Regenerative action potentials induced in the neurogenic heart of Limulus polyphemus

Abstract

• 1. Intracellular recordings from Limulus heart showed 3–5 steps on the rising phase of the response (undershooting) correlated with three to five inflection points on the phase-plane (V vs. V) display. This suggests that the normal neurogenic response represents the summation of junctional potentials. Tetrodotoxin (TTX) rapidly abolishes all electrical activity.

• 2. The resting potential averaged —41 mV; the cells became depolarized at [K⁺]_o levels above 100 mM and [K⁺]_i was estimated to be 600–800 mM.

• 3. In zero Ca²⁺, deganglionated hearts developed spontaneous contractions and electrical activity, including pacemaker potentials and plateau components, which were not affected by TTX. These myogenic responses were dependent on [Na⁺]₀, and had no steps on the rising phase. Ba²⁺ prolonged the plateau, and Ba²⁺ and Sr²⁺ increased the rate of rise and amplitude of the action potentials.

• 4. All-or-none overshooting action potentials immune to TTX also occurred spontaneously or could be electrically triggered when Ba²⁺ (5–20 mM) was added to deganglionated hearts bathed in isosmotic choline chloride or sucrose solutions (i.e. Na⁺- and Ca²⁺-free); these action potentials had a maximum rate of rise ranging from 0·3 to 1·0 V/sec and had a propagation velocity of 0·9–7 cm/sec.

• 5. Hence, the normally inexcitable sarcolemma can be transformed into a spontaneously regenerative membrane during which Na⁺, Ba²⁺ and Sr²⁺ may carry inward current.