Acetylcholine (ACh) is the neurotransmitter of the parasympathetic nervous system that modulates cardiac function, and its high concentrations may induce atrial fibrillation. We compared the ACh action on the mechanical function of single cardiomyocytes from the left atria (LA) and the right atria (RA). We exposed single rat LA and RA cardiomyocytes to 1, 10, and 100 µM ACh for 10–15 min and measured the parameters of sarcomere shortening–relengthening and cytosolic calcium ([Ca²⁺]_i) transients during cell contractions. We also studied the effects of ACh on cardiac myosin function using an in vitro motility assay and analyzed the phosphorylation level of sarcomeric proteins. In LA cardiomyocytes, ACh decreased the time to peak sarcomere shortening, time to 50% relengthening, and time to peak [Ca²⁺]_i transients. In RA cardiomyocytes, ACh affected the time of shortening and relengthening only at 10 µM. In the in vitro motility assay, ACh reduced to a greater extent the sliding velocity of F-actin over myosin from LA cardiomyocytes, which was accompanied by a more pronounced decrease in phosphorylation of the myosin regulatory light chain (RLC) in LA cardiomyocytes than in RA cardiomyocytes. Our findings indicate that ACh plays an important role in modulating the contractile function of LA and RA, provoking more pronounced changes in the time course of sarcomere shortening–relengthening and the kinetics of actin–myosin interaction in LA cardiomyocytes.

17β-estradiol (E2) is the most active metabolite of estrogen with a wide range of physiological action on cardiac muscle. Previous studies have reported E2 effects predominantly for the ventricles, while the E2 impact on the atria has been less examined. In this study, we focused on the direct E2 effects on atrial and ventricular contractility at the cellular and molecular levels. Single atrial and ventricular cardiomyocytes (CM) from adult (24 weeks-old) female Wistar rats were incubated with 10 nM E2 for 15 min. Sarcomere length and cytosolic [Ca²⁺]_i transients were measured in mechanically non-loaded CM, and the tension–length relationship was studied in CM mechanically loaded by carbon fibers. The actin–myosin interaction and sarcomeric protein phosphorylation were analyzed using an in vitro motility assay and gel electrophoresis with Pro-Q Diamond phosphoprotein stain. E2 had chamber-specific effects on the contractile function of CM with a pronounced influence on ventricular CM. The characteristics of [Ca²⁺]_i transients did not change in both atrial and ventricular CM. However, in ventricular CM, E2 reduced the amplitude and maximum velocity of sarcomere shortening and decreased the slope of the passive tension–length relationship that was associated with increased TnI and cMyBP-C phosphorylation. E2 treatment accelerated the cross-bridge cycle of both atrial and ventricular myosin that was associated with increased phosphorylation of the myosin essential light chain. This study shows that E2 impairs the mechanical function of the ventricular myocardium while atrial contractility remains mostly preserved. Hormonal replacement therapy (HRT) with estrogen is by far the most effective therapy for treating climacteric symptoms experienced during menopause. Here we found a chamber specificity of myocardial contractile function to E2 that should be taken into account for the potential side effects of HRT.

Obesity is a major risk factor for heart failure and atrial fibrillation. This study investigated the effects of diet-induced obesity on the molecular and cellular mechanisms of cardiomyocyte contractility in the left and right atria (LA and RA). Female Wistar rats were fed a Western diet (WD) for 18 weeks. Sarcomere dynamics and calcium transients were measured in unloaded cardiomyocytes. Actin–myosin interactions and contractile protein phosphorylation were assessed via an in vitro motility assay and phosphoprotein-specific gel electrophoresis. WD-fed rats developed obesity, hypertension, and metabolic alterations in the absence of echocardiographic or histological evidence of cardiac remodeling or systolic dysfunction. In LA cardiomyocytes, contractile dysfunction was indicated by increased calcium transient amplitude coupled with reduced shortening amplitude and relengthening velocity. This functional impairment correlated with a slowed myosin cross-bridge cycle and dephosphorylation of cMyBP-C. In contrast, RA cardiomyocytes displayed only molecular changes in response to obesity, including altered phosphorylation of most sarcomeric proteins and a decelerated cross-bridge cycle, but showed no evident contractile dysfunction. Thus, an 18-week WD reflects the early stages of contractile impairment, where functional deficits are specific to the LA, while RA alterations are confined to the molecular level.

Atrial fibrillation (AF) is the most common arrhythmia worldwide. AF is associated with a deficiency in nitric oxide (NO) production, which contributes to disturbances in the electrical and mechanical function of the atrial myocardium. NO donors are considered promising for the treatment and prevention of AF, but their effects on atrial contractility are unclear. This study examines the direct impact of a low-molecular-weight NO donor, spermine-NONOate (NOC-22), on the contractile function of atrial cardiomyocytes in paroxysmal AF. To study whether an NO donor-induced increase in NO level causes chamber-specific changes in atrial contractility, we measured sarcomere length (SL) dynamics in contracting single cardiomyocytes from the rat left and right atria (LA, RA) using a 7-day acetylcholine-CaCl₂-induced AF model. We showed that in control rats NOC-22 provoked alternans of sarcomere shortening in both LA and RA cardiomyocytes. In AF, NOC-22 decreased the sarcomere-shortening amplitudes and velocities of sarcomere shortening–relengthening and increased the magnitude of sarcomere-shortening alternans only in RA cardiomyocytes. The negative effects of NO donors on RA contractility warrant careful consideration of their use in AF treatment.