Histological and neurophysiological changes induced by carrageenan in skeletal muscle of cat and rat.

Infiltration of the rat gastrocnemius-soleus (GS) muscle with carrageenan induced a myositis which was characterized histologically by an accumulation of polymorphonuclear leukocytes around capillaries and small arterioles. In chloralose-anaesthetized cats having an inflamed GS-muscle, the discharge behaviour of single muscle receptors with group III and IV afferent fibres was recorded. Concerning background activity, only group III receptors showed a significant increase, whereas a significant lowering in mechanical threshold was present only among group IV receptors. Both high- and low-threshold mechanosensitive receptors showed signs of a sensitization. In contrast to group IV receptors in the cat, rat group IV receptors showed a significantly higher level of background activity in inflamed muscle but no increased responsiveness to mechanical stimuli. Acetylsalicylic acid (ASA) influenced only some of the receptors in the inflamed tissue. Leukotriene D4 did not act as a sensitizing substance but depressed the activity of group IV muscle receptors.