The effect of temperature on the direct muscle twitch response and the action of drugs on the isolated denervated rat diaphragm.

The effect of temperature on the direct twitch response of the normal and denervated rat diaphragm, and the sensitivity of the contractural response of the denervated muscle to acetylcholine, suxamethonium, decamethonium, and potassium salts, was studied. Cooling the normal diaphragm from 39 to 20 degrees increased the size of the direct twitch, but below this temperature the twitch response of the muscle was progressively depressed. The denervated muscle twitch was, by contrast, maximal at 39 degrees and decreases as the temperature was lowered. This property of the denervated muscle first appeared about the third day after denervation. The contractural response of the denervated diaphragm to applied acetylcholine was enhanced by cooling, while the response to potassium salts was depressed. The depression of the contracture induced by potassium followed closely the depression of the direct muscle twitch of the denervated muscle over the same temperature range.