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

This study investigated the pattern of autonomicinnervation of the heart of the fat-tailed dunnart(Sminthopsis crassicaudata) using isolated cardiac preparations.While the pattern of autonomic innervation ofthe atria was consistent with that found in other mammals,the ventricles displayed an unusual pattern ofmammalian cardiac innervation. Transmural stimulationof the intramural nerves of isolated right ventricularpreparations caused a decrease in the force of contractionof 46.8±3.2% followed by a rebound increase inthe force of contraction beyond basal levels of40.9±6.9%. These responses could be blocked independentlyby the application of the muscarinic receptorantagonist hyoscine and b-adrenoreceptor antagonistpropranolol respectively and could also be mimickedby the application of the agonists acetylcholine (Ach)and noradrenaline (NA). These findings indicated thepresence of a functional cholinergic innervation of theventricles that was capable of reducing the force ofcontraction below basal levels. This pattern of innervationhas only been found previously in one othermammal, the bent-winged bat (Miniopterus schreibersii).Given that both of these species are heterotherms, it ispossible that such a pattern of innervation may relate tothe control of cardiac output during torpor. Thesefindings are the first that demonstrate the homogeneityof a physiological control mechanism in a so-called‘shallow, daily torpidator’ (S. crassicaudata) and a ‘deephibernator’ (M. schreibersii) that is absent in mammalianhomeotherms. These findings are consistent withrecent work suggesting that there may be little differencebetween these types of heterothermy.