| Summary: | The performance of the isolated hearts of a gastropod, Busycon canaliculatum (L.), and a bivalve, Mercenaria mercenaria(L.), were examined at different perfusion levels around the expected physiological ranges. Both hearts followed the Frank-Starling relationship with regard to stroke volume versus preload, but the heart-rate response was species-dependent. The argument is developed that the molluscs might functionally apply Starling's Law of the heart to accommodate increased output during exercise. At the expected in vivo filling pressures the power output of the two hearts was the same (15-30 × 10?6Wg?1 ventricular tissue), but the Mercenaria filling levels were considerably lower. This clearly indicates that the cardiac muscle of each of the two species has evolved to operate at specific pressure ranges. Electrical recordings from the surface of the myocardium in the perfused Busycon hearts confirm that the shape changes reported in the literature, dealing with stretched myocardium, also occur for changes in the whole heart at realistic loading pressures. These results support previous conclusions that the cardiac output is controlled by the duration of the action potential plateau.
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