Evidence of primary cilia in the developing rat heart.

Background: A transient increase in cytosolic Ca²⁺ (the "Ca²⁺ transient") determines the degree and duration of myocyte force development in the heart. However, we have previously observed that, under the same experimental conditions, the Ca²⁺ transients from isolated cardiac myocytes are reduced in amplitude in comparison to those from multicellular cardiac preparations. We therefore questioned whether the enzymatic cell isolation procedure might remove structures that modulate intracellular Ca²⁺ in some way. Primary cilia are found in a diverse range of cell types, and have an abundance of Ca²⁺-permeable membrane channels that result in Ca²⁺ influx when activated. Although primary cilia are reportedly ubiquitous, their presence and function in the heart remain controversial. If present, we hypothesized they might provide an additional Ca²⁺ entry pathway in multicellular cardiac tissue that was lost during cell isolation. The aim of our study was to look for evidence of primary cilia in isolated myocytes and ventricular tissue from rat hearts.

Methods: Immunohistochemical techniques were used to identify primary cilia-specific proteins in isolated myocytes from adult rat hearts, and in tissue sections from embryonic, neonatal, young, and adult rat hearts. Either mouse anti-acetylated α-tubulin or rabbit polyclonal ARL13B antibodies were used, counterstained with Hoechst dye. Selected sections were also labelled with markers for other cell types found in the heart and for myocyte F-actin.

Results: No evidence of primary cilia was found in either tissue sections or isolated myocytes from adult rat ventricles. However, primary cilia were present in tissue sections from embryonic, neonatal (P2) and young (P21 and P28) rat hearts.

Conclusion: The lack of primary cilia in adult rat hearts rules out their contribution to myocyte Ca²⁺ homoeostasis by providing a Ca²⁺ entry pathway. However, evidence of primary cilia in tissue from embryonic and very young rat hearts suggests they have a role during development.