Effect of temperature on acetylcholine - induced contraction of isolated rat tracheal muscle

Abstract

The tracheal strip-chain preparation of the rat was used to study the effect of temperature on electrically or acetylcholine (ACh)- induced contraction. The preparation was suspended in an organ bath containing Krebs bicarbonate solution for isometric tension recording. Decrease of bath temperature from 37°C to 20°C (cooling) had no effect on basal tone,but markedly augmented the contractile responses to cholinergic nerve stimulation (0.5Hz) and ACh (3×10-6 M). The potentiation of ACh-induced contraction was also obtained in the presence of physostigmine (10-7M) or tetrodotoxin (5×10-7M). Cooling caused a parallel shift to the left of the dose -response curve of ACh. At 37°C and 20°C, the doses of ACh required to produce 50% of maximum response were 17.6+3.5 and 3.5±0.4μM, respectively. However, the affinity of the muscarinic receptors determined from pA2 values for the competitive antagonist atropine was significantly smaller at 20°C than at 37°C. On the other hand, ACh-induced contraction of the trachea incubated with isotonic KCl solution or with Ca-free EGTA (0.1mM) solution was increased at 20°C. From these observations, it is concluded that increased responsiveness of the rat tracheal muscle to ACh at lower temperature is mainly due to an acceleration of Ca release from intracellular stored site.