PT  - JOURNAL ARTICLE
AU  - Leah M. Costello
AU  - Daniel García-Párraga
AU  - Jose Luis Crespo-Picazo
AU  - Jonathan Codd
AU  - Holly A. Shiels
AU  - William Joyce
TI  - Absence of atrial smooth muscle in the heart of the loggerhead sea turtle (<em>Caretta caretta</em>): a re-evaluation of its role in diving physiology
AID  - 10.1101/2022.08.01.502306
DP  - 2022 Jan 01
TA  - bioRxiv
PG  - 2022.08.01.502306
4099  - http://biorxiv.org/content/early/2022/08/03/2022.08.01.502306.short
4100  - http://biorxiv.org/content/early/2022/08/03/2022.08.01.502306.full
AB  - Contraction of atrial smooth muscle in the hearts of semi-aquatic emydid turtles regulates ventricular filling, and it has been proposed that it could regulate stroke volume during characteristic rapid transitions in cardiac output associated with diving. For this hypothesis to be supported, atrial smooth muscle should be widely distributed in diving Testudines. To further understand the putative function and evolutionary significance of endocardial smooth muscle in Testudines, we studied the hearts of loggerhead sea turtles, Caretta caretta (n=7), using immunohistochemistry and histology. Surprisingly, we found no evidence of prominent atrial smooth muscle in C. caretta. However, smooth muscle was readily identified in the sinus venosus. Our results suggest atrial smooth muscle does not contribute to the diving capabilities of C. caretta, indicating that the possible roles of smooth muscle in emydid turtle hearts requires a re-evaluation. In sea turtles, the sinus venosus may instead contribute to regulate cardiac filling.Competing Interest StatementThe authors have declared no competing interest.