Meeting Abstract
Tentacles and arms of squid differ in that the two tentacles extend in 15-30 ms during prey capture, whereas the eight arms undergo slower bending without changing length. Fibers of the transverse muscle mass that generate tentacular extension are cross-striated and have a shortening velocity 10-fold higher than the obliquely-striated fibers in the transverse muscle mass of the arms that generate slower movements. Given the explosive elongation of tentacles, we hypothesized that electrical excitability of transverse tentacle fibers would be characterized by an all-or-nothing action potential mechanism. Whole-cell patch voltage-clamp experiments were carried out on enzymatically dissociated muscle-fiber fragments (5-20 pF capacitance) from small blocks of transverse muscle dissected from tentacles and arms. Na current (INa) density (normalized to cell capacitance) in tentacle fibers was 10-fold higher than that in arm fibers. INa was tetrodotoxin-sensitive and similar to that in squid giant axons. Voltage-dependent IK density was about 4-fold higher in tentacle fibers than in arms, and kinetics of activation and inactivation were slower in tentacles. In general, neither fiber type had significant ICa, although fibers of both types occasionally showed non-inactivating ICa. Under current-clamp conditions only transverse tentacle fibers generated all-or-nothing action potentials. Excitability of transverse tentacle fibers is thus appropritate for a rapid tentacular strike.
