The effects of acetylcholine (ACh) and potassium (24203369) on tension and membrane potential were studied to determine the role of electromechanical and pharmacomechanical coupling in ACh induced contractions of dog trachealis muscle. Potassium causes sustained contractions of the muscle via membrane depolorization, which appears to be related in a linear manner to the log of the external potassium concentration. From the potassium experiments a curve relating membrane potential and tension was obtained. The resting membrane potential of the trachealis is -60 plus or minus 1 mV. At a dose of 100nmol ACh the membrane is depolarized to about -55 mV and then at 1 micromole ACh there was more depolarization to about - 40 mV. At higher drug dose there was no further change in membrane potential. The relationship between the dose of ACh and tension was markedly different. Contraction began at 100nmol ACh but did not saturate until a dose of 1mmol was reached. At 100nmol ACh only pharmacomechanical coupling was involved in the contraction. At higher drug doses both pharmacomechanical and electromechanical coupling were involved, with pharmacomechanical coupling playing a greater role. The maximal contribution of electromechanical coupling was approximately 30% of the contraction at 1mmol ACh.
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