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

A high-frequency burst of pulses at the onset of a. subtetanic train of sti mulation allows skeletal muscle to hold force at a higher level than expect ed from the extra pulses alone because of the catchlike property of muscle. The present study tested the hypothesis that the presence and degree of fo rce increase induced by a high-frequency burst are strongly modulated by th e 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 train s. 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 co mpared with that produced during subtetanic trains of an equal number of pu lses 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 freque ncy 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 15-2 5 Hz. The use of incorrect patterns of burst stimulation could also precipi tate F-area decreases, which were observed with the four-pulse, 5-ms IPI pa radigm. The time required to reach 80% of maximal force (T-80%) became shor ter for each of the pulse burst stimulation patterns, with maximal reductio n of Diff-T-80% occurring at a subsequent train frequency of 20 Hz in all c ases. These data indicate that extra-pulse burst stimulation paradigms need to incorporate the optimal combinations of extra-pulse number, IPI, and th e frequency of the subsequent subtetanic train to take greatest advantage o f the catchlike property of muscle.