The force-frequency relationship and postrest recovery of force development in rat myocardium: The effect of muscle length alteration.

This study investigated the influence of muscle length and frequency modulation on force development of rat papillary muscle. Twitch characteristics data were collected and force-frequency curves were derived at nine pacing frequencies for three muscle lengths (100%, 95%, and 90% of the length which elicits the greatest amount of active force (Lmax)). Examination of the twitch characteristics determined length modulation significantly altered the properties of a single muscle twitch. Force-frequency curves were derived by plotting the peak force development values against nine pacing frequencies at the three previously mentioned muscle lengths. Statistical analysis of the force-frequency curves determined that the shorter muscle length curves were significantly reduced and non-parallel compared to the longer muscle lengths. Force-frequency curves derived at shorter muscle lengths displayed a curve which was not typical of the negative force-frequency relation (at higher frequencies the curves displayed a positive force staircase). Postrest recovery experiments utilizing the drug ryanodine were implemented to determine if trans-sarcolemmal $\rm Ca\sp{++}$ influx may be responsible for the length-dependent alteration in the force-frequency curves. Ryanodine was used to help examine trans-sarcolemmal $\rm Ca\sp{++}$ influx during postrest force recovery for it diminishes the influence of activator $\rm Ca\sp{++}$ from the sarcoplasmic reticulum. Force recovery after a 30 second rest period was examined across three frequencies at the same three previously mentioned muscle lengths. The results from this experimentation demonstrated that relatively more calcium may be entering the muscle cell via the sarcolemmal at shorter muscle lengths when compared to greater muscle lengths at higher frequencies. A mechanism which may be responsible for greater relative trans-sarcolemmal $\rm Ca\sp{++}$ influx at shorter muscle lengths compared to longer muscle lengths at higher frequencies is an increase in $\rm Ca\sp{++}$ influx by the sarcolemmal $\rm Ca\sp{++}$ slow channels and the reversal of function of the $\rm Na\sp+/Ca\sp{++}$ exchange. (Abstract shortened by UMI.)Dept. of Kinesiology. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1993 .K66. Source: Masters Abstracts International, Volume: 33-04, page: 1192. Adviser: Kenji Kennd. Thesis (M.H.K.)--University of Windsor (Canada), 1994