DIFFERENTIAL SENSITIVITY TO ANDROGENS WITHIN A SEXUALLY DIMORPHIC MUSCLE OF MALE FROGS (XENOPUS-LAEVIS)

Male frogs use their forelimb flexor muscles to clasp females during t he mating behavior known as amplexus. We investigated the effects of t estosterone on a principal forelimb flexor, the flexor carpi radialis muscle (FCR), using morphological and histochemical techniques. Male X enopus laevis were surgically manipulated to produce high or low level s of circulating testosterone for an 8-week period. After this treatme nt, measurement of fibers in muscle cross-sections revealed that avera ge fiber size was positively correlated with testosterone level. This effect was not the same for all muscle fibers, however. Fibers in the shoulder region were more sensitive to testosterone than fibers in oth er regions of the muscle. Histochemical staining of cross-sections sho wed that the patterns of staining for myosin ATPase or succinic dehydr ogenase (SDH) were not influenced by testosterone levels, but total SD H activity was increased by testosterone treatment. When sensitivity t o testosterone was correlated with ATPase activity, fibers with high A TPase activity were found to be more sensitive to testosterone than fi bers with low activity, regardless of position within the muscle. Most fibers with high ATPase activity were located in the shoulder region of the muscle. These fibers are innervated by different motor axons th an are fibers in the elbow region of the muscle, and contractions of s houlder (but not elbow) region fibers, elicited by stimulation of moto r axons, are slowed by testosterone treatment (Regnier and Herrera, 19 93, J. Physiol. 461:565-581). (C) 1993 John Wiley & Sons, Inc.