Abstract
The effects of streptomycin on neuromuscular transmission were investigated on frog cutaneous pectoris muscles. The half-inhibition doses of peak end-plate current amplitude are 3 x 10(-5) and 8.5 x 10(-5) M in the presence of 0.9 and 1.8 mM extracellular calcium, respectively. The quantal content of the end-plate current was reduced by 50% in the presence of 3 x 10(-5) M streptomycin in Ringer's solution containing 0.35 mM Ca and 2 mM Mg. Miniature end-plate currents under these conditions were reduced by only 20%, suggesting that the presynaptic blocking action predominates over the postsynaptic action. A much higher concentration of streptomycin (3.5 x 10(-4) M) was required to achieve 50% block of peak transient depolarizations induced by iontophoretic application of acetylcholine. The postsynaptic action involves primarily a blocking action on acetylcholine receptors since the drug did not alter the linearity of current-voltage relationship for end-plate currents at membrane potentials more positive than -50 mV. An additional weak blocking action on the acetylcholine-activated channels exhibited a slight voltage and concentration dependence, giving rise to a slight prolongation of the end-plate current and curvature of the current-voltage relation at membrane potentials more negative than -50 mV. Thus, under normal conditions the predominant action of streptomycin at the neuromuscular junction is to reduce transmitter release. A secondary competitive inhibition on the acetylcholine receptor and a weak blocking action on the ionic channels may also contribute to the overall block.
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|