GENDER DOES NOT INFLUENCE NEUROMUSCULAR PROPERTIES IN DIMORPHIC SKELETAL-MUSCLES OF THE TOAD

The aim of this work was to study gender differences on the physiology of the dimorphic brachial musculature involved in the clasp reflex of the toad (Bufo marinus L.). The neuromuscular transmission, the sensi tivity to acetylcholine (ACh) and the cholinesterase activity were com pared on the forelimb sternoradialis muscles (SR) from male and female toads. The interosseous muscles of the first finger were used to comp are the properties of the nicotinic receptor/ionic channel complex (AC hR). All the muscles studied were dimorphic, i.e. significantly smalle r in the female than in the male frog in otherwise similar size animal s. The SR of either sex contracted to bath application of ACh with sim ilar EC50. In physiological solution the frequency of the miniature en d-plate potentials (mepps) was very low (0.1 s(-1)) and no gender diff erence was detected. The mepp amplitudes were 0.62 +/- 0.03 and 0.58 /- 0.03 mV in SR from male and female toads, respectively. To increase exocytosis the muscles were incubated in hypertonic solution (158 mM NaCl). Under this condition mepp frequency was increased by five and s even times and mepp amplitude increased by 1.3 and 1.6 times in SR fro m male and female toads, respectively. The cholinesterase activity mea sured by the colorimetric method, did not differ in SR from male and f emale toads. In muscle fibers dissociated from the dimorphic interosse ous muscles of male and female toads, the ionic channel conductance wa s 43 +/- 5.3 and 44 +/- 4.5 pS, respectively. The mean channel open ti me was voltage-dependent and not significantly different in preparatio ns from both genders. These observations indicate that neither the ACh -nicotinic receptor interaction, nor the AChR complex kinetics and the nicotinic excitation-contraction coupling or the cholinesterase activ ity differ in dimorphic muscles from Bufo genders. No gender differenc e was detected in neuromuscular transmission of the studied muscle. On ly a slight increase in mepp frequency and amplitude could be detected when the muscles were incubated in hypertonic solution. (C) 1998 Else vier Science Inc. All rights reserved.