Cardiac performance correlates of relative heart ventricle mass in amphibians
This study used an in situ heart preparation to analyze the power output and stroke work of spontaneously beating hearts of four anurans (Rhinella marina, Lithobates catesbeianus, Xenopus laevis, Pyxicephalus edulis) and three urodeles (Necturus maculosus, Ambystoma tigrinum, Amphiuma tridactylum) that span a representative range of relative ventricle mass (RVM) found in amphibians. Previous research has documented that RVM correlates with dehydration tolerance and maximal aerobic capacity in amphibians. The power output (mW g−1 ventricle mass) and stroke work (mJ g−1 ventricle muscle mass) were independent of RVM and were indistinguishable from previously published results for fish and reptiles. RVM was significantly correlated with maximum power output (Pmax, mW kg−1 body mass), stroke volume, cardiac output, afterload pressure (PO) at Pmax, and preload pressure (PI) at Pmax. PI at Pmax and PO at Pmax also correlated very closely with each other. The increases in both PI and PO at maximal power outputs in large hearts suggest that concomitant increases in blood volume and/or increased modulation of vascular compliance either anatomically or via sympathetic tone on the venous vasculature would be necessary to achieve Pmax in vivo. Hypotheses for variation in RVM and its concomitant increased Pmax in amphibians are developed.