Depressive effects of free fatty acids on the development of tension in different muscle types.

The depression of twitch and tetanic tensions, contractures, and spontaneous activities of various muscle preparations by a selected free fatty acid (Na-octanoate, NaC8) depends on the concentration, the temperature, the time of exposure, and the type of muscle. Among skeletal muscles, there is neither a significant difference between frog and rat twitch muscles nor between fast- and slow-twitch rat muscles at room temperature. Small differences seem to occur between frog tonic and phasic fibres as well as between fast- and slow-twitch rat muscles at 32 degrees C. Smooth muscles are more sensitive than skeletal muscles but less sensitive than papillary muscles in which 0.5 mmol/l NaC8 decrease the contractility by about 50%. The different sensitivity of the various muscle types might be caused by unequal alterations of membrane properties responsible for excitation-contraction coupling. It is supposed that free fatty acids interact with the calcium binding sites of the membranes.