New optical spike in frog twitch muscle fibers injected with a potential probe dye NK2935.

Abstract

Absorbance responses posterior to action potentials were recorded from frog twitch fibers loaded by an impermeant potential probe dye NK2935 that was chemically identical with WW781. Extracellularly loaded NK2935-related responses (EOS) increased in absorbance around 640 nm wavelengths and were like action potential in waveform but were slightly preceded by action potential. Model analysis showed that EOS consisted of two components, namely 39% non-delay component reflected with sarcolemma action potential and 61 % first-order delay component reflected with transverse tubule (T) action potential (delay-constant i1 = 0.48 ms). The T-disrupted fibers showed only the non-delayed response. Intracellularly injected NK2935-related responses (IOS) were also composed of a single spike of the same direction swing as in the EOS case described by the term "ipsidirection," but were clearly preceded by EOS and followed by Ca transient, birefringence, transparency and latency relaxation responses. The T-disrupted fibers showed a small "sidedness" response characterized by both opposite direction swing, namely "contradirection" and non-delay against EOS. Model analysis showed that IOS consisted of two main components, namely second-order delay component with ipsidirection (x2 =0.8 ms) and the further high-order delay component with contradirection. Based on spatio-temporal heterogeneity of NK2935 binding inside fibers and potential-absorbance relation in sarcolemma, the former origin was thought to be in junctional sarcoplasmic reticulum (jSR) responding to T excitation with a transient "positive polarization" estimated roughly 20 mV. Similarly, the latter was possibly in free SR where a slow "negative polarization" estimated -0.9 mV might appear associated with Ca release.