SUMMARYAn evaluation is made of the relative efficiency (precision of the final estimate per unit time of measurement on a given set of sections) of different methods for planar analysis aimed at estimating aggregate, overall stereological parameters (such as Vv, Sv). The methods tested are point‐counting with different densities of test points (4 ≤ PT ≤ 900 per picture), semiautomatic computer image analysis with MOP and automatic image analysis with Quantimet, for obtaining Vv and Sv estimates. One biological sample as well as three synthetic model structures with known coefficients of variation between sections are used. The standard error of an estimate is mainly determined by the coefficient of variation between sampling units (= sections in the present paper) so that measuring each sample unit with a very high precision is not necessary. Automatic image analysis and point‐counting with a 100‐point grid were the most efficient methods for reducing the relative standard errors of the Vv and Sv estimates to equivalent levels in the synthetic models. Using a 64‐point grid was as precise, and about 11 times faster than using a tracing device for obtaining the estimate of Vv in the biological sample.

Mole rats (Spalax ehrenbergi superspecies) perform the heavy work of digging their subterranean burrows in Israel under highly hypoxic͞hypercapnic conditions. Unlike most other mammals, they can achieve high levels of metabolic rate under these conditions, while their metabolic rate at low work rates is depressed. We explored, by comparing mole rats with white rats, whether and how this is related to adaptations in the design of the respiratory system, which determines the transfer of O 2 from the lung to muscle mitochondria. At the same body mass, mole rats were found to have a significantly smaller total skeletal muscle mass than ordinary white rats (؊22%). In contrast, the fractional volume of muscle mitochondria was larger by 46%. As a consequence, both species had the same total amount of mitochondria and achieved, under normoxia, the same V O 2 max . Whereas the O 2 transport capacity of the blood was not different, we found a larger capillary density (؉31%) in the mole rat muscle, resulting in a reduced diffusion distance to mitochondria. The structural pulmonary diffusing capacity for O 2 was greater in the mole rat (؉44%), thus facilitating O 2 uptake in hypoxia. We conclude that structural adaptations in lung and muscle tissue improve O 2 diffusion conditions and serve to maintain high metabolic rates in hypoxia but have no consequences for achieving V O 2 max under normoxic conditions.The purpose of the present investigation was to estimate determinants of oxygen flow from lungs to skeletal muscle mitochondria in mole rats and to compare them to white rats of the same body mass. The blind mole rats of the Spalax ehrenbergi superspecies (1) live underground in Israel and are known to have many adaptations to the subterranean hypoxic environment, in particular to low O 2 concentrations in the burrow (1-11). The resting normoxic mole rat has a low, arrhythmic heart frequency with 142 beats per min, compared with 314 beats per min in white rats (8). In hypoxia, the heart rate of mole rats increases by 230%, almost twice that of white rats under similar condition (12). Efficient O 2 unloading is achieved when tissue P O 2 is very low, indicating that a large O 2 saturation difference can still exist in hypoxia (13). Adaptation of mole rats to hypoxia is further supported by the finding that general metabolism seems not affected by chronic exposure to hypoxia, in contrast to white rats (11).Under hypoxic conditions, O 2 uptake in the lung is a critical step, and the question arises whether the pulmonary gas exchanger shows adaptive features resulting in a higher diffusing capacity.We advance the hypothesis that the capacity to do aerobic work under low O 2 pressures is due to adaptations in the design of the respiratory system, such that structures determining oxygen uptake in the lung and oxygen supply to muscle cells are adjusted to ensure adequate O 2 supply at very low ambient O 2 concentrations. This hypothesis is tested by comparing mole rats to white rats. The physiology of the white rat conf...

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