Effects of testosterone on a sexually dimorphic frog muscle: Repeated in vivo observations and androgen receptor distribution

Abstract

In the present study the sexually dimorphic, androgen-sensitive flexor carpi radialis muscle (FCR) in male Xenopus laevis was viewed repeatedly in vivo to assess the influence of testosterone on muscle fiber size over a period of up to 12 weeks. Regions of the muscle innervated by different spinal nerves responded differently to testosterone treatment. Muscle fibers innervated by spinal nerve 2 (SN2) hypertrophied within 7 days in frogs that had been castrated and given testosterone-filled implants. This initial hypertrophy was followed by a return to normal fiber size a week later, after which fiber size slowly increased again. In castrated males with empty implants, muscle fibers innervated by SN2 gradually atrophied. Fibers innervated by spinal nerve 3 (SN3) were not affected by androgen replacement or withdrawal. The sartorius, a control muscle that is neither sexually dimorphic nor particularly androgen sensitive, was also unaffected. The in vivo observations were confirmed by measurements of muscle fiber cross-sectional areas in frozen sections of whole forelimbs. At 8 and 12 weeks after castration, cross-sectional areas of fibers innervated by SN2 were significantly larger in frogs provided with testosterone than in castrates without testosterone. No difference was found in the SN3 region or in the anconeus caput scapulare (triceps), another control muscle. Immunocytochemistry employing an antibody against the androgen receptor (AR) indicated that the receptor is present in myonuclei of all muscles of the forelimb. While no difference in labeling intensity was detected, the number of AR-containing nuclei per muscle fiber cross-section was higher in fibers innervated by SN2 than in those innervated by SN3, and was yet lower in the triceps. This suggests that regulation of androgen sensitivity may occur via muscle fiber ARs, although an influence of the nerve may also contribute.

Current research in amphibians: Studies integrating endocrinology, behavior, and neurobiology

“…Similar androgendependent sex differences are found in the oblique muscles of the body wall (Tiagen et al, 1985;Girgenrath and Marsh, 2003), and in the flexor carpi radialis, the forelimb flexor muscle used by males to clasp females while they oviposit (Herrera and Regnier, 1991;Regnier and Herrera, 1993;Dorlöchter et al, 1994;Sidor and Blackburn, 1998).…”

Isometric contractile properties of sexually dimorphic forelimb muscles in the marine toad Bufo marinus Linnaeus 1758: functional analysis and implications for amplexus

“…These differences appear to develop under the influence of hormonal regulation (Kelley, 1997;Kelley, 2004). A few studies have shown that the size dimorphism in forelimb muscles is also affected by testosterone (Dorlochter et al, 1994;Muller et al, 1969;Regnier and Herrera, 1993a;Regnier and Herrera, 1993b). But less is known about any differences in neuronal or muscle fiber cellular/molecular mechanisms in the limb muscles.…”

Effects of testosterone on synaptic efficacy at neuromuscular junctions in a sexually dimorphic muscle of male frogs.

“…The effects of testosterone on the contractile properties of the flexor carpi radialis indicate that androgens play a role in adapting fibres in the muscle to the functional demands of clasping and further suggest that SN2 motor units and SN3 motor units are selectively important to the prolonged and brief patterns of flexion, respectively (Regnier & Herrera, 1993a). Indeed, the orientation of fibres innervated by SN2 with respect to the elbow joint (Dorlochter et al 1994) suggests that they are capable of producing greater rotational force (torque) and, therefore, are more important for the prolonged phase of clasping than those innervated by SN3 (Regnier & Herrera, 1993 b). The present results provide additional evidence for the differential roles of SN2 and SN3 motor units in clasping behaviour and suggest that testosterone enhances synaptic efficacy such that transmission is maintained in vivo.…”

“…Repeated in vivo observations of shoulder region fibres in the flexor carpi radialis have indicated that changes in muscle fibre size induced by testosterone manipulation can occur over 1-2 weeks (Dorlochter et al 1994). To determine whether testosterone-associated changes in synaptic efficacy were occurring in less than 8 weeks, the percentage of subthreshold fibres innervated by SN2 was examined in muscles from frogs treated for 2 weeks (Fig.…”

“…All forelimb bones except the humerus were removed prior to recording. The identification and anatomy of the flexor carpi radialis is discussed in Dorlochter et al (1994). For all experiments, nerves were stimulated with suction electrodes.…”