Couplings, EJSR, Gap junctions, ECC, in Ratite Heart Conduction Fibers

Abstract

The ultrastructure of avian hearts has provided seminal information regarding striated muscle function. Extended junctional SR (EJSR) in avian hearts suggests that excitation-contraction coupling (ECC) is accomplished through a diffusible transmitter substance, but raises questions regarding the function of “couplings” in general. Other information gleaned from small avian hearts has elucidated adaptations of the individual and appositional geometry of the conduction fibers (CF) to functional demands imposed by different heart sizes and rates, as well as the relationship between the size of gap junctions (GJ), cell input resistance and coupling resistance between cells. The hearts of ratites (ostrich, emu, rhea) are very large (> 1 kg) and beat at about 50/min. Their working myocytes are almost identical to those found in chickens.We investigated the following questions: 1. Do ratite CF contain “couplings”, EJSR and corbular SR? 2. Are there differences in geometry between the CF of small, fast beating, and large, slow beating hearts as is the case in mammals?