Physiological properties of zebrafish embryonic red and white muscle fibers during early development

The zebrafish is a model organism for studies of vertebrate muscle differen tiation and development. However, an understanding of fish muscle physiolog y during this period is limited. We examined the membrane, contractile, ele ctrical coupling, and synaptic properties of embryonic red (ER) and white ( EW) muscle fibers in developing zebrafish from 1 to 5 days postfertilizatio n. Resting membrane potentials were -73 mV in 1 day ER and -78 mV in 1 day EW muscle and depolarized 17 and 7 mV, respectively, by 5 days. Neither fib er type exhibited action potentials. Current-voltage relationships were lin ear in EW fibers and day 1 ER fibers but were outwardly rectifying in some ER fibers at 3 to 5 days. Both ER and EW fibers were contractile at all age s examined (1 to 5 days) and could follow trains of electrical stimulation of up to 30 Hz without fatiguing for up to 5 min. Synaptic activity consist ing of miniature endplate potentials (mEPPs) was observed at the earliest a ges examined (1.2-1.4 days) in both ER and EW fibers. Synaptic activity inc reased in frequency, and mEPP amplitudes were larger by 5 days. Miniature E PP rise times and half-widths decreased in ER fibers by 5 days, while EW fi ber mEPPs showed fast kinetics as early as 1.2-1.4 days. ER and EW muscle f ibers showed extensive dye coupling but not heterologous (red-white) coupli ng. Dye coupling decreased by 3 days yet remained at 5 days. Somites were e lectrically coupling, and this allowed filtered synaptic potentials to spre ad from myotome to myotome. It is concluded that at early developmental sta ges the physiological properties of ER and EW muscle are similar but not id entical and are optimized to the patterns of swimming observed at these sta ges.