Limulus and heart rhythm

Abstract

Great interest in the comparative physiology of hearts and their functions in Animalia has emerged with classic papers on Limulus polyphemus and mollusks. The recurrent cardiac activity-heart rate-is the most important physiological parameter and when present the kardia (Greek) is vital to the development of entire organs of the organisms in the animal kingdom. Extensive studies devoted to the regulation of cardiac rhythm in invertebrates have revealed that the basics of heart physiology are comparable to mammals. The hearts of invertebrates also beat spontaneously and are supplied with regulatory nerves: either excitatory or inhibitory or both. The distinct nerves and the source of excitation/inhibition at the level of single neurons are described for many invertebrate genera. The vertebrates and a majority of invertebrates have myogenic hearts, whereas the horseshoe crab L. polyphemus and a few other animals have a neurogenic cardiac rhythm. Nevertheless, the myogenic nature of heartbeat is precursor, because the contraction of native and stem-cellderived cardiomyocytes does occur in the absence of any neural elements. Even in L. polyphemus, the heart rhythm is myogenic at embryonic stages.

Probability that there is a mammalian counterpart of cardiac clock in insects

“…Insects do not have SAN, although they possess cardiac myocytes that generate rhythms. In many crustaceans, cardiac rhythm is generated by a small network of neurons residing in the heart (Kodirov et al, 2019). In vertebrates SAN myocytes are drivers of HR among all cardiac cells.…”

“…Previously, it was stated that “the HR was Ca 2+ and concentration‐dependent, although the effects started to saturate around 4 mM” (Kodirov et al, 2019) in contrary to assumptive approximately 8 mM based on Figure 3 (Feliciano et al, 2011). As the 4 mM is a standard concentration and beforehand there are only 1 and 2 mM tested doses, the exponential increase in HR can not be substantiated precisely.…”

“…Since specifically ZD7288 did not affect the HR, there is either no dedicated pacemaker region in invertebrates or HCN channels are absent, or both are distinct compared to vertebrates' (Kodirov et al, 2019). In particular, and in regard to HR − heart rate, the Figure 2 was covered entirely as 'BDM almost obliterates the contraction within 10 min, but the electrical activity changes only in rate and amplitude (Feliciano et al, 2011).…”

“…Note that the aforementioned study was extensively reviewed leading to conclusion that the ‘heart was insensitive to carbamylcholine, CsCl, isoproterenol, norepinephrine, and ZD7288 in established concentrations. Since specifically ZD7288 did not affect the HR, there is either no dedicated pacemaker region in invertebrates or HCN channels are absent, or both are distinct compared to vertebrates’ (Kodirov et al, 2019). In particular, and in regard to HR − heart rate, the Figure 2 was covered entirely as ‘BDM almost obliterates the contraction within 10 min, but the electrical activity changes only in rate and amplitude (Feliciano et al, 2011).…”

“…The depolarizing and repolarizing polarity distribution of electrograms also differ compared to control. However, 100 μM Cd 2+ completely and reversibly diminishes both parameters’ (Kodirov et al, 2019). Regrettably, Figure 2b is based on cut, copy and paste approach along the horizontal stretching − “decreased rate” to mimic hypothetical insufficient wash‐out effects (Figures 2−4).…”