Activity-dependent intracellular Ca2+ transients in unmyelinated nerve fibres of the isolated adult rat vagus nerve

 Confocal laser scanning microscopy was used to follow changes in the free intracellular calcium concentration (Ca²⁺]_i) in nerve fibres and adjacent Schwann cells in isolated rat vagus nerves. Ca²⁺]_i was monitored by the Ca²⁺-sensitive fluorescent dyes Calcium Green-1 and Fura Red. Intracellular Ca²⁺ transients were observed during repetitive (1–50 Hz) supramaximal electrical stimulation or by bath application of ATP. Trains of action potentials were more effective at elongated, fibre-like structures of the vagus nerves, whereas ATP-induced Ca²⁺ transients were found predominantly in regions of Schwann cell bodies. Activity-induced Ca²⁺ signals were unaffected by pharmacological manipulation of intracellular Ca²⁺ stores, during long-lasting application of purinergic receptor agonists, or by substitution of extracellular Na⁺ with Li⁺. However, they were abolished in the presence of Ca²⁺-free bathing solution or after the blocking of Ca²⁺ channels with Cd²⁺. Ca²⁺ transients were also observed during Ca²⁺ action potentials. Such ”Ca²⁺ spikes” were elicited by electrical stimulation in the presence of a combination of tetrodotoxin and K⁺ channel blockers. These data suggest that voltage-dependent Ca²⁺ channels, activated during short trains of Na⁺ action potentials, produce an increase in intra-axonal Ca²⁺] of rat vagus nerves. We did not find evidence for activity-dependent Ca²⁺ transients in the Schwann cells surrounding the unmyelinated axons. Received: 16 September 1997 / Received after revision: 25 November 1997 / Accepted: 27 November 1997