Local Ca2+ coupling between mitochondria and sarcoplasmic reticulum following depolarization in guinea pig urinary bladder smooth muscle cells

Abstract

Spatiotemporal changes in cytosolic Ca2+ concentration ([Ca2+]c) trigger a number of physiological functions in smooth muscle cells (SMCs). We previously imaged Ca2+-induced Ca2+ release following membrane depolarization as local Ca2+ transients, Ca2+ hotspots, in subplasmalemmal regions. In this study, the physiological significance of mitochondria on local Ca2+ signaling was examined. Cytosolic and mitochondrial Ca2+ images following depolarization or action potentials were recorded in single SMCs from the guinea pig urinary bladder using a fast-scanning confocal fluorescent microscope. Depolarization- and action potential-induced [Ca2+]c transients occurred at several discrete sites in subplasmalemmal regions, peaked within 30 ms, and then spread throughout the whole-cell. In contrast, Ca2+ concentration in the mitochondria matrix ([Ca2+]m) increased after a delay of ~50 ms from the start of depolarization, and then peaked within 500 ms. Following repolarization, [Ca2+]c returned to the resting level with a half-decay time of ~500 ms, while [Ca2+]m recovered more slowly (∼1.5 s). Carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, a mitochondrial uncoupler, abolished depolarization-induced [Ca2+]m elevations and slowed [Ca2+]c changes. Importantly, short depolarization-induced changes in [Ca2+]m and transmembrane potential in mitochondria coupled to Ca2+ hotspots were significantly larger than those in other mitochondria. Total internal reflection fluorescence imaging revealed that a subset of mitochondria closely localized with ryanodine receptors and voltage-dependent Ca2+ channels. These results indicate that particular mitochondria are functionally coupled to ion channels and sarcoplasmic reticulum fragments within the local Ca2+ microdomain, and thus, strongly contribute to [Ca2+]c regulation in SMCs.