Avian cardiomyocyte architecture and what it reveals about the evolution of the vertebrate heart.

Bird cardiomyocytes are long, thin and lack t-tubules, similar to ectothermic non-avian reptiles. Paradoxically, birds can achieve greater contractile rates and developed pressures than mammals, whose wide cardiomyocytes contain a dense transverse (t)-tubular network allowing for uniform excitation-contraction coupling and strong contractile force. To address this apparent contradiction, this paper functionally links recent electrophysiological studies on bird cardiomyocytes with decades of ultrastructure measurements. It shows that the strong transsarcolemmal Ca2+ influx via the L-type Ca2+ current (ICaL) and the high gain of Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR), coupled with the internal SR Ca2+ release relay system, facilitates the strong fast contractions in the long thin bird cardiomyocytes, without the need for t-tubules. The maintenance of the elongated myocyte morphology following the post-hatch transition from ectothermy to endothermy in birds is discussed in relation to cardiac load, myocyte ploidy level, and the cardiac regeneration potential of adult cardiomyocytes. Overall, the paper shows how little we know about cellular Ca2+ dynamics in the bird heart and suggests how increased research efforts in this area would provide vital information, and evolutionary perspective, in our quest to understand the role of myocyte architecture in the evolution of the vertebrate heart.