A calcium dependent post-tetanic hyperpolarization of primary afferent terminals

Abstract

In decerebrated spinal cats, the effects of iontophoretically applied calcium antagonists, cobalt, manganese and verapamil, and of strontium, which reportedly can act like a calcium agonist, were tested on post-tetanic depression of group I afferent terminal excitability. The actions of these agents on the duration of action potentials in the afferent terminals were determined by a recently described method (8). The calcium antagonists reduced the maximum post-tetanic depression of the antidromic compound action potentials and accelerated the recovery of these potentials from the depression. Strontium, on the other hand, had the opposite effects. The duration of afferent terminal action potentials appeared to increase following a tetanic stimulation. This enhancement in the duration of the action potentials was facilitated by strontium and counteracted by calcium antagonists. These observations indicate that calcium influx into primary afferent terminals is increased following a tetanic stimulation and that post-tetanic hyperpolarization of primary afferent terminals may be, at least partly, dependent on the increased accumulation of calcium in the terminals.