Tissue restoration is the process whereby multiple damaged cell types are replaced to restore the histoarchitecture and function to the tissue. Several theories have been proposed to explain the phenomenon of tissue restoration in amphibians and in animals belonging to higher orders. These theories include dedifferentiation of damaged tissues, transdifferentiation of lineage-committed progenitor cells, and activation of reserve precursor cells. Studies by Young et al. and others demonstrated that connective tissue compartments throughout postnatal individuals contain reserve precursor cells. Subsequent repetitive single cell-cloning and cell-sorting studies revealed that these reserve precursor cells consisted of multiple populations of cells, including tissue-specific progenitor cells, germ-layer lineage stem cells, and pluripotent stem cells. Tissue-specific progenitor cells display various capacities for differentiation, ranging from unipotency (forming a single cell type) to multipotency (forming multiple cell types). However, all progenitor cells demonstrate a finite life span of 50 to 70 population doublings before programmed cell senescence and cell death occurs. Germ-layer lineage stem cells can form a wider range of cell types than a progenitor cell. An individual germ-layer lineage stem cell can form all cells types within its respective germ-layer lineage (i.e., ectoderm, mesoderm, or endoderm). Pluripotent stem cells can form a wider range of cell types than a single germ-layer lineage stem cell. A single pluripotent stem cell can form cells belonging to all three germ layer lineages. Both germ-layer lineage stem cells and pluripotent stem cells exhibit extended capabilities for self-renewal, far surpassing the limited life span of progenitor cells (50-70 population doublings). The authors propose that the activation of quiescent tissue-specific progenitor cells, germ-layer lineage stem cells, and/or pluripotent stem cells may be a potential explanation, along with dedifferentiation and transdifferentiation, for the process of tissue restoration. Several model systems are currently being investigated to determine the possibilities of using these adult quiescent reserve precursor cells for tissue engineering.
In rats anesthetized with alpha-chloralose, doses of 0.1, 0.5, and 1 g/kg of ethanol produced an upward shift of baroreflex curves constructed by plotting the heart rate response against mean arterial pressure following evoked rises in mean arterial pressures by phenylephrine or angiotensin II. Whereas the upward shift of baroreceptor curves may be related, at least in part, to a higher base-line heart rate after ethanol, the data showed that the 1 g/kg dose of ethanol significantly depressed baroreflex sensitivity, suggesting that higher doses of ethanol impair baroreflex-mediated bradycardia. The phenylephrine, but not the angiotensin II or the nitroprusside, dose-response curves were shifted to the right after ethanol, indicating a decreased pressor responsiveness and suggesting that ethanol may have alpha-adrenergic blocking activity. This effect was also obtained in conscious rats. That this effect was not influenced by changes in baroreflex sensitivity was supported by the finding that a similar shift of the phenylephrine pressor-response curve was obtained in bilaterally vagotomized and hexamethonium-treated rats. Whether this effect of ethanol on baroreflex control of heart rate was influenced by anesthesia was investigated in conscious rats; the 1 g/kg dose of ethanol that produced the most significant decrease in baroreflex sensitivity was used in these experiments. Ethanol was still able to significantly inhibit baroreflex sensitivity in conscious rats, but the upward shift of the baroreflex curve and the elevated base-line heart rate no longer occurred.(ABSTRACT TRUNCATED AT 250 WORDS)
Experiments were performed to determine the mechanism of vasopressinergic pulmonary vasodilation in isolated, salt-perfused rat lungs. Administration of a 50-ng bolus of arginine vasopressin (AVP) to lungs preconstricted with the synthetic thromboxane analogue U-46619 resulted in a 66% reversal of pulmonary vasoconstriction. Administration of the known endothelium-dependent vasodilator ATP resulted in a parallel decrease in pressure. The vasodilatory responses to both agents were significantly attenuated by pretreatment with the nitric oxide synthesis inhibitor N omega-nitro-L-arginine (L-NNA). In addition to attenuating the vasodilatory response to these agents, L-NNA pretreatment caused a significant augmentation of the pressor response to U-46619 without affecting baseline pulmonary arterial pressure. The attenuation of vasopressinergic pulmonary vasodilation by L-NNA was completely reversed by addition of excess substrate for NO production (50 mM L-arginine) but was unaffected by addition of equimolar amounts of D-arginine. Finally, L-NNA pretreatment failed to attenuate the vasodilatory actions of sodium nitroprusside and isoproterenol. We conclude that AVP dilates the preconstricted pulmonary vasculature via the release of nitric oxide.
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