Contractile properties of the axolotl ventricle at 17 and 21 °C

Mammalian cardiac muscle exhibits a number of intrinsic response systems which adjust function to changing conditions. These include the Frank–Starling response and the slow force response which are activated upon myocardial stretch, and the force–frequency response, which is evident upon changes in cardiac frequency. In this study we have examined the effect of myocardial stretch and changes in pacing frequency on isolated ventricular muscle preparations from the ectothermic amphibian, the axolotl (Ambystoma mexicanum). We preformed these studies at two physiologically relevant temperatures 17 °C and 21 °C. We found that increasing the length of the muscle increased active force via the Frank–Starling response at both temperatures, which is consistent with cardiac muscle preparations in other vertebrates. We found no evidence for the slow force response at either temperature suggesting that in axolotl, unlike mammals but similar to fish, the slow force response is not associated with the Frank–Starling response. Increasing contraction frequency caused a decrease in active force across all frequencies tested (0.5−2.0 Hz)—a monophasic negative force–frequency response, independent of temperature.