Ca2+ influx through muscle-type nicotinic AChRs contributes to contractions and development of slow muscle cells in early developmental stages

Although the difference between the characteristics of fast and slow muscles has been extensively studied, it is still not fully understood. Here, we propose that nicotinic acetylcholine receptors (AChRs) expressed in slow muscles of zebrafish have high Ca ²⁺ permeability compared to that of AChRs of fast muscles. To analyze the significance of the Ca ²⁺ influx through AChRs in slow muscles, we generated a transgenic (Tg) zebrafish line that expresses Ca ²⁺ -impermeable AChRs in its slow muscles. The locomotor activities of the Tg zebrafish were markedly decreased at 1-3 days post fertilization (dpf) compared to those of zebrafish expressing Ca ²⁺ -permeable AChRs in its slow muscles. Ca ²⁺ imaging suggested that Ca ²⁺ influx via AChRs is crucial for the Ca ²⁺ response during muscle contraction in 2 dpf larvae, as slow muscle cells of the Tg line lacked a sustained Ca ²⁺ response. Furthermore, we found that slow muscles of the Tg line became thinner compared to those expressing Ca ²⁺ -permeable AChRs. These short Ca ²⁺ responses and thinner slow muscles may have induced locomotion impairment in the Tg line. These results suggested the physiological roles of the Ca ²⁺ influx through AChRs in slow muscles and provided new insights into the characterization of fast and slow muscles.