Inhibition of Cardiac Muscle Contractions by Ultraviolet Light Irradiation
Contraction of bullfrog atrial muscle, frog ventricular muscle, and rat papillary muscle were inhibited by ultraviolet (UV) light irradiation of weak intensity (75 mW/cm2). The muscle preparation was superfused with the standard Ringer or Tyrode solution and repetitively stimulated at a rate of 1/2 sec or 1/5 sec. Shortly after irradiation on muscle with UV light, the twitch tension started to decrease to the steady level of 90% of the control tension within a few min. The tension recovered completely from inhibition if the UV irradiation was interrupted. This inhibition was enhanced when the superfusion of muscle preparation was stopped. Although there were no qualitative differences in twitch inhibitions among three kinds of muscle preparations, the rat papillary muscle was a little more sensitive to UV light than other muscles. The action potential recorded with a single sucrose gap method was prolonged in its tail without changes in plateau level and duration during UV irradiation. It is suggested that a certain injurious substance(s) which inhibits contraction is released by UV irradiation, rather than that UV damages the cellular structure. The substance has not yet been identified, but it is probably related to free radicals.