biocides in one treatment help to offset the physiological resistance of biofilms because of their dual mechanism of action [35]. 

SAMs of tliiolates on gold are generally resistant to strong acids or bases [175, 178 and 179], are not destroyed by solvents [180] and can witlistand physiological environments [181, 182 and 183]. However, tliey also show some degradation if exposed to tire ambient atmosphere for sufficiently extended periods [184]. 

Repeated exposure to ethyl alcohol results in the development of a tolerance as evidenced by decreasing symptomatic reactions. It has been demonstrated that the symptoms of exposure are less clear and the time required to produce them is greater in subjects accustomed to alcohol. There is no proof, however, of physiological adaptation in humans in terms of metaboHc changes or resistance to cellular injuries. The subject of the interaction of alcohol with other dmgs has received much attention (277). 

Silicone rubbers find use because of their excellent thermal and electrical properties, their physiological inertness and their low compression set. Use is, however, restricted because of their poor hydrocarbon oil and solvent resistance (excepting the fluorosilicones), the low vulcanisate strength and the somewhat high cost. 

As recently as 1970, only about 30 naturally occurring organohalogen compounds were known. It was simply assumed that chloroform, halogenated phenols, chlorinated aromatic compounds called PCBs, and other such substances found in the environment were industrial pollutants. Now, only a third of a century later, the situation js quite different. More than 5000 organohalogen compounds have been found to occur naturally, and tens of thousands more surely exist. From a simple compound like chloromethane to an extremely complex one like vancomycin, a remarkably diverse range of organohalogen compounds exists in plants, bacteria, and animals. Many even have valuable physiological activity. Vancomycin, for instance, is a powerful antibiotic produced by the bacterium Amycolatopsis orientalis and used clinically to treat methicillin-resistant Staphylococcus aureus (MRSA). 

Although blood pressure control follows Ohm s law and seems to be simple, it underlies a complex circuit of interrelated systems. Hence, numerous physiologic systems that have pleiotropic effects and interact in complex fashion have been found to modulate blood pressure. Because of their number and complexity it is beyond the scope of the current account to cover all mechanisms and feedback circuits involved in blood pressure control. Rather, an overview of the clinically most relevant ones is presented. These systems include the heart, the blood vessels, the extracellular volume, the kidneys, the nervous system, a variety of humoral factors, and molecular events at the cellular level. They are intertwined to maintain adequate tissue perfusion and nutrition. Normal blood pressure control can be related to cardiac output and the total peripheral resistance. The stroke volume and the heart rate determine cardiac output. Each cycle of cardiac contraction propels a bolus of about 70 ml blood into the systemic arterial system. As one example of the interaction of these multiple systems, the stroke volume is dependent in part on intravascular volume regulated by the kidneys as well as on myocardial contractility. The latter is, in turn, a complex function involving sympathetic and parasympathetic control of heart rate intrinsic activity of the cardiac conduction system complex membrane transport and cellular events requiring influx of calcium, which lead to myocardial fibre shortening and relaxation and affects the humoral substances (e.g., catecholamines) in stimulation heart rate and myocardial fibre tension. 

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