Catchlike property of rat diaphragm: subsequent train frequency effects in variable-train stimulation

A high-frequency burst of pulses at the onset of a subtetanic train of stimulation allows skeletal muscle to hold force at a higher level than expected from the extra pulses alone because of the catchlike property of muscle. The present study tested the hypothesis that the presence and degree of force increase induced by a high-frequency burst are strongly modulated by the subsequent train frequency. Rat diaphragm muscle strips (studied in vitro at 37 degrees C) underwent two-, three-, or four-pulse bursts [interpulse interval (IPI) of 5 or 10 ms] at the onset of 10- to 50-Hz subtetanic trains. Force was quantified during the train with respect to its peak value (F(peak)), mean value (F(mean)), and force-time integral (F(area)), and it was compared with that produced during subtetanic trains of an equal number of pulses without preceding pulse bursts (Diff-F(peak), Diff-F(mean), Diff-F(area)). F(peak) and F(mean) increased with two-, three-, and four-pulse bursts, and Diff-F(peak) and Diff-F(mean) increased progressively with decreasing frequency of the subtetanic train. F(area), the best reflection of catchlike force augmentation, was increased mainly by the four-pulse bursts with an IPI of 10 ms, and Diff-F(area) was maximal at subsequent train frequencies of 15-25 Hz. The use of incorrect patterns of burst stimulation could also precipitate F(area) decreases, which were observed with the four-pulse, 5-ms IPI paradigm. The time required to reach 80% of maximal force (T(80%)) became shorter for each of the pulse burst stimulation patterns, with maximal reduction of Diff-T(80%) occurring at a subsequent train frequency of 20 Hz in all cases. These data indicate that extra-pulse burst stimulation paradigms need to incorporate the optimal combinations of extra-pulse number, IPI, and the frequency of the subsequent subtetanic train to take greatest advantage of the catchlike property of muscle.