EFFECTS OF CONTRACTION AND LACTIC-ACID ON THE DISCHARGE OF GROUP-III MUSCLE AFFERENTS IN CATS

1. In barbiturate-anesthetized cats we examined the interaction Of lac tic acid and static contraction on the discharge of group III muscle a fferents. Only afferents whose receptive fields were located in the tr iceps surae muscles were studied. 2. Twelve of 20 afferents were stimu lated by a 60-s static contraction. The majority of firing occurred wi thin the first few seconds of contraction. Thirteen of 20 afferents we re stimulated by femoral arterial injections of 24 mM lactic acid (1-4 ml) with the muscle at rest. Repeated injections of lactic acid with the muscle at rest led to tachyphylaxis. Lactic acid was then injected (24 mM; 4 ml) during the last 15 s of static contraction. In eight of nine afferents that were tachyphylactic to lactic acid with the muscl e at rest, we noted a restored sensitivity to lactic acid during contr action. 3. In separate experiments we examined the effects of dichloro acetate (DCA) on the responses of group III muscle afferents to static contraction. DCA reduces the production of lactic acid by increasing levels of the active form of the enzyme pyruvate dehydrogenase. 4. DCA lowered arterial and venous lactate concentrations at rest and during contraction. DCA significantly decreased (31%; P < 0.05) the response s of the afferents to contraction. This effect was most prominent with in the first 10 s of contraction and was not due to a reduced level of mechanical stimulation after DCA, because peak tension levels were th e same during the two bouts of contraction. 5. In conclusion, these ex periments further characterize the important role lactic acid plays in stimulating group III muscle afferents during contraction. Increased delivery of lactic acid in the absence of contraction enhances their d ischarge, and reduced lactic acid levels during contraction decreases their discharge. In addition, repeated exposure of the afferent to lac tic acid leads to a reduction in discharge frequency, which can be rev ersed by muscle contraction.