Reduced safety factor for neuromuscular transmission and abnormal quantal secretion precede neuromuscular weakness induced by dithiobiuret

Abstract

Daily treatment of rats with 2,4-dithiobiuret (DTB. 1 mg/kg/day, ip) causes a flaccid neuromuscular weakness first observable in the hindlimbs after 5–6 days of treatment. With continued exposure, neuromuscular weakness appears to encompass the other muscles of the body; death is presumed to result from paralysis of respiratory muscles. The purpose of the present study was to investigate the early effects on neuromuscular transmission which precede neuromuscular weakness caused by DTB, particularly as they relate to the apparent differential muscle sensitivity, using conventional intracellular microelectrode recording techniques. Experiments were conducted using the hemidiaphragm muscle isolated from male rats treated for 7–8 days, with 1 mg/kg/day DTB ip, a regimen which resulted in hindlimb, but not diaphragmatic paralysis, or with 0,9% NaCl (1 ml/kg/day) as control. Analysis of quantal content of end plate potentials (EPPs) from hemidiaphragms of DTB-treated rats indicated no difference from control. Exposure of hemidiaphragm preparations from DTB-paralyzed rats to solutions containing elevated Mg 2+ and lowered Ca 2+ concentrations (6 and 1 m m, respectively) resulted in a decreased quantal content of the EPP compared to that of similarly treated control preparations. When miniature EPPs (MEPPs) were evoked from nerve terminals by elevating [K −] e, in the presence of 6 m m Mg 2+ and 1 m m Ca 2+, the mean peak frequency evoked by K +-induced depolarization was reduced in the DTB-treated group; however, the time at which peak frequency was attained was the same for the control and DTB-treated group. Mean MEPP amplitude but not resting MEPP frequency was altered in the presence of high [Mg 2+] for diaphragms of the DTB-treated group. Neither MEPP amplitude nor frequency was altered in diaphragms exposed to normal concentrations of Ca and Mg. Prolongation of rise and decay times of MEPPs occurred from end plates of DTB-treated rats irrespective of whether low [Ca 2/high [Mg 2+] solutions were used. However, these effects were more pronounced when low [Ca 2+/high [Mg 2+] solutions were used. Diaphragm-derived end plates of the DTB-treated group were also characterized frequently by the presence of very large amplitude MEPPs with prolonged decay times. The overall percentage of the total population of MEPPs which these abnormal MEPPs made up in the DTB-treated rats was increased dramatically by exposure to low [Ca 2+]/high [Mg 2+] solutions. Thus, differential muscle sensitivity occurs during paralysis induced by chronic DTB treatment, with the diaphragm being somewhat more resistant than hindlimbs to neuromuscular weakness. This effect appears to be associated with the safety factor associated with acetylcholine release and reflects an impairment of presynaptic processes resulting in a diminished number of quanta liberated in response to motor nerve stimulation. Additional neuromuscular symptoms precede the onset of muscle weakness.