A direct negative inotropic effect of acetylcholine on rat ventricular myocytes. Am. J. Physiol. 265 (Heart Circ. Physiol. 34): H1393-H1400, 1993.-Acetylcholine (ACh) decreasedth e contraction of rat ventricular cells within 20s. ACh (3.1x lossM) produced a half-maximal effect and lo+ M ACh p ro d uced a maximal effect (a 23.8 t 5.4% decrease; meant SE, n=11). During a 3-min exposure to ACh, the inotropic effect faded. Parallel changes were observed in action potential duration: ACh caused an immediate shortening of the action potential, but then the effect faded with time. The changes in action potential duration were the cause of the changes in contraction, because ACh had no effect on contraction when the contractions were triggered by voltage-clamp pulses of constant duration. The changes in action potential duration were the result of the activation of a K+ current (i K ACh) by ACh. During an exposure to ACh, this current faded as a result of desensitization. iK,ACh was 6.3 times smaller in ventricular than in atria1 cells. This may explain why the negative inotropic effect of ACh on atria1 cells was greater: 1.0