Regulation of Ca2 + signaling by acute hypoxia and acidosis in rat neonatal cardiomyocytes

Abstract Ischemic heart disease is an arrhythmogenic condition, accompanied by hypoxia, acidosis, and impaired Ca 2 + signaling. Here we report on effects of acute hypoxia and acidification in rat neonatal cardiomyocytes cultures. Results Two populations of neonatal cardiomyocyte were identified based on inactivation kinetics of L-type I Ca : rapidly-inactivating I Ca (τ ~ 20 ms) myocytes (prevalent in 3–4-day cultures), and slow-inactivating I Ca (τ ≥ 40 ms) myocytes (dominant in 7-day cultures). Acute hypoxia (pO 2 Ca reversibly in both cell-types to different extent and with different kinetics. This disparity disappeared when Ba 2 + was the channel charge carrier, or when the intracellular Ca 2 + buffering capacity was increased by dialysis of high concentrations of EGTA and BAPTA, suggesting critical role for calcium-dependent inactivation. Suppressive effect of acute acidosis on I Ca (~ 40%, pH 6.7), on the other hand, was not cell-type dependent. Isoproterenol enhanced I Ca in both cell-types, but protected only against suppressive effects of acidosis and not hypoxia. Hypoxia and acidosis suppressed global Ca 2 + transients by ~ 20%, but suppression was larger, ~ 35%, at the RyR2 microdomains, using GCaMP6-FKBP targeted probe. Hypoxia and acidosis also suppressed mitochondrial Ca 2 + uptake by 40% and 10%, respectively, using mitochondrial targeted Ca 2 + biosensor (mito-GCaMP6). Conclusion Our studies suggest that acute hypoxia suppresses I Ca in rapidly inactivating cell population by a mechanism involving Ca 2 + -dependent inactivation, while compromised mitochondrial Ca 2 + uptake seems also to contribute to I Ca suppression in slowly inactivating cell population. Proximity of cellular Ca 2 + pools to sarcolemmal Ca 2 + channels may contribute to the variability of inactivation kinetics of I Ca in the two cell populations, while acidosis suppression of I Ca appears mediated by proton-induced block of the calcium channel.