Debra A. Gordon

Motor-unit force potentiation in adult cats during a standard fatigue test.

1. The purpose of this study was to examine the time course of tetanic force during a standard fatigue test and to distinguish between the appearance of potentiation and fatigue among the four motor‐unit types of a cat hindlimb muscle. 2. Motor units of the tibialis posterior muscle in the adult cat were assigned to four categories (i.e. types S, FR, FI, FF) based on conventional criteria (Burke, Levine, Tsairis & Zajac, 1973). The mean (+/‐ S.D.) time course of peak force was constructed for each motor‐unit type and, within each type, for those units that potentiated (a greater than 3% increase in peak force compared to the initial value) and those that did not potentiate. 3. The average time courses of force differed between motor‐unit types. There was, however, considerable variability within each motor‐unit type. For the same relative force output, the forces exerted by slow‐twitch units were less variable than those exerted by fast‐twitch units. In addition, the variability among slow‐twitch units was relatively constant during the fatigue test while variability among fast‐twitch units either increased or decreased with time. 4. For a given motor‐unit type, the average time course of force did not depend on whether force in each tetanus was expressed as a peak value, an average peak value, or a force‐time integral. 5. Some motor units within each type exhibited potentiation. Most of the variability in the time course of the peak force for each motor‐unit type could be accounted for by the potentiating units. Motor units that exhibited only force decline (i.e. fatigue), regardless of unit type, had less variable time courses of peak force. Since potentiation was transient in some unit types, it was assumed that at least two opposing processes (i.e. fatigue and potentiation) occurred simultaneously in these units (see also, Krarup, 1981; Rankin, Enoka, Volz & Stuart, 1988; Garner, Hicks & McComas, 1989). 6. It is concluded that the expression of force potentiation throughout a fatiguing regimen is variable among motor units and that this is not related to conventional motor‐unit types. This dissociation suggests that the mechanisms that form the basis for the conventional distinction between motor‐unit types are different from those which lead to force potentiation.

Force development and relaxation in single motor units of adult cats during a standard fatigue test.

1. The purpose of this study was to investigate tetanic force development and relaxation in single motor units that were subjected to a standard fatigue test. 2. Motor units of tibialis posterior, a hindlimb muscle in the adult cat, were assigned to four categories (i.e. types S, FR, FI, FF) using conventional criteria. 3. Based on the first tetanus of the fatigue test, type S units took significantly longer to develop force and to relax than the fast‐twitch units. Within the fast‐twitch subpopulations, type FR and FI units were significantly slower to develop force and to relax than were type FF units, but there were no significant differences between type FR and FI units. 4. After 120 s of the fatigue test, the rates of force development were faster than initial values in type S and FR units, but were largely unchanged for the type FI and FF units. Most relaxation parameters were unaffected by stimulation in type S and FR units, but all parameters became significantly slower in type FI and FF units. 5. The average time courses of force development and relaxation showed that during 240 s of the fatigue test, type S units exhibited either a progressive increase in a parameter or no change at all. In contrast, fast‐twitch units displayed profiles that included initial increases in a force development or relaxation parameter followed by variable amounts of decline that corresponded to fatigability. 6. It is concluded that repetitive activation affects the development and relaxation of tetanic force in all motor‐unit types. Average changes in these parameters tended to parallel the conventional classification of motor units into four categories.

Characterization of a panel of rat ventral prostate epithelial cell lines immortalized in the presence or absence of androgens

We have transfected rat ventral prostate (RVP) epithelial cells with a plasmid containing the SV40 large T-antigen in an attempt to establish a panel of cell lines that will be useful in molecular genetic studies of prostate cell function. Since the distribution of cell types in the RVP is dramatically affected by androgen withdrawal and replacement, cells isolated from normal, castrated, or castrated rats that were given daily injections of testosterone were used in these experiments. Cell lines were established in media that were supplemented or depleted of androgens to accommodate the possible requirements of different prostate cell types. Numerous cell lines were isolated which retain characteristics of RVP epithelial cells and five of these cell lines were studied in detail. All five cell lines express the SV40 large T-antigen, supporting the role of this viral protein in immortalization. The RVP cell lines were shown to contain high levels of functional glucocorticoid receptors, but very low levels of androgen binding activity even though androgen receptor RNA could be detected. It was determined that the decreased androgen receptor activity in the RVP cells was apparently due to low receptor expression based on the results of transient transfection assays using androgen receptor cDNA. Taken together, the biochemical, cytological, and morphological characterizations of the RVP cell lines suggest that they may all have been derived from basal prostate epithelial cells despite the initial differences in androgen status of the animal and the level of androgens in the culture media.

Application of Cross-Sectional Single-Fiber Microchemistry to the Study of Motor-Unit Fatigability

The glycogen-depletion technique for histologically identifying physiologically defined motor units, as proposed by Krnjevic and Miledi (1958) and first used by Edstrom and Kugelberg (1968), has provided a means of demonstrating associations between the physiological and biochemical properties of single motor units. Such associations have been demonstrated using largely qualitative histochemistry (e.g., Burke et al., 1971; Edstrom and Kugelberg, 1968; for review. Burke, 1981) but, most recently, quantitative biochemistry (Nemeth et al., 1981). The latter approach provides the advantage of yielding quantitative information on biochemical activites suitable for detailed comparison with physiological parameters. We have recently reported the development of a technique which permits biochemical analysis of cross-sections of fibers belonging to single mammalian motor units (Nemeth et al., 1985).

Measurement of axonal conduction velocity in single mammalian motor axons

In deeply anesthetized cats, determinations of motor-axonal conduction velocity (CV) were made using extracellular potentials recorded from single, functionally isolated motor axons innervating the muscle tibialis posterior. Axons were activated by suprathreshold electrical stimulation at the ventral-root level. Action potentials were recorded with 3 bipolar electrodes located on the muscle nerve at the level of the popliteal fossa. The most proximal and distal of the bipolar muscle-nerve electrodes were 16.4-22.0 mm apart. Estimates were made of CV from ventral root to muscle nerve (conventional CV) and between the proximal and the distal pairs of muscle-nerve electrodes (muscle-nerve CV). An evaluation was based on comparison of these CVs, estimates of uncertainties in time and distance measurements and simulations of the effects of recording conditions on CV estimates. The analysis indicated that the uncertainty in the conventional CV measurement of mammalian motor axons is at least +/- 2%. However, variability may be as great as 20% between muscle-nerve CV measurements from different experiments, probably due to such factors as regional variation in CV and differences in recording configuration.