Fig. 6.

Biochemical and bioelectrical integration during early regeneration via H₂O₂. (A) Drug targets and H₂O₂ effects on bioelectricity at distinct steps during a putative H₂O₂ ‘life cycle’. From top to bottom, steps are production of H₂O₂ from NADPH oxidases, availability and diffusion of produced H₂O₂, and H₂O₂-mediated influences on bioelectric activities or parameters (V_m, TEP, EF, J_I and Na_V). A representative cell potentially from wound epithelium (overlaying bud) and/or mesenchymal bud of a tadpole amputated at stage 40-41. It is also possible that one cell produces H₂O₂ and another takes it up to regulate its bioelectric state. For simplicity, we present everything in the same hypothetical cell. e⁻, electrons; Nox/Duox, NADPH oxidase families (Nox1, Nox2, Nox4 and Nox5 produce superoxide anion (O₂^.−), whereas Duox1 and Duox2 produce H₂O₂ directly); AQP, aquaporin; Na_V, voltage-gated Na⁺ channel; Cys-SH, cysteine (Cys) residues of proteins containing thiol (-SH) groups suitable for oxidation; P, promoters; E, enhancers; TF, transcription factors; H/ARE, hypoxia and antioxidant responsive elements; black arrows, ‘life cycle’ of H₂O₂, from upstream O₂ to downstream effect; blue arrows, redox-mediated bioelectric outcome; green arrow, pharmacological activation; red lines, pharmacological inhibition. Drugs are in italic. (B) H₂O₂ is a switcher in the electrical equivalent circuit. Simplified circuit in a membrane patch of, potentially, a wound epithelium (overlaying bud) and/or mesenchymal bud cell of a tadpole amputated during the regenerative (stage 40-41) or refractory (stage 45-46) period. In the absence of H₂O₂ (switcher of Na_V channels), putative Na⁺ current (charge carrier) does not cross the membrane through Na_V channels and current reversal does not occur at 6 hpa. Suppressed redox state mimics the refractory period circuit. Exogenous H₂O₂ rescues (from depleted ROS) and induces (from the refractory period) J_I reversal. Na⁺, charge carrier; C_m, capacitance of membrane; E_Na⁺, electromotive force driving Na⁺; I_Na⁺, current of Na⁺; R_Na⁺, resistance to Na⁺ flux (synonymous of Na_V channels); S_H2O2, switcher H₂O₂. Arrows inside circles indicate net inward (down arrow) or outward (up arrow) current measured extracellularly.