DIFFERENCES BETWEEN PROPERTIES OF

JOURNAL OF PHYSIOLOGY AND PHARMACOLOGY 2006, 57, 1, 83–93 www.jpp.krakow.pl

J. CELICHOWSKI, H. DRZYMA£A

DIFFERENCES BETWEEN PROPERTIES OF MALE AND FEMALE MOTOR UNITS IN THE RAT MEDIAL GASTROCNEMIUS MUSCLE

Department of Neurobiology, University School of Physical Education in Poznañ, Poland

Differences between motor units in hindlimb locomotor muscles of male and female

Wistar rats were studied. The contractile and action potential properties of various

types of motor units as well as proportions of these units in the medial gastrocnemius

muscle were analyzed. Experiments were based on functional isolation and electrical stimulation of axons of single motor units. Composition of motor units was different for male and female subjects, with higher number of the fast fatigable and lower

number of slow type units in male animals. The contraction and the half-relaxation times were significantly longer in male motor units, what might be due to differences

in muscle size. Slower contraction of male motor units likely corresponds to lower firing rates of their motoneurons. On the other hand, no significant differences between sexes were observed with respect to force parameters of motor units (the

twitch and the maximum tetanus forces), except the fast resistant units (higher force values in male muscles). The mass of the muscle was approximately 1.5 time bigger in male rats. However, the mean ratio of motor unit tetanus force to the muscle mass

was almost twice smaller in this group, what indirectly suggests that muscles of male rats are composed of higher number of motor units. Finally, female muscles appeared

to have higher fatigue resistance as the effect of higher proportion of resistant units

(slow and fast resistant) and higher values of the fatigue index in respective motor unit

types.

The

motor

unit

action

potentials

in

female

rats

had

slightly

lower

amplitudes and shorter time parameters although this difference was significant only for fast resistant units.

Key

w o r d s : sexual

dimorphism,

skeletal

muscle,

gastrocnemius,

motor

unit,

male

and

female rats

INTRODUCTION

The sexual dimorphism within various species of mammals concerns size and appearance of animals (1, 2). As a rule males have consistently bigger total body

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mass and size (3). Differences in body and muscle mass reflect fundamental morphological variability between males and females. Leslie et al. (3) found that the foot muscle, flexor digitorum brevis, was sexually dimorphic, and was larger in adult male rats than in females. The proportion of muscle fibers in muscles of mastication of several mammals is also sexually dimorphic. English et al. (4, 5) found higher proportion and size of fast fibers in male rabbit masseter muscle. Mouse masseter is composed predominantly of three types of fast fibers (IIa, IIx and IIb): Eason et al. (6) reported higher proportion of IIb fibers and smaller participation of IIa muscle fibers in mouse male muscle in comparison to females. Finally, this group of authors showed in rabbit masseter that the contractile properties of its motor units differed significantly between two sexes. The median value of the twitch contraction times was shorter in males than in females and the twitch forces of motor units in males were significantly higher (5). It has been well known that the body mass of rat males is nearly two-times higher than of females. However, in the literature there is limited information concerning

sexual

dimorphism

in

rat

locomotor

muscles

(3)

and

no

data

concerning the motor unit properties in these muscles. Therefore, the purpose of this paper was to study the sexual differences between contractile properties and proportions of motor units in one of the rat hindlimb locomotor muscles - the medial gastrocnemius.

MATERIAL AND METHODS

The present study was performed on 16 adult, 6-months old Wistar rats (all of them coming from one breeding), under pentobarbital anesthesia (initial dose of 60 mg/kg, i.p., supplemented during the experiments as required, with doses of 10 mg/kg). The depth of anaestesia was verified by controlling pinna reflexes. The mean weight of males and females amounted to 468 ± 60 g and 246 ± 21 g, respectively. The principles of laboratory animal care, as approved by European Union and the Polish Law on Animal Protection were followed. After the experiments, the animals were killed with an overdose of pentobarbital (180 mg/kg). Detailed surgical procedures were described elsewhere (7). Briefly, the medial gastrocnemius muscle was partly isolated from surrounding tissues. The innervation and blood supply to the examined muscle were left intact, whereas other muscles were denervated by cutting remaining branches of the sciatic nerve. The laminectomy was performed over L2-S1 segments. Dorsal and ventral roots were cut proximally to the spinal cord. The animals were immobilized with steel clamps on the tibia, sacral bone and the L1 vertebra. The isolated spinal cord and ventral and dorsal roots of spinal nerves were covered with warm paraffin oil in a small pool made by skin around the laminectomy. The oil and animal core temperature were kept at constant level (37 ± 1°C) by automatic heating system. The studied muscle was connected to the inductive force transducer through the Achilles tendon. The isometric force of contractions was recorded in muscle stretched up to the passive tension of 100 mN. Under these conditions the majority of its motor units can develop the highest twitch force (8). L5 and L4 ventral root were split into very fine bundles of axons that were placed on the silver electrode and electrically stimulated. The "all or none" twitch contractions and muscle action potentials were used as criteria for a single motor unit isolation. Muscle action potentials were recorded using two silver wire electrodes inserted into the muscle.

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Both the contractile force and muscle fiber action potentials were stored on a computer disc using an AD converter (sampling rate of 1 kHz and 20 kHz for force and action potentials, respectively). All investigated motor units were stimulated according to the following protocol: 1) 5 stimuli at 1 Hz (5 single twitches were recorded and then the averaged twitch was estimated), 2) series of stimuli at 10, 20, 30, 40, 50, 60, 75, 100 and 150 Hz frequencies and 500 ms duration, 3) the fatigue test (tetani evoked by trains of 14 stimuli at 40 Hz frequency, repeated every second for 4 minutes) (9). 10 s time intervals were applied between all above elements of the protocol. The studied motor units were classified as fast-twitch according to the presence of "sag" in the 40 Hz unfused tetanus whereas "non-sagging" units were classified as slow-twitch (S) (9, 10). The fatigue index was calculated as the ratio of tetanic force reached by a motor unit two minutes after its force potentiated to the maximum at the beginning of the fatigue test to this maximal initial force (11). Fast units were divided into fast resistant (FR) and fast fatigable (FF), according to values of the fatigue index which exceeded 0.5 for FR units and was lower than 0.5 for FF units (10, 12). In the present experiments 229 motor units were studied, 121 in males and 108 in females. The contraction time (CT, from the beginning of a twitch to the peak of this force record), the halfrelaxation time (HRT, from the peak to the moment when the force decreased to the half of the peak value) and the twitch force (TwF, the peak force amplitude) were calculated from the averaged twitch

record

(13).

For

the

motor

unit

action

potentials

(MUAPs)

the

three

parameters

were

measured: the peak-to-peak amplitude, the peak-to-peak duration and the total duration (14).

RESULTS

Table 1 presents mean values, standard deviations and variability ranges for body weight, muscle mass and also percentage participation of the muscle in the body mass. Although the body and muscle masses were higher for males, the relation of the muscle mass to the body mass was higher for females. Mechanical properties of 121 medial gastrocnemius motor units in male rats and of 108 motor units in female rats were studied. The mean values and their standard deviations for the analyzed contractile properties of these motor units are summarized in Tab. 2. In males, for all three types of motor units the contraction and half-relaxation times were longer than in females. These differences were statistically significant in both types of fast units. The twitch force and the maximum tetanus force were higher for males although these differences were statistically significant only for FR units. The mean ratio of the twitch-to-tetanus

Table 1. Mean values, standard deviations and variability ranges for male and female body mass, muscle mass and also percentage of the muscle mass in the body mass. *** - difference significant, P