The cardiac innervation of a marsupial heterotherm,the fat-tailed dunnart (Sminthopsis crassicaudata)

This study investigated the pattern of autonomic innervation of the heart of the fat-tailed dunnart (Sminthopsis crassicaudata) using isolated cardiac preparations. While the pattern of autonomic innervation of the atria was consistent with that found in other mammals, the ventricles displayed an unusual pattern of mammalian cardiac innervation. Transmural stimulation of the intramural nerves of isolated right ventricular preparations caused a decrease in the force of contraction of 46.8+/-3.2% followed by a rebound increase in the force of contraction beyond basal levels of 40.9+/-6.9%. These responses could be blocked independently by the application of the muscarinic receptor antagonist hyoscine and beta-adrenoreceptor antagonist propranolol respectively and could also be mimicked by the application of the agonists acetylcholine (Ach) and noradrenaline (NA). These findings indicated the presence of a functional cholinergic innervation of the ventricles that was capable of reducing the force of contraction below basal levels. This pattern of innervation has only been found previously in one other mammal, the bent-winged bat (Miniopterus schreibersii). Given that both of these species are heterotherms, it is possible that such a pattern of innervation may relate to the control of cardiac output during torpor. These findings are the first that demonstrate the homogeneity of a physiological control mechanism in a so-called 'shallow, daily torpidator' (S. crassicaudata) and a 'deep hibernator' (M. schreibersii) that is absent in mammalian homeotherms. These findings are consistent with recent work suggesting that there may be little difference between these types of heterothermy.