Differences in Effects of Length-Dependent Regulation of Force and Ca2+ Transient in the Myocardial Trabeculae of the Rat Right Atrium and Ventricle

Abstract

The comparative differences in the fundamental mechanisms of contractility regulation and calcium handling of atrial and ventricular myocardium remain poorly studied. An isometric force–length protocol was performed for the entire range of preloads in isolated rat right atrial (RA) and ventricular (RV) trabeculae with simultaneous measurements of force (Frank-Starling mechanism) and Ca2+ transients (CaT). Differences were found between length-dependent effects in RA and RV muscles: (a) the RA muscles were stiffer, faster, and presented with weaker active force than the RV muscles throughout the preload range; (b) the active/passive force—length relationships were almost linear for the RA and RV muscles; (c) the value of the relative length-dependent growth of passive/active mechanical tension did not differ between the RA and RV muscles; (d) the time-to-peak and amplitude of CaT did not differ between the RA and RV muscles; (e) the CaT decay phase was essentially monotonic and almost independent of preload in the RA muscles, but not in the RV muscles. Higher peak tension, prolonged isometric twitch, and CaT in the RV muscle may be the result of higher Ca2+ buffering by myofilaments. The molecular mechanisms that constitute the Frank-Starling mechanism are common in the rat RA and RV myocardium.

Differences in Mechanical, Electrical and Calcium Transient Performance of the Isolated Right Atrial and Ventricular Myocardium of Guinea Pigs at Different Preloads (Lengths)

“…The amplitude and relative area under the CaT difference curve in the III phase increase with increasing cardiac muscle stretch for both RA strips and RV trabeculae of guinea pigs, with no significant differences between RA and RV. However, such differences are found in the data we obtained in rats [34].…”

“…In the atria, on the other hand, it is possible to cut a strip of tissue with the same shape in the same location, thus reducing the inhomogeneity of the strip shape. It is worth noting that, in contrast to guinea pig muscle preparations, the active tension of rat RV trabeculae is significantly greater than the active tension of rat RA strips [34]. In our experiments, a higher level of passive tension was observed for atrial strips compared to ventricular trabeculae over the entire length-range studied.…”

“…It has been shown that, with the increasing length of guinea pig atrial and ventricular preparations, the initial phase of the CaT decay is accelerated, and the final phase of the CaT decay is slowed (Figure 6A,C). Previously, we had noted a similar effect in isolated rat preparations, where the final phase was termed the "bump" [34,55]. The non-monotonic CaT decay reflects the complex kinetics of calcium ion sequestration in cardiomyocytes.…”

“…In light of morphological differences in the structure of the sarcoplasmic reticulum, a comparison of the temporal and spatial characteristics of CaTs in atrial and ventricular cardiomyocytes may provide clues as to how cellular organisation affects the behaviour of the electromechanical coupling mechanism at different preloads [33]. With regard to the present work, it should be emphasised that there is a lack of data studying the effects of sequential changes in the myocardial length of the different heart chambers of different animals, including the guinea pig, on electromechanical coupling, with the exception of our work on the rat heart [34]. The present study was performed on guinea pig myocardial preparations because their myocardium differs from the rat myocardium in electromechanical coupling and is similar to the human myocardium [21,35,36].…”

“…Sci. 2023, 24, x FOR PEER REVIEW 3 of 17 the rat heart [34]. The present study was performed on guinea pig myocardial preparations because their myocardium differs from the rat myocardium in electromechanical coupling and is similar to the human myocardium [21,35,36].…”