Maximum metabolism and the aerobic factorial scope of endotherms.

Minimum and maximum metabolism in response to cold were measured in 30 species of Australian monotremes, marsupials, eutherians and birds. In marsupials and the echidna, maximum metabolism was also determined during treadmill locomotion. These data were used to determine, for the first time, the relationships between maximum metabolism and body mass in the four endothermic groups and to compare aerobic factorial scopes (the ratio of maximum to minimum metabolism) elicited by cold and locomotion. The effect of body mass on maximum metabolism is the same in marsupials and eutherians (the therians) but is significantly less in birds. At the same body mass, there is no difference between the two therian groups for either minimum or maximum metabolism induced by either cold or locomotion. Aerobic scope during cold is significantly higher in marsupials (8.3) than in eutherians (5.1), birds (5.4) and monotremes (5.4). Aerobic scope during locomotion in all groups is almost twice that observed in cold conditions.

Energetic cost of locomotion as a function of ambient temperature and during growth in the marsupial Potorous tridactylus.

In the marsupial, the potoroo, multiple regression analysis shows that ambient temperature makes a minor (2%) contribution towards variation in oxygen consumption with speed. This suggests that the heat generated during running is substituted for heat which would otherwise have to be generated for temperature regulation. Maximum levels of oxygen consumption are also temperature-independent over the range 5-25 degrees C, but plasma lactate concentrations at the conclusion of exercise significantly increase with ambient temperature. Adult potoroos show a linear increase in oxygen consumption with speed, and multiple regression indicates that the most significant factor affecting energy use during running is stride length. Juvenile potoroos have an incremental cost of locomotion about 40% lower than that predicted on the basis of body mass. The smaller animals meet the demands of increasing speed by increasing stride length rather than stride frequency, as would be expected in a smaller species. Our results show that juvenile potoroos diverge significantly from models based only on adult animals in incremental changes in stride frequency, length and the cost of transport, suggesting that they are not simply scaled-down adults.

Serum effects confound the neuroleptic radioreceptor assay.

The neuroleptic radioreceptor assay (NRRA) is used widely to monitor total neuroleptic-like activity (NLA) in patients taking one or more antipsychotic drugs. The original report of Creese and Snyder (1) stated that serum alone caused a small effect on binding which was negligible compared to normal daily variations in the assay. Conversely, in studies with striata from rat or cow brain, we found that sera from healthy, drug free volunteers, when used at 50 microL/1 mL assay volume, caused marked inhibition of binding. Although any sample of serum causes reproducible inhibition with a given preparation of bovine or rat striatal membranes, the effects of various serum samples may differ markedly when several striatal membrane preparations are compared. Moreover, samples taken from people at different times may also vary, although less than the interindividual differences. Despite this variance, the slopes of log-logit plots were equal to 1 either in the presence or absence of serum. Because of the differences in the interaction of individual sera with different membrane preparations, it is difficult to compensate accurately for this serum effect by simply including control serum in the standard curve. Thus, the use of the NRRA as a quantitative tool in the clinical pharmacology of neuroleptics may be limited by this non-specific effect of serum, and this finding may offer one explanation for some of the inconsistencies found in comparing the NRRA with direct analytical methods.