Effect of neurogenic inactivity on posttetanic responses of rat fast muscle

At a short-term tetanic stimulation of fast muscle in response to subsequent single stimulation there is recorded a temporary increase of the strength of single contractions that returns to the initial background after 6–10 min. This phenomenon is called posttetanic potentiation (PTP) and is recorded only in fast muscles. The goal of the present work was a study of effect of motor innervation on the course of PTP in rat m. extensor digitorum longus (m. Edl). It has been established that the first signs of effect of motor denervation on the PTP course after section of sciatic nerve in the area of popliteal fossa are recorded as early as at the 4th day after denervation and are expressed in a decrease of strength of single contractions after cessation of tetanic stimulation. These changes reach its maximum at the 14–15th day after denervation when effect of PTP in denervated muscle does not appear at all. Pharmacological analysis of the studied phenomenon has shown that dantrolen (10 μM) suppresses amplitude of the single contraction, but does not prevent the appearance of PTP in intact muscle. In the denervated m. Edl, instead of the appearance of PTP, after a brief slight increase, a gradual decrease of the strength of contraction is recorded. Thus, it can be concluded that no significant PTP changes are present under action of dantrolen. It has been established that after the 10-min muscle incubation in Ringer’s solution with caffeine (4 mM), strength of the single contraction in intact and denervated muscles increases by approximately equal value. Tetanization of intact muscle increases strength of the single contraction approximately by 7% more than this occurs after incubation with caffeine, i.e., this substance reduces the capability of muscle for the appearance of PTP. On denervated muscle, caffeine increases strength of singly contraction, but does not potentiate development of PTP. The obtained data allow concluding about the existence of different mechanisms underlying the pretetatnic contraction and posttetanic potentiation of the single contraction. The main difference between two types of contractions can be recruiting of additional DICR-channels in the process of contraction under conditions of PTP.