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The "fluidity" of brain synaptosomal membrane preparations of arctic and hot-springs fish species, two temperate water fish species acclimated to different seasonal temperatures, and two mammals was estimated using the fluorescence polarization technique. At all measurement temperatures, the fluidity decreased in the order: arctic sculpin, 50-acclimated goldfish, 250-acclimated goldfish, desert pupfish, and rat. This correlated with increasing adaptation or body (i.e., cellular) temperatures of 0°, 50, 250, 340, and 370 and suggested a partial compensation of membrane fluidity for environmental temperature that occurs over the evolutionary time period as well as during laboratory (seasonal) acclimation. Evolutionary adaptation of relatively stenothermal species to constant thermal environments resulted in a more complete compensation than laboratory (seasonal) acclimation. Each compensation is accompanied by differences in the saturation of membrane phosphoglycerides. At increased cellular temperatures the proportion of saturated fatty acids increased and the unsaturation index decreased; the correlation between these indices and the measured expression of membrane dynamic structure was highly significant. It is concluded that the homeoviscous compensation of synaptic membrane function is an important component of temperature adaptation.Biological membranes resemble a two-dimensional, hydrophobic fluid whose dynamic nature has important consequences for a number of membrane-associated functional properties (1). A variety of organisms possess the ability to modulate the fluidity of their constituent cellular membranes in compensation for the direct effects of altered environmental temperature, a phenomenon termed "homeoviscous adaptation" (2). In Tetrahymena (3) and the synaptosomal membranes of the goldfish Carassius auratus (4), partial compensation is achieved after laboratory acclimation at different temperatures, but it is somewhat less than that required to maintain a constant "fluidity" at all environmental temperatures; partial compensation may be associated with the eurythermal properties of these animals. Bacterial membranes show a complete compensation (2, 5).Fishes that inhabit relatively constant thermal environments, particularly such extremes as polar seas or thermal springs, may be expected to exhibit a more complete homeoviscous adaptation because for them the maintenance of a eurythermal ability has no evolutionary significance. Indeed, stenothermal species often exhibit a high degree of adaptation to their respective environments such that they perish at temperatures only slightly removed from normal (6). To test the hypothesis of homeoviscous adaptation, we present here comparative studies of the fluidity and biochemical composition of synaptosomal membranes isolated from fish that live in arctic or hot-springs environments, other fish adapted to a wide range of temperatures, and rat and hamster. Synaptosomal memThe costs of publication of this article were defrayed in part by the pay...

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Electrical slow waves were recorded by intracellular electrodes and by quasi-intracellular pressure and suction electrodes from muscle fibers at different levels in edgewise preparations of cat jejunum. Simultaneous recordings from longitudinal and circular muscle layers showed similar resting potentials from either muscle layer near the boundary zone, and lower resting potentials in cells of circular muscle near the submucosa. Slow waves were maximal in amplitude at the boundary between the two layers and spread electrotonically away from the boundary in both layers. Bipolar recordings were of opposite polarity on the two sides of the boundary. Amplitudes of slow waves from inner circular fibers were significantly lower than from outer circular fibers. Small strips of each muscle layer were prepared with or without the attached interstitial cells of Cajal plexus as identified by methylene blue staining. Either muscle layer showed slow waves from regions where interstitial cells of Cajal (ICC) were observed after the recording. No slow waves were recorded from either layer from regions where ICC were not observed. Strips containing ICC but not strips lacking ICC could be driven electrically. Since blocking of neurons does not abolish slow waves and since regions of muscle lacking ICC do not have slow waves, it is concluded that the interstitial cells (ICC-I) are most likely the boundary elements essential for slow waves in either layer of intestinal muscle.

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SUMMARY Numerous examples are cited of physiological characters which, like morphological characters, vary genotypically, ontogenetically or phenotypically. Physiological variation is useful in describing interspecific relations, intraspecific variation and the limits of ecological range. Physiological variation of a character permits the establishment of populations at environmental limits and thus predisposes a race for genetic fixation of a character (Baldwin effect). Such variation is primary in providing one mechanism of reproductive isolation. Secondary physiological variation functions in habitat selection, particularly after reproductive isolation. Criteria for physiological variation are given by measurement of internal state in relation to environmental stress. In regulators a criterion is the point of failure of homeostatic mechanisms; in adjusters a criterion is the limit of tolerance after maximum acclimation. Both regulation and adjustment are adaptive to environmental change; in both types, phenotypic variation must be distinguished from genotypic variation. Examples of physiological variation in osmotic adaptation, in different populations of the same species, are given for species of Paramecium, Euplotes, Asterias, Gammarus, Callinectes, Anopheles, Onchorhynchus and Gasterosteus. Genetically different races have been established for Gasterosteus aculeatus and Euplotes vassus. Ionic balance has been little studied as a basis for variation of populations but specific requirements for some elements, such as copper and calcium, may have led to local differences. Stress of temperature extremes has induced many physiological variants. Closely related large homoiotherms differ in insulating capacity, smaller species differ also in metabolic response, particularly to cold. Poikilotherms differ in tolerance limits after acclimation, in ‘selected’ temperatures, in metabolic level according to their temperature history, in Q10, in critical temperatures for reproduction and development. Physiological races with respect to temperature tolerance or selection are cited for species of Notropis, Micropterus, Gambusia, Pandalus and Carabus. Examples of metabolic differences in populations are found in aquatic but not terrestrial poikilotherms. Examples of races with respect to optimal temperature of development are found in species of Paramecium, Crassostrea, Urosalpinx, Daphnia, Drosophila, Lymantria and Rana. Criteria of variation with respect to oxygen are the half‐saturation value of transport pigments, the critical oxygen concentration, metabolic level, tolerance of products of anaerobiosis, auxiliary breathing mechanisms and stimulation of pigment development by hypoxia. Correlations of these functions with history of oxygen availability are cited, but data are inadequate to establish genetic races. Selection of specific food plants depends on secondary products, not primary food; many subspecies and races are known by food plant selection, especially among bugs, gall flies and butterflies. Preference can ...

10. Removal of external potassium produced a reversible sequence of events almost identical to those following ouabain application.11. Replacement of 50 % of the external sodium chloride with sucrose produced no changes in slow-wave activity with respect to rates of rise or fall, maximum amplitude or frequency. Sucrose replacement of all external sodium chloride eliminated slow waves after 5 min; however, activity could be restored by a slight hyperpolarization. Longer exposures to the modified bath abolished activity.12. Following a conditioning exposure to potassium-free Krebs solution, readmission of potassium at normal concentration produced a mean hyperpolarization of 20-5 mV and in spontaneous preparations an arrest of activity.13. Pump current in sodium-loaded, non-spontaneously active preparations was measured by voltage clamp and was observed to be voltagedependent.14. The results of this study indicate that an electrogenic pump is present in longitudinal muscle of cat duodenum, and that oscillations in the level of pump current produce slow waves.

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