Mechanism of inotropic action by hypotonic solution in the frog atrial muscle

The effects of hypotonic solution on the mechanical activities and action potential of the bullfrog atrium were investigated. Exposure of muscle to hypotonic solutions (70% of normal solution) produced initially a transient increase in twitch after which twitch declined below the control level. The response is independent of the kinds of salts withdrawn to make the medium hypotonic and of the presence of beta-blocker (5 X 10(-7) M propranolol). The resting potential and the plateau level of action potential were little changed initially. When the twitch declined, a small amount of depolarization and a shortening of action potential duration were observed; however, the plateau level of action potential was not reduced. The initial increase in twitch was not observed, and only the gradual decline of twitch remained in the caffeine containing hypotonic solution. The weight of muscle increased 10% in the hypotonic solution. The resting tension was also increased transiently and then declined to reach a maintained plateau with exposure to hypotonic solution. In the 0-Ca2+ or caffeine containing medium, the transient component of contracture was suppressed but the plateau tension remained. It is suggested that the initial transient increase of twitch by the perfusion of the hypotonic solution was induced by the Ca2+ released from the sarcoplasmic reticulum (SR), and the resultant decline of twitch resulted from the depletion of Ca2+ from the SR and/or from the shortening of action potential duration.