Temperature-dependent tonic contraction of smooth muscle in Antarctic fishes Notothenia neglecta and Chaenocephalus aceratus - Role of calcium ions and responses to acetylcholine

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The isolated intestinal smooth muscle of Antarctic fishes Notothenia neglecta and the icefish Chaenocephalus aceratus displayed a sustained tonic contraction depending upon the temperature of the nutritive bath. The contraction was maximal at 2℃, relaxing proportionally with rise of the temperature, from 2℃ to 18℃. The contractile responses to acetylcholine gradually increased in proportion to the rise of temperature. By chelating calcium ions, EDTA caused a reversible relaxation of the contracted muscle at 2.0℃. This effect was eliminated by washing out the drug from the muscle nutritive bath. Similar results were found with the administration of verapamil, a calcium channel blocker. However, after this drug, tonic contraction was not restored by washout. Such contraction was partially inhibited by the muscarinic blocking drug atropine. Results show that the tonic contraction is an active response, dependent on the entry of calcium into the cell and mediated by cholinergic receptors. The temperature-related tonic contraction is not a general event in poikilotherm animals, since it did not occur in the Bufo marinus intestinal smooth muscle, in similar experiments carried out as a control, in a tropical environment. The responsiveness of the smooth muscle to acetylcholine appeared to increase with temperature, suggesting lack of adaptation of the organ to low Antarctic temperature.
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