# Protein carbonylation causes sarcoplasmic reticulum Ca2+ overload by increasing intracellular Na+ level in ventricular myocytes

## TL;DR

This study shows that protein carbonylation from methylglyoxal in diabetic conditions disrupts heart cell calcium regulation by increasing sodium levels, leading to potential heart disease.

## Contribution

The study reveals a novel mechanism by which protein carbonylation disrupts cardiac calcium regulation through increased intracellular sodium.

## Key findings

- Methylglyoxal increases sarcoplasmic reticulum Ca2+ load and slows cytosolic Ca2+ extrusion via Na+/Ca2+ exchanger.
- Methylglyoxal-induced sodium influx activates tetrodinoxin-sensitive pathways, leading to spontaneous Ca2+ waves.
- Elevated sodium levels from methylglyoxal mimic the effects of Na+-K+ ATPase inhibition on calcium regulation.

## Abstract

Diabetes is commonly associated with an elevated level of reactive carbonyl species due to alteration of glucose and fatty acid metabolism. These metabolic changes cause an abnormality in cardiac Ca2+ regulation that can lead to cardiomyopathies. In this study, we explored how the reactive α-dicarbonyl methylglyoxal (MGO) affects Ca2+ regulation in mouse ventricular myocytes. Analysis of intracellular Ca2+ dynamics revealed that MGO (200 μM) increases action potential (AP)-induced Ca2+ transients and sarcoplasmic reticulum (SR) Ca2+ load, with a limited effect on L-type Ca2+ channel-mediated Ca2+ transients and SERCA-mediated Ca2+ uptake. At the same time, MGO significantly slowed down cytosolic Ca2+ extrusion by Na+/Ca2+ exchanger (NCX). MGO also increased the frequency of Ca2+ waves during rest and these Ca2+ release events were abolished by an external solution with zero [Na+] and [Ca2+]. Adrenergic receptor activation with isoproterenol (10 nM) increased Ca2+ transients and SR Ca2+ load, but it also triggered spontaneous Ca2+ waves in 27% of studied cells. Pretreatment of myocytes with MGO increased the fraction of cells with Ca2+ waves during adrenergic receptor stimulation by 163%. Measurements of intracellular [Na+] revealed that MGO increases cytosolic [Na+] by 57% from the maximal effect produced by the Na+-K+ ATPase inhibitor ouabain (20 μM). This increase in cytosolic [Na+] was a result of activation of a tetrodotoxin-sensitive Na+ influx, but not an inhibition of Na+-K+ ATPase. An increase in cytosolic [Na+] after treating cells with ouabain produced similar effects on Ca2+ regulation as MGO. These results suggest that protein carbonylation can affect cardiac Ca2+ regulation by increasing cytosolic [Na+] via a tetrodotoxin-sensitive pathway. This, in turn, reduces Ca2+ extrusion by NCX, causing SR Ca2+ overload and spontaneous Ca2+ waves.